1 Foundation Classes {#occt_user_guides__foundation_classes}
2 =================================
6 @section occt_fcug_1 Introduction
8 @subsection occt_fcug_1_1 Foundation Classes Overview
10 This manual explains how to use Open CASCADE Technology (**OCCT**) Foundation Classes. It provides basic documentation on foundation classes. For advanced information on foundation classes and their applications, see our offerings on our web site at <a href="http://www.opencascade.org/support/training/">www.opencascade.org/support/training/</a>
11 Foundation Classes provide a variety of general-purpose services such as automated dynamic memory management (manipulation of objects by handle), collections, exception handling, genericity by downcasting and plug-in creation.
12 Foundation Classes include the following:
15 Root classes are the basic data types and classes on which all the other classes are built. They provide:
16 * fundamental types such as Boolean, Character, Integer or Real,
17 * safe handling of dynamically created objects, ensuring automatic deletion of unreferenced objects (see the Standard_Transient class),
18 * configurable optimized memory manager increasing the performance of applications that intensively use dynamically created objects,
19 * extended run-time type information (RTTI) mechanism facilitating the creation of complex programs,
20 * management of exceptions,
21 * encapsulation of C++ streams.
22 Root classes are mainly implemented in the *Standard* and *MMgt* packages.
25 Strings are classes that handle dynamically sized sequences of characters based on both ASCII (normal 8-bit character type) and Unicode (16-bit character type).
26 Strings may also be manipulated by handles, and consequently be shared.
27 Strings are implemented in the *TCollection* package.
30 Collections are the classes that handle dynamically sized aggregates of data.
31 Collection classes are *generic*, that is, they define a structure and algorithms allowing to hold a variety of objects which do not necessarily inherit from a unique root class (similarly to C++ templates). When you need to use a collection of a given type of object, you must *instantiate* it for this specific type of element. Once this declaration is compiled, all functions available on the generic collection are available on your *instantiated class*.
33 Collections include a wide range of generic classes such as run-time sized arrays, lists, stacks, queues, sets and hash maps.
34 Collections are implemented in the *TCollection* and *NCollection* packages.
36 ### Collections of Standard Objects
38 The *TColStd* package provides frequently used instantiations of generic classes from the *TCollection* package with objects from the *Standard* package or strings from the *TCollection* package.
40 ### Vectors and Matrices
43 These classes provide commonly used mathematical algorithms and basic calculations (addition, multiplication, transposition, inversion, etc.) involving vectors and matrices.
45 ### Primitive Geometric Types
47 Open CASCADE Technology primitive geometric types are a STEP-compliant implementation of basic geometric and algebraic entities.
49 * Descriptions of elementary geometric shapes:
54 * Planes and elementary surfaces,
55 * Positioning of these shapes in space or in a plane by means of an axis or a coordinate system,
56 * Definition and application of geometric transformations to these shapes:
60 * Scaling transformations
61 * Composed transformations
62 * Tools (coordinates and matrices) for algebraic computation.
64 ### Common Math Algorithms
66 Open CASCADE Technology common math algorithms provide a C++ implementation of the most frequently used mathematical algorithms.
68 * Algorithms to solve a set of linear algebraic equations,
69 * Algorithms to find the minimum of a function of one or more independent variables,
70 * Algorithms to find roots of one, or of a set, of non-linear equations,
71 * Algorithms to find the eigen-values and eigen-vectors of a square matrix.
75 A hierarchy of commonly used exception classes is provided, all based on class Failure, the root of exceptions.
76 Exceptions describe exceptional situations, which can arise during the execution of a function. With the raising of an exception, the normal course of program execution is abandoned. The execution of actions in response to this situation is called the treatment of the exception.
80 These are various classes supporting date and time information and fundamental types representing most physical quantities such as length, area, volume, mass, density, weight, temperature, pressure etc.
82 ### Application services
84 Foundation Classes also include implementation of several low-level services that facilitate the creation of customizable and user-friendly applications with Open CASCADE Technology. These include:
85 * Unit conversion tools, providing a uniform mechanism for dealing with quantities and associated physical units: check unit compatibility, perform conversions of values between different units and so on (see package *UnitsAPI*).
86 * Basic interpreter of expressions that facilitates the creation of customized scripting tools, generic definition of expressions and so on (see package *ExprIntrp*)
87 * Tools for dealing with configuration resource files (see package *Resource*) and customizable message files (see package *Message*), making it easy to provide a multi-language support in applications
88 * Progress indication and user break interfaces, giving a possibility even for low-level algorithms to communicate with the user in a universal and convenient way.
91 @subsection occt_fcug_1_2 Fundamental Concepts
92 An object-oriented language structures a system around data types rather than around the actions carried out on this data. In this context, an **object** is an **instance** of a data type and its definition determines how it can be used. Each data type is implemented by one or more classes, which make up the basic elements of the system.
94 In Open CASCADE Technology the classes are usually defined using CDL (CASCADE Definition Language) that provides a certain level of abstraction from pure C++ constructs and ensures a definite level of similarity in the implementation of classes. See *CDL User’s Guide* for more details.
96 This chapter introduces some basic concepts most of which are directly supported by CDL and used not only in Foundation Classes, but throughout the whole OCCT library.
98 @subsubsection occt_fcug_1_2_1 Modules and toolkits
99 The whole OCCT library is organized in a set of modules. The first module, providing most basic services and used by all other modules, is called Foundation Classes and described by this manual.
101 Every module consists primarily of one or several toolkits (though it can also contain executables, resource units etc.). Physically a toolkit is represented by a shared library (e.g. .so or .dll). The toolkit is built from one or several packages.
103 @subsubsection occt_fcug_1_2_2 Packages
104 A **package** groups together a number of classes which have semantic links. For example, a geometry package would contain Point, Line, and Circle classes. A package can also contain enumerations, exceptions and package methods (functions). In practice, a class name is prefixed with the name of its package e.g.
106 Data types described in a package may include one or more of the following data types:
110 * Pointers to other object classes
111 Inside a package, two data types cannot bear the same name.
113 @image html /user_guides/foundation_classes/images/foundation_classes_image003.png "Contents of a package"
114 @image latex /user_guides/foundation_classes/images/foundation_classes_image003.png "Contents of a package"
116 **Methods** are either **functions** or **procedures**. Functions return an object, whereas procedures only communicate by passing arguments. In both cases, when the transmitted object is an instance manipulated by a handle, its identifier is passed. There are three categories of methods:
117 * **Object constructor** Creates an instance of the described class. A class will have one or more object constructors with various different arguments or none.
118 * **Instance method** Operates on the instance which owns it.
119 * **Class method** Does not work on individual instances, only on the class itself.
121 @subsubsection occt_fcug_1_2_3 Classes
122 The fundamental software component in object-oriented software development is the class. A class is the implementation of a **data type**. It defines its **behavior** (the services offered by its functions) and its **representation** (the data structure of the class – the fields, which store its data).
124 #### Categories of Classes
125 Classes fall into three categories:
127 * Deferred classes. A **deferred class** cannot be instantiated. The purpose of having such classes is to have a given behavior shared by a hierarchy of classes and dependent on the implementation of the descendants. This is a way of guaranteeing a certain base of inherited behavior common to all the classes based on a particular deferred class. The C++ equivalent of a deferred CDL class is an abstract class.
128 * Generic classes. A **generic class** offers a set of functional behaviors to manipulate other data types. Instantiation of a generic class requires that a data type is given for its argument(s). The generic classes in CDL perform the same mission as template classes in C++.
130 @subsubsection occt_fcug_1_2_4 Genericity
131 Generic classes are implemented in two steps. First you declare the generic class to establish the model, then you instantiate this class by giving information about the generic types.
133 #### Declaring a Generic Class
135 The generic classes in Open CASCADE Technology are similar by their intent to C++ templates with explicit instantiation.
136 A generic class is declared in CDL as operating on data items of non-fixed types which are declared as arguments of the generic class. It is possible to put a restriction on these data types to be of subtype of some definite class. Definition of the generic class does not create new class type in C++ terms; it only defines a pattern for generation (instantiation) of the real classes.
138 #### Instantiation of a Generic Class
140 When a generic class is instantiated, its argument types are substituted by actually existing data types (elementary types or classes). The result of instantiation is a new C++ class with an arbitrary name (specified in the instantiating declaration). By convention, the name of the instantiated class is usually constructed from the name of the generic class and names of actual argument types. As for any other class, the name of the class instantiating a generic type is prefixed by the name of the package in which instantiation is declared.
142 class Array1OfReal instantiates Array1 from TCollection (Real);
145 This declaration located in a CDL file of the *TColStd* package defines a new C++ class *TColStd_Array1OfReal* as the instantiation of generic class *TCollection_Array1* for *Real* values.
146 More than one class can be instantiated from the same generic class with the same argument types. Such classes will be identical by implementation, but considered as two different classes by C++.
147 No class can inherit from a generic class.
148 A generic class can be a deferred class. A generic class can also accept a deferred class as its argument. In both these cases, any class instantiated from it will also be deferred. The resulting class can then be inherited by another class.
150 #### Nested Generic Classes
152 It often happens that many classes are linked by a common generic type. This is the case when a base structure furnishes an iterator. In this context, it is necessary to make sure that the group of linked generic classes is indeed instantiated for the same type of object. In order to group the instantiation, you may declare certain classes as being nested.
153 When generic class is instantiated, its nested classes are instantiated as well. The name of the instantiation of the nested class is constructed from the name of that nested class and name of the main generic class, connected by ‘Of’.
155 class MapOfReal instantiates Map from TCollection (Real,MapRealHasher);
157 This declaration in *TColStd* defines not only class *TColStd_MapOfReal*, but also class *TColStd_MapIteratorOfMapOfReal*, which is instantiated from nested class *MapIterator* of the generic class *TCollection_Map*. Note that instantiation of the nested class is separate class, it is not nested class to the instantiation of the main class.
158 **Nested classes**, even though they are described as non-generic classes, are generic by construction being inside the class they are a member of.
160 @subsubsection occt_fcug_1_2_5 Inheritance
161 The purpose of inheritance is to reduce the development workload. The inheritance mechanism allows a new class to be declared already containing the characteristics of an existing class. This new class can then be rapidly specialized for the task in hand. This avoids the necessity of developing each component “from scratch”.
162 For example, having already developed a class *BankAccount* you could quickly specialize new classes: *SavingsAccount, LongTermDepositAccount, MoneyMarketAccount, RevolvingCreditAccount*, etc....
164 The corollary of this is that when two or more classes inherit from a parent (or ancestor) class, all these classes guarantee as a minimum the behavior of their parent (or ancestor). For example, if the parent class BankAccount contains the method Print which tells it to print itself out, then all its descendent classes guarantee to offer the same service.
166 One way of ensuring the use of inheritance is to declare classes at the top of a hierarchy as being **deferred**. In such classes, the methods are not implemented. This forces the user to create a new class which redefines the methods. This is a way of guaranteeing a certain minimum of behavior among descendent classes.
168 @subsubsection occt_fcug_1_2_6 Categories of Data Types
169 The data types in Open CASCADE Technology fall into two categories:
170 * Data types manipulated by handle (or reference)
171 * Data types manipulated by value
173 @image html /user_guides/foundation_classes/images/foundation_classes_image004.png "Manipulation of data types"
174 @image latex /user_guides/foundation_classes/images/foundation_classes_image004.png "Manipulation of data types"
176 A data type is implemented as a class. The class not only defines its data representation and the methods available on instances, but it also suggests how the instance will be manipulated.
177 * A variable of a type manipulated by value contains the instance itself.
178 * A variable of a type manipulated by handle contains a reference to the instance.
179 The first examples of types manipulated by values are the predefined **primitive types**: *Boolean, Character, Integer, Real*, etc.
181 A variable of a type manipulated by handle which is not attached to an object is said to be **null**. To reference an object, we instantiate the class with one of its constructors. For example, in C++:
184 Handle(myClass) m = new myClass;
187 In Open CASCADE Technology, the Handles are specific classes that are used to safely manipulate objects allocated in the dynamic memory by reference, providing reference counting mechanism and automatic destruction of the object when it is not referenced.
189 @subsubsection occt_fcug_1_2_7 Exceptions
190 The behavior of any object is implemented by the methods, which were defined in its class declaration. The definition of these methods includes not only their signature (their programming interface) but also their domain of validity.
192 This domain is expressed by **exceptions**. Exceptions are raised under various error conditions. This mechanism is a safeguard of software quality.
194 @subsubsection occt_fcug_1_2_8 Persistence and Data Schema
195 The data schema is the structure used by an application to store its data. Data schemas consist of persistent classes.
197 An object is called **persistent** if it can be permanently stored. Thus, the object can be reused at a later date by the application, which created it, or by another application.
199 In order for an object to be persistent for CDL, its type must be declared as inheriting from the class *Standard_Persistent* or have a parent class inheriting from the *Standard_Persistent* class. Note that classes inheriting from *Standard_Persistent* are handled by a reference.
201 Objects instantiated from classes which inherit from the Standard_Storable class cannot themselves be stored individually, but they can be stored as fields of an object which inherits from *Standard_Persistent*. Note that objects inheriting from *Standard_Storable* are handled by a value.
203 @section occt_fcug_2 Basics
204 This chapter deals with basic services such as memory management, programming with handles, primitive types, exception handling, genericity by downcasting and plug-in creation.
205 @subsection occt_fcug_2_1 Data Types
206 @subsubsection occt_fcug_2_1_1 Primitive Types
207 The primitive types are predefined in the language and they are **manipulated by value**.
208 Some of these primitives inherit from the **Storable** class. This means they can be used in the implementation of persistent objects, either contained in entities declared within the methods of the object, or they form part of the internal representation of the object.
210 The primitives inheriting from *Standard_Storable* are the following:
211 * **Boolean** is used to represent logical data. It may have only two values: *Standard_True* and *Standard_False*.
212 * **Character** designates any ASCII character.
213 * **ExtCharacter** is an extended character.
214 * **Integer** is a whole number.
215 * **Real** denotes a real number (i.e. one with whole and a fractional part, either of which may be null).
216 * **ShortReal** is a real with a smaller choice of values and memory size.
217 There are also non-Storable primitives. They are:
218 * **CString** is used for literal constants.
219 * **ExtString** is an extended string.
220 * **Address** represents a byte address of undetermined size.
221 The services offered by each of these types are described in the **Standard** Package.
222 The table below presents the equivalence existing between C++ fundamental types and OCCT primitive types.
224 **Table 1: Equivalence between C++ Types and OCCT Primitive Types**
226 | C++ Types | OCCT Types |
227 | :--------- | :----------- |
228 | int | Standard_Integer |
229 | double | Standard_Real |
230 | float | Standard_ShortReal |
231 | unsigned int | Standard_Boolean |
232 | char | Standard_Character |
233 | short | Standard_ExtCharacter |
234 | char\* | Standard_CString |
235 | void\* | Standard_Address |
236 | short\* | Standard_ExtString |
238 \* The types with asterisk are pointers.
241 **Reminder of the classes listed above:**
243 * **Standard_Integer** : fundamental type representing 32-bit integers yielding negative, positive or null values. *Integer* is implemented as a *typedef* of the C++ *int* fundamental type. As such, the algebraic operations +, -, *, / as well as the ordering and equivalence relations <, <=, ==, !=, >=, > are defined on it.
244 * **Standard_Real** : fundamental type representing real numbers with finite precision and finite size. **Real** is implemented as a *typedef* of the C++ *double* (double precision) fundamental type. As such, the algebraic operations +, -, *, /, unary- and the ordering and equivalence relations <, <=, ==, !=, >=, > are defined on reals.
245 * **Standard_ShortReal** : fundamental type representing real numbers with finite precision and finite size. *ShortReal* is implemented as a *typedef* of the C++ *float* (simple precision) fundamental type. As such, the algebraic operations +, -, *, /, unary- and the ordering and equivalence relations <, <=, ==, !=, >=, > are defined on reals.
246 * **Standard_Boolean** : fundamental type representing logical expressions. It has two values: *false* and *true*. *Boolean* is implemented as a *typedef* of the C++ *unsigned int* fundamental type. As such, the algebraic operations *and, or, xor* and *not* as well as equivalence relations == and != are defined on Booleans.
247 * **Standard_Character** : fundamental type representing the normalized ASCII character set. It may be assigned the values of the 128 ASCII characters. *Character* is implemented as a *typedef* of the C++ *char* fundamental type. As such, the ordering and equivalence relations <, <=, ==, !=, >=, > are defined on characters using the order of the ASCII chart (ex: A B).
248 * **Standard_ExtCharacter** : fundamental type representing the Unicode character set. It is a 16-bit character type. *ExtCharacter* is implemented as a *typedef* of the C++ *short* fundamental type. As such, the ordering and equivalence relations <, <=, ==, !=, >=, > are defined on extended characters using the order of the UNICODE chart (ex: A B).
249 * **Standard_CString** : fundamental type representing string literals. A string literal is a sequence of ASCII (8 bits) characters enclosed in double quotes. *CString* is implemented as a *typedef* of the C++ *char* fundamental type.
250 * **Standard_Address** : fundamental type representing a generic pointer. *Address* is implemented as a *typedef* of the C++ *void* fundamental type.
251 * **Standard_ExtString** is a fundamental type representing string literals as sequences of Unicode (16 bits) characters. *ExtString* is implemented as a *typedef* of the C++ *short* fundamental type.
253 @subsubsection occt_fcug_2_1_2 Types manipulated by value
254 There are three categories of types which are manipulated by value:
257 * Types defined by classes not inheriting from *Standard_Persistent* or *Standard_Transient*, whether directly or not.
258 Types which are manipulated by value behave in a more direct fashion than those manipulated by handle and thus can be expected to perform operations faster, but they cannot be stored independently in a file.
260 @image html /user_guides/foundation_classes/images/foundation_classes_image005.png "Manipulation of a data type by value"
261 @image latex /user_guides/foundation_classes/images/foundation_classes_image005.png "Manipulation of a data type by value"
263 Types that are known to the schema (i.e. they are either **primitives** or they inherit from *Storable*) and are manipulated by value, can be stored inside a persistent object as part of the representation. Only in this way can a “manipulated by value” object be stored in a file.
265 @subsubsection occt_fcug_2_1_3 Types manipulated by reference (handle)
266 There are two categories of types which are manipulated by handle:
267 * Types defined by classes inheriting from the *Persistent* class, which are therefore storable in a file.
268 * Types defined by classes inheriting from the *Transient* class.
270 @image html /user_guides/foundation_classes/images/foundation_classes_image006.png "Manipulation of a data type by reference"
271 @image latex /user_guides/foundation_classes/images/foundation_classes_image006.png "Manipulation of a data type by reference"
273 @subsubsection occt_fcug_2_1_4 Summary of properties
275 The following table summarizes how various data types are handled and stored.
277 | Type | Manipulated by handle | Manipulated by value |
278 | :------- | :-------------------- | :-------------------- |
279 | storable | Persistent | Primitive, Storable (if nested in a persistent class)|
280 |temporary | Transient | Other |
283 @subsection occt_fcug_2_2 Programming with Handles
284 @subsubsection occt_fcug_2_2_1 Handle Definition
285 A handle may be compared with a C++ pointer. Several handles can reference the same object. Also, a single handle may reference several objects, but only one at a time. To have access to the object it refers to, the handle must be de-referenced just as with a C++ pointer.
287 Transient and Persistent classes may be manipulated either with handles or with values. Handles which reference non-persistent objects are called non-storable handles; therefore, a persistent object cannot contain a non-storable handle.
289 #### Organization of Classes
291 Classes used with handles are persistent or transient.
293 Classes that inherit from *Standard_Transient* are transient while classes that inherit from *Standard_Persistent* are persistent.
295 In this chapter we will discuss only transient classes and relevant handles. Persistent classes and their handles are organized in a similar manner.
297 Class *Standard_Transient* is a root of a big hierarchy of OCCT classes that are said to be operable by handles. It provides a reference counter field, inherited by all its descendant classes, that is used by associated *Handle()* classes to track a number of handles pointing to this instance of the object.
299 For every class derived (directly or indirectly) from *Transient*, CDL extractor creates associated class *Handle()* whose name is the same as the name of that class prefixed by *Handle_*. Open CASCADE Technology provides preprocessor macro *Handle()* that produces a name of a *Handle()* class for a given transient class name.
304 A handle is characterized by the object it references.
306 Before performing any operation on a transient object, you must declare the handle. For example, if Point and Line are two transient classes from the Geom package, you would write:
308 Handle(Geom_Point) p1, p2;
310 Declaring a handle creates a null handle that does not refer to any object. The handle may be checked to be null by its method *IsNull()*. To nullify a handle, use method *Nullify()*.
312 To initialize a handle, either a new object should be created or the value of another handle can be assigned to it, on condition that their types are compatible.
314 **Note** that handles should only be used for object sharing. For all local operations, it is advisable to use classes manipulated by values.
316 @subsubsection occt_fcug_2_2_2 Type Management
321 Open CASCADE Technology provides a means to describe the hierarchy of data types in a generic way, with a possibility to check the exact type of the given object at run-time (similarly to C++ RTTI). For every class type derived from *Standard_Transient*, CDL extractor creates a code instantiating single instance of the class *Standard_Type* (type descriptor) that holds information on that type: its name and list of ancestor types.
322 That instance (actually, a handle on it) is returned by the virtual method *DynamicType()* of the class derived from *Standard_Transient*. The other virtual method *IsKind()* provides a means to check whether a given object has specified type or inherits it.
324 In order to refer to the type descriptor object for a given class type, use macros *STANDARD_TYPE()* with argument being a name of the class.
328 The type used in the declaration of a handle is the static type of the object, the type seen by the compiler. A handle can reference an object instantiated from a subclass of its static type. Thus, the dynamic type of an object (also called the actual type of an object) can be a descendant of the type which appears in the handle declaration through which it is manipulated.
330 Consider the persistent class *CartesianPoint*, a sub-class of *Point*; the rule of type conformity can be illustrated as follows:
333 Handle (Geom_Point) p1;
334 Handle (Geom_CartesianPoint) p2;
335 p2 = new Geom_CartesianPoint;
336 p1 = p2; // OK, the types are compatible
340 The compiler sees p1 as a handle to *Point* though the actual object referenced by *p1* is of the *CartesianPoint* type.
342 #### Explicit Type Conversion
345 According to the rule of type conformity, it is always possible to go up the class hierarchy through successive assignments of handles. On the other hand, assignment does not authorize you to go down the hierarchy. Consequently, an explicit type conversion of handles is required.
347 A handle can be converted explicitly into one of its sub-types if the actual type of the referenced object is a descendant of the object used to cast the handle. If this is not the case, the handle is nullified (explicit type conversion is sometimes called a “safe cast”). Consider the example below.
350 Handle (Geom_Point) p1;
351 Handle (Geom_CartesianPoint) p2, p3;
352 p2 = new Geom_CartesianPoint;
353 p1 = p2; // OK, standard assignment
354 p3 = Handle (Geom_CartesianPoint)::DownCast (p1);
355 // OK, the actual type of p1 is CartesianPoint, although the static type of the handle is Point
358 If conversion is not compatible with the actual type of the referenced object, the handle which was “cast” becomes null (and no exception is raised). So, if you require reliable services defined in a sub-class of the type seen by the handle (static type), write as follows:
361 void MyFunction (const Handle(A) & a)
363 Handle(B) b = Handle(B)::Downcast(a);
365 // we can use “b” if class B inherits from A
368 // the types are incompatible
372 Downcasting is used particularly with collections of objects of different types; however, these objects should inherit from the same root class.
374 For example, with a sequence of transient objects *SequenceOfTransient* and two classes A and B that both inherit from *Standard_Transient*, you get the following syntax:
379 Handle (Standard_Transient) t;
380 SequenceOfTransient s;
386 // here, you cannot write:
389 a = Handle (A)::Downcast (t)
391 // types are compatible, you can use a
394 // the types are incompatible
398 @subsubsection occt_fcug_2_2_3 Using Handles to Create Objects
399 To create an object which is manipulated by handle, declare the handle and initialize it with the standard C++ **new** operator, immediately followed by a call to the constructor. The constructor can be any of those specified in the source of the class from which the object is instanced.
402 Handle (Geom_CartesianPoint) p;
403 p = new Geom_CartesianPoint (0, 0, 0);
406 Unlike for a pointer, the **delete** operator does not work on a handle; the referenced object is automatically destroyed when no longer in use.
408 @subsubsection occt_fcug_2_2_4 Invoking Methods
409 Once you have a handle on a persistent or transient object, you can use it like a pointer in C++. To invoke a method which acts on the referenced object, you translate this method by the standard *arrow* operator, or alternatively, by function call syntax when this is available.
411 To test or to modify the state of the handle, the method is translated by the *dot* operator.
412 The example below illustrates how to access the coordinates of an (optionally initialized) point object:
415 Handle (Geom_CartesianPoint) centre;
416 Standard_Real x, y, z;
417 if (centre.IsNull()) {
418 centre = new PGeom_CartesianPoint (0, 0, 0);
420 centre->Coord(x, y, z);
423 The example below illustrates how to access the type object of a Cartesian point:
426 Handle(Standard_Transient) p = new Geom_CartesianPoint(0.,0.,0.);
427 if ( p->DynamicType() == STANDARD_TYPE(Geom_CartesianPoint) )
428 cout << ;Type check OK; << endl;
430 cout << ;Type check FAILED; << endl;
433 *NullObject* exception will be raised if a field or a method of an object is accessed via a *Null* handle.
435 #### Invoking Class Methods
437 A class method is called like a static C++ function, i.e. it is called by the name of the class of which it is a member, followed by the “::” operator and the name of the method.
439 For example, we can find the maximum degree of a Bezier curve:
443 n = Geom_BezierCurve::MaxDegree();
446 @subsubsection occt_fcug_2_2_5 Handle deallocation
447 Before you delete an object, you must ensure it is no longer referenced. To reduce the programming load related to this management of object life, the delete function in Open CASCADE Technology is secured by a **reference counter** of classes manipulated by handle. A handle automatically deletes an object when it is no longer referenced. Normally you never call the delete operator explicitly on instances of subclasses of *Standard_Transient*.
449 When a new handle to the same object is created, the reference counter is incremented. When the handle is destroyed, nullified, or reassigned to another object, that counter is decremented. The object is automatically deleted by the handle when reference counter becomes 0.
451 The principle of allocation can be seen in the example below.
456 Handle (TColStd_HSequenceOfInteger) H1 = new TColStd_HSequenceOfInteger;
457 // H1 has one reference and corresponds to 48 bytes of memory
459 Handle (TColStd_HSequenceOfInteger) H2;
460 H2 = H1; // H1 has two references
462 Handle (TColStd_HSequenceOfInteger) H3;
464 // Here, H1 has three references
467 // Here, H1 has two references
469 // Here, H1 has 1 reference
471 // Here, H1 has no reference and the referred TColStd_HSequenceOfInteger object is deleted.
476 Cycles appear if two or more objects reference each other by handles (stored as fields). In this condition automatic destruction will not work.
478 Consider for example a graph, whose objects (primitives) have to know the graph object to which they belong, i.e. a primitive must have a reference to complete graph object. If both primitives and the graph are manipulated by handle and they refer to each other by keeping a handle as a field, the cycle appears.
479 The graph object will not be deleted when the last handle to it is destructed in the application, since there are handles to it stored inside its own data structure (primitives).
481 There are two approaches how to avoid such situation:
482 * Use C++ pointer for one kind of references, e.g. from a primitive to the graph
483 * Nullify one set of handles (e.g. handles to a graph in primitives) when a graph object needs to be destroyed
485 @subsubsection occt_fcug_2_2_6 Creating Transient Classes without CDL
487 Though generation of Handle class and related C++ code is normally performed by CDL extractor, it is also possible to define a class managed by handle without CDL. To facilitate that, several macros are provided in the file Standard_DefineHandle.hxx:
489 * **DEFINE_STANDARD_HANDLE(class_name,ancestor_name)** - declares Handle class for a class *class_name* that inherits class *ancestor_name* (for instance, *Standard_Transient*). This macro should be put in a header file; the declaration of the handle to a base class must be available (usually put before or after the declaration of the class *class_name*, or into a separate header file).
490 * **IMPLEMENT_STANDARD_HANDLE(class_name,ancestor_name)** - implements method *DownCast()* of the *Handle* class. Should be located in a C++ file (normally the file where methods of the class *class_name* are implemented).
491 * **DEFINE_STANDARD_RTTI(class_name)** - declares methods required for RTTI in the class *class_name* declaration; should be in public: section.
492 * **IMPLEMENT_STANDARD_RTTIEXT(class_name,ancestor_name)** - implements above methods. Usually put into the C++ file implementing class class_name.
493 Note that it is important to ensure correctness of macro arguments, especially the ancestor name, otherwise the definition may be inconsistent (no compiler warnings will be issued in case of mistake).
495 In *Appli_ExtSurface.hxx* file:
497 #include <Geom_Surface.hxx>
498 class Appli_ExtSurface : public Geom_Surface
502 DEFINE_STANDARD_RTTI(Appli_ExtSurface)
504 DEFINE_STANDARD_HANDLE(Appli_ExtSurface,Geom_Surface)
507 In *Appli_ExtSurface.cxx* file:
509 #include <Appli_ExtSurface.hxx>
510 IMPLEMENT_STANDARD_HANDLE(Appli_ExtSurface,Geom_Surface)
511 IMPLEMENT_STANDARD_RTTIEXT(Appli_ExtSurface,Geom_Surface)
515 @subsection occt_fcug_2_3 Memory Management in Open CASCADE Technology
516 In the course of a work session, geometric modeling applications create and delete a considerable number of C++ objects allocated in the dynamic memory (heap). In this context, performance of standard functions for allocating and deallocating memory may be not sufficient. For this reason, Open CASCADE Technology employs a specialized memory manager implemented in the Standard package.
518 @subsubsection occt_fcug_2_3_1. Usage
519 To use the Open CASCADE Technology memory manager to allocate memory in a C code, just use method *Standard::Allocate()* instead of *malloc()* and method *Standard::Free()* instead of *free()*. In addition, method *Standard::Reallocate()* is provided to replace C function *realloc()*.
521 In C++, operators *new()* and *delete()* for a class may be defined so as to allocate memory using *Standard::Allocate()* and free it using *Standard::Free()*. In that case all objects of that class and all inherited classes will be allocated using the OCCT memory manager.
523 CDL extractor defines *new()* and *delete()* in this way for all classes declared with CDL. Thus all OCCT classes (apart from a few exceptions) are allocated using the OCCT memory manager.
524 Since operators *new()* and *delete()* are inherited, this is also true for any class derived from an OCCT class, for instance, for all classes derived from *Standard_Transient*.
526 **Note** that it is possible (though not recommended unless really unavoidable) to redefine *new()* and *delete()* functions for some class inheriting Standard_Transient. If that is done, the method *Delete()* should be also redefined to apply operator *delete* to *this* pointer. This will ensure that appropriate *delete()* function will be called, even if the object is manipulated by a handle to a base class.
528 @subsubsection occt_fcug_2_3_2 Configuring the memory manager
529 The OCCT memory manager may be configured to apply different optimization techniques to different memory blocks (depending on their size), or even to avoid any optimization and use C functions *malloc()* and *free()* directly.
530 The configuration is defined by numeric values of the following environment variables:
531 * *MMGT_OPT*: if set to 0 (default) every memory block is allocated in C memory heap directly (via *malloc()* and *free()* functions). In this case, all other options except for *MMGT_CLEAR* are ignored; if set to 1 the memory manager performs optimizations as described below; if set to 2, Intel ® TBB optimized memory manager is used.
532 * *MMGT_CLEAR*: if set to 1 (default), every allocated memory block is cleared by zeros; if set to 0, memory block is returned as it is.
533 * *MMGT_CELLSIZE*: defines the maximal size of blocks allocated in large pools of memory. Default is 200.
534 * *MMGT_NBPAGES*: defines the size of memory chunks allocated for small blocks in pages (operating-system dependent). Default is 1000.
535 * *MMGT_THRESHOLD*: defines the maximal size of blocks that are recycled internally instead of being returned to the heap. Default is 40000.
536 * *MMGT_MMAP*: when set to 1 (default), large memory blocks are allocated using memory mapping functions of the operating system; if set to 0, they will be allocated in the C heap by *malloc()*.
538 @subsubsection occt_fcug_2_3_3 Implementation details
539 When *MMGT_OPT* is set to 1, the following optimization techniques are used:
540 * Small blocks with a size less than *MMGT_CELLSIZE*, are not allocated separately. Instead, a large pools of memory are allocated (the size of each pool is *MMGT_NBPAGES* pages). Every new memory block is arranged in a spare place of the current pool. When the current memory pool is completely occupied, the next one is allocated, and so on.
542 In the current version memory pools are never returned to the system (until the process finishes). However, memory blocks that are released by the method *Standard::Free()* are remembered in the free lists and later reused when the next block of the same size is allocated (recycling).
544 * Medium-sized blocks, with a size greater than *MMGT_CELLSIZE* but less than *MMGT_THRESHOLD*, are allocated directly in the C heap (using *malloc()* and *free()*). When such blocks are released by the method *Standard::Free()* they are recycled just like small blocks.
546 However, unlike small blocks, the recycled medium blocks contained in the free lists (i.e. released by the program but held by the memory manager) can be returned to the heap by method *Standard::Purge()*.
548 * Large blocks with a size greater than *MMGT_THRESHOLD*, including memory pools used for small blocks, are allocated depending on the value of *MMGT_MMAP*: if it is 0, these blocks are allocated in the C heap; otherwise they are allocated using operating-system specific functions managing memory mapped files. Large blocks are returned to the system immediately when *Standard::Free()* is called.
550 #### Benefits and drawbacks
552 The major benefit of the OCCT memory manager is explained by its recycling of small and medium blocks that makes an application work much faster when it constantly allocates and frees multiple memory blocks of similar sizes. In practical situations, the real gain on the application performance may be up to 50%.
554 The associated drawback is that recycled memory is not returned to the operating system during program execution. This may lead to considerable memory consumption and even be misinterpreted as a memory leak. To minimize this effect, the method Standard::Purge() shall be called after the completion of memory-intensive operations.
555 The overhead expenses induced by the OCCT memory manager are:
556 * size of every allocated memory block is rounded up to 8 bytes (when MMGT_OPT is 0 (default), the rounding is defined by the CRT; the typical value for 32-bit platforms is 4 bytes)
557 * additional 4 bytes (or 8 on 64-bit platforms) are allocated in the beginning of every memory block to hold its size (or address of the next free memory block when recycled in free list) only when MMGT_OPT is 1
560 Note that these overheads may be greater or less than overheads induced by the C heap memory manager, so overall memory consumption may be greater in either optimized or standard modes, depending on circumstances.
562 As a general rule, it is advisable to allocate memory through significant blocks. In this way, you can work with blocks of contiguous data, and processing is facilitated for the memory page manager.
564 OCCT memory manager uses mutex to lock access to free lists, therefore it may have less performance than non-optimized mode in situations when different threads often make simultaneous calls to the memory manager.
565 The reason is that modern implementations of *malloc()* and *free()* employ several allocation arenas and thus avoid delays waiting mutex release, which are possible in such situations.
567 @subsection occt_fcug_2_4 Exception Handling
568 Exception handling provides a means of transferring control from a given point in a program being executed to an **exception handler** associated with another point previously executed.
570 A method may raise an exception which interrupts its normal execution and transfers control to the handler catching this exception.
572 Open CASCADE Technology provides a hierarchy of exception classes with a root class being class Standard_Failure from the Standard package. The CDL extractor generates exception classes with standardized interface.
574 Open CASCADE Technology also provides support for converting system signals (such as access violation or division by zero) to exceptions, so that such situations can be safely handled with the same uniform approach.
576 However, in order to support this functionality on various platforms, some special methods and workarounds are used. Though the implementation details are hidden and handling of OCCT exceptions is done basically in the same way as with C++, some peculiarities of this approach shall be taken into account and some rules must be respected.
578 The following paragraphs describe recommended approaches for using exceptions when working with Open CASCADE Technology.
580 @subsubsection occt_fcug_2_4_1 Raising an Exception
582 #### “C++ like” Syntax
584 To raise an exception of a definite type method Raise() of the appropriate exception class shall be used.
586 DomainError::Raise(“Cannot cope with this condition”);
588 raises an exception of *DomainError* type with the associated message “Cannot cope with this condition”, the message being optional. This exception may be caught by a handler of a *DomainError* type as follows:
595 // handle DomainError exceptions here
601 Exceptions should not be used as a programming technique, to replace a “goto” statement for example, but as a way to protect methods against misuse. The caller must make sure its condition is such that the method can cope with it.
604 * No exception should be raised during normal execution of an application.
605 * A method which may raise an exception should be protected by other methods allowing the caller to check on the validity of the call.
607 For example, if you consider the *TCollection_Array1* class used with:
608 * *Value* function to extract an element
609 * *Lower* function to extract the lower bound of the array
610 * *Upper* function to extract the upper bound of the array.
612 then, the *Value* function may be implemented as follows:
615 Item TCollection_Array1::Value (const Standard_Integer&index) const
617 // where r1 and r2 are the lower and upper bounds of the array
618 if(index r1 || index > r2) {
619 OutOfRange::Raise(“Index out of range in Array1::Value”);
621 return contents[index];
625 Here validity of the index is first verified using the Lower and Upper functions in order to protect the call.
626 Normally the caller ensures the index being in the valid range before calling Value(). In this case the above implementation of Value is not optimal since the test done in Value is time-consuming and redundant.
628 It is a widely used practice to include that kind of protections in a debug build of the program and exclude in release (optimized) build. To support this practice, the macros Raise_if() are provided for every OCCT exception class:
630 <ErrorTypeName>_Raise_if(condition, “Error message”);
632 where ErrorTypeName is the exception type, condition is the logical expression leading to the raise of the exception, and Error message is the associated message.
634 The entire call may be removed by defining one of the pre-processor symbols No_Exception or No_<ErrorTypeName> at compile-time:
637 #define No_Exception /* remove all raises */
640 Using this syntax, the Value function becomes:
643 Item TCollection_Array1::Value (const Standard_Integer&index) const
645 OutOfRange_Raise_if(index r1 || index > r2,
646 “index out of range in Array1::Value”);
647 return contents[index];
651 @subsubsection occt_fcug_2_4_2 Handling an Exception
653 When an exception is raised, control is transferred to the nearest handler of a given type in the call stack, that is:
654 * the handler whose try block was most recently entered and not yet exited,
655 * the handler whose type matches the raise expression.
657 A handler of T exception type is a match for a raise expression with an exception type of E if:
658 * T and E are of the same type, or
659 * T is a supertype of E.
661 In order to handle system signals as exceptions, make sure to insert macro OCC_CATCH_SIGNALS somewhere in the beginning of the relevant code. The recommended location for it is first statement after opening brace of try {} block.
663 As an example, consider the exceptions of type *NumericError, Overflow, Underflow* and *ZeroDivide*, where *NumericError* is the parent type of the three others.
672 catch(Standard_Overflow) { // first handler
675 catch(Standard_NumericError) { // second handler
681 Here, the first handler will catch exceptions of *Overflow* type and the second one - exceptions of *NumericError* type and all exceptions derived from it, including *Underflow* and *ZeroDivide*.
683 The handlers are checked in order of appearance, from the nearest to the most distant try block, until one matches the raise expression. For a try block, it would be a mistake to place a handler for a base exception type ahead of a handler for its derived type since that would ensure that the handler for the derived exception would never be invoked.
692 g(i);// i is accessible
694 // statement here will produce compile-time errors !
695 catch(Standard_NumericError) {
696 // fix up with possible reuse of i
698 // statement here may produce unexpected side effect
704 The exceptions form a hierarchy tree completely separated from other user defined classes. One exception of type *Failure* is the root of the entire exception hierarchy. Thus, using a handler with *Failure* type catches any OCCT exception. It is recommended to set up such a handler in the main routine.
706 The main routine of a program would look like this:
709 #include <Standard_ErrorHandler.hxx>
710 #include <Standard_Failure.hxx>
711 #include <iostream.h>
712 int main (int argc, char* argv[])
719 catch(Standard_Failure) {
720 Handle(Standard_Failure) error = Standard_Failure::Caught ();
727 In this example function *Caught* is a static member of *Failure* that returns an exception object containing the error message built in the raise expression. Note that this method of accessing a raised object is used in Open CASCADE Technology instead of usual C++ syntax (receiving the exception in catch argument).
729 Though standard C++ scoping rules and syntax apply to try block and handlers, note that on some platforms Open CASCADE Technology may be compiled in compatibility mode when exceptions are emulated by long jumps (see below). In this mode it is required that no statement precedes or follows any handler. Thus it is highly recommended to always include a try block into additional {} braces. Also this mode requires that header file *Standard_ErrorHandler.hxx* be included in your program before a try block, otherwise it may fail to handle Open CASCADE Technology exceptions; furthermore *catch()* statement does not allow passing exception object as argument.
731 #### Catching signals
733 In order for the application to be able to catch system signals (access violation, division by zero, etc.) in the same way as other exceptions, the appropriate signal handler shall be installed in the runtime by the method *OSD::SetSignal()*.
735 Normally this method is called in the beginning of the main() function. It installs a handler that will convert system signals into OCCT exceptions.
737 In order to actually convert signals to exceptions, macro *OCC_CATCH_SIGNALS* needs to be inserted in the source code. The typical place where this macro is put is beginning of the *try{}* block which catches such exceptions.
739 @subsubsection occt_fcug_2_4_3 Implementation details
741 The exception handling mechanism in Open CASCADE Technology is implemented in different ways depending on the preprocessor macros *NO_CXX_EXCEPTIONS* and *OCC_CONVERT_SIGNALS*, which shall be consistently defined by compilation procedures for both Open CASCADE Technology and user applications:
743 1. On Windows and DEC, these macros are not defined by default, and normal C++ exceptions are used in all cases, including throwing from signal handler. Thus the behavior is as expected in C++.
745 2. On SUN and Linux, macro *OCC_CONVERT_SIGNALS* is defined by default. The C++ exception mechanism is used for catching exceptions and for throwing them from normal code. Since it is not possible to throw C++ exception from system signal handler function, that function makes a long jump to the nearest (in the execution stack) invocation of macro *OCC_CATCH_SIGNALS*, and only there the C++ exception gets actually thrown. The macro *OCC_CATCH_SIGNALS* is defined in the file *Standard_ErrorHandler.hxx*. Therefore, including this file is necessary for successful compilation of a code containing this macro.
747 This mode differs from standard C++ exception handling only for signals:
749 * macro *OCC_CATCH_SIGNALS* is necessary (besides call to *OSD::SetSignal()* described above) for conversion of signals into exceptions;
750 * the destructors for automatic C++ objects created in the code after that macro and till the place where signal is raised will not be called in case of signal, since no C++ stack unwinding is performed by long jump.
752 3. On SUN and Linux Open CASCADE Technology can also be compiled in compatibility mode (which was default till Open CASCADE Technology 6.1.0). In that case macro *NO_CXX_EXCEPTIONS* is defined and the C++ exceptions are simulated with C long jumps. As a consequence, the behavior is slightly different from that expected in the C++ standard.
754 While exception handling with NO_CXX_EXCEPTIONS is very similar to C++ by syntax, it has a number of peculiarities that should be taken into account:
756 * try and catch are actually macros defined in the file *Standard_ErrorHandler.hxx*. Therefore, including this file is necessary for handling OCCT exceptions;
757 * due to being a macro, catch cannot contain a declaration of the exception object after its type; only type is allowed in the catch statement. Use method *Standard_Failure::Caught()* to access an exception object;
758 * catch macro may conflict with some STL classes that might use catch(...) statements in their header files. So STL headers should not be included after *Standard_ErrorHandler.hxx*;
759 * Open CASCADE Technology try/catch block will not handle normal C++ exceptions; however this can be achieved using special workarounds;
760 * the try macro defines a C++ object that holds an entry point in the exception handler. Therefore if exception is raised by code located immediately after the try/catch block but on the same nesting level as *try*, it may be handled by that *catch*. This may lead to unexpected behavior, including infinite loop. To avoid that, always surround the try/catch block in \{\} braces;
761 * the destructors of the C++ objects allocated on the stack after handler initialization are not called by exception raising.
763 In general, for writing platform-independent code it is recommended to insert macros *OCC_CATCH_SIGNALS* in try \{\} blocks or other code where signals may happen. For compatibility with previous versions of Open CASCADE Technology the limitations described above for *NO_CXX_EXCEPTIONS* shall be assumed.
765 @subsection occt_fcug_2_5 Plug-In Management
767 @subsubsection occt_fcug_2_5_1 Distribution by Plug-Ins
769 A plug-in is a component that can be loaded dynamically into a client application, not requiring to be directly linked to it. The plug-in is not bound to its client, i.e. the plug-in knows only how its connection mechanism is defined and how to call the corresponding services.
771 A plug-in can be used to:
772 * implement the mechanism of a *driver*, i.e dynamically changing a driver implementation according to the current transactions (for example, retrieving a document stored in another version of an application),
773 * restrict processing resources to the minimum required (for example, it does not load any application services at run-time as long as the user does not need them),
774 * facilitate modular development (an application can be delivered with base functions while some advanced capabilities will be added as plug-ins when they are available).
776 The plug-in is identified with the help of the global universal identifier (GUID). The GUID includes lower case characters and cannot end with a blank space.
778 Once it has been loaded, the call to the services provided by the plug-in is direct (the client is implemented in the same language as the plug-in).
780 #### C++ Plug-In Implementation
782 The C++ plug-in implements a service as an object with functions defined in an abstract class (this abstract class and its parent classes with the GUID are the only information about the plug-in implemented in the client application). The plug-in consists of a sharable library including a method named Factory which creates the C++ object (the client cannot instantiate this object because the plug-in implementation is not visible).
783 Foundation classes provide in the package **Plugin** a method named Load(), which enables the client to access the required service through a library.
785 That method reads the information regarding available plug-ins and their locations from the resource file Plugin found by environment variable CSF_PluginDefaults:
788 $CSF_PluginDefaults/.Plugin
791 The *Load* method looks for the library name in the resource file or registry through its GUID, for example, on UNIX:
793 ! METADATADRIVER whose value must be OS or DM.
796 a148e300-5740-11d1-a904-080036aaa103.Location:
799 a148e300-5740-11d1-a904-080036aaa103.CCL:
800 /adv_44/CAS/BAG/FW-K4C/inc/FWOS.ccl
803 a148e301-5740-11d1-a904-080036aaa103.Location:
805 a148e301-5740-11d1-a904-080036aaa103.CCL:
806 /adv_44/CAS/BAG/DESIGNMANAGER-K4C/inc/DMAccess.ccl|/
807 adv_44/CAS/BAG/DATABASE-K4C/inc/FWDMCommands.ccl
808 a148e301-5740-11d1-a904-080036aaa103.Message: /adv_44/CAS/
809 BAG/DESIGNMANAGER-K4C/etc/locale/DMAccess
812 5ff7dc00-8840-11d1-b5c2-00a0c9064368.Location:
813 libCDMShapeDriversPlugin.so
814 5ff7dc01-8840-11d1-b5c2-00a0c9064368.Location:
815 libCDMShapeDriversPlugin.so
816 5ff7dc02-8840-11d1-b5c2-00a0c9064368.Location:
817 libCDMShapeDriversPlugin.so
818 5ff7dc03-8840-11d1-b5c2-00a0c9064368.Location:
819 libCDMShapeDriversPlugin.so
820 5ff7dc04-8840-11d1-b5c2-00a0c9064368.Location:
821 libCDMShapeDriversPlugin.so
824 d0d722a2-b4c9-11d1-b561-0000f87a4710.location: FWOSPlugin
825 d0d722a2-b4c9-11d1-b561-0000f87a4710.CCL: /adv_44/CAS/BAG/
826 VIEWERS-K4C/inc/CCLPlotters.ccl
827 d0d722a2-b4c9-11d1-b561-0000f87a4710.Message: /adv_44/CAS/
828 BAG/VIEWERS-K4C/etc/locale/CCLPlotters
831 e3708f72-b1a8-11d0-91c2-080036424703.Location:
832 libBRepExchangerPlugin.so
833 e3708f72-b1a8-11d0-91c2-080036424703.CCL: /adv_44/CAS/BAG/
838 Then the *Load* method loads the library according to the rules of the operating system of the host machine (for example, by using environment variables such as *LD_LIBRARY_PATH* with Unix and *PATH* with Windows). After that it invokes the *Factory* method to return the object which supports the required service.
839 The client may then call the functions supported by this object.
841 #### C++ Client Plug-In Implementation
843 To invoke one of the services provided by the plug-in, you may call the *Plugin::ServiceFactory* global function with the *Standard_GUID* of the requested service as follows:
846 Handle(FADriver_PartStorer)::DownCast
847 (PlugIn::ServiceFactory
848 (PlugIn_ServiceId(yourStandardGUID)))
851 Let us take *FAFactory.cxx* as an example:
854 #include <FAFactory.ixx>
856 #include <FADriver_PartRetriever.hxx>
857 #include <FADriver_PartStorer.hxx>
858 #include <FirstAppSchema.hxx>
859 #include <Standard_GUID.hxx>
860 #include <Standard_Failure.hxx>
861 #include <FACDM_Application.hxx>
862 #include <Plugin_Macro.hxx>
867 StorageDriver(“45b3c690-22f3-11d2-b09e-0000f8791463”);
869 RetrievalDriver(“45b3c69c-22f3-11d2-b09e-0000f8791463”);
871 Schema(“45b3c6a2-22f3-11d2-b09e-0000f8791463”);
873 //======================================================
874 // function : Factory
876 //======================================================
878 Handle(Standard_Transient) FAFactory::Factory(const Standard_GUID& aGUID)
880 if(aGUID == StorageDriver) {
881 cout “FAFactory : Create store driver” endl;
882 static Handle(FADriver_PartStorer) sd = new FADriver_PartStorer();
886 if(aGUID == RetrievalDriver) {
887 cout “FAFactory : Create retrieve driver” endl;
888 static Handle(FADriver_PartRetriever)
889 rd = new FADriver_PartRetriever();
893 if(aGUID == Schema) {
894 cout “FAFactory : Create schema” endl;
895 static Handle(FirstAppSchema) s = new FirstAppSchema();
899 Standard_Failure::Raise(“FAFactory: unknown GUID”);
900 Handle(Standard_Transient) t;
905 #### Without using the Software Factory
907 To create a factory without using the Software Factory, define a *dll* project under Windows or a library under UNIX by using a source file as specified above. The *FAFactory* class is implemented as follows:
910 #include <Handle_Standard_Transient.hxx>
911 #include <Standard_Macro.hxx>
912 class Standard_Transient;
916 Standard_EXPORT static Handle_Standard_Transient
917 Factory(const Standard_GUID& aGUID) ;
923 @section occt_fcug_3 Collections, Strings and Unit Conversion
925 @subsection occt_fcug_3_1 Collections
927 @subsubsection occt_fcug_3_1_1 Overview
929 The **Collections** component contains the classes that handle dynamically sized aggregates of data. They include a wide range of collections such as arrays, lists and maps.
931 Collections classes are *generic*, that is, they can hold a variety of objects which do not necessarily inherit from a unique root class. When you need to use a collection of a given type of object you must *instantiate* it for this specific type of element. Once this declaration is compiled, all the functions available on the generic collection are available on your *instantiated class*.
933 * Each collection directly used as an argument in OCCT public syntax is instantiated in an OCCT component.
934 * The *TColStd* package (**Collections of Standard Objects** component) provides numerous instantiations of these generic collections with objects from the **Standard** package or from the **Strings** component.
935 The **Collections** component provides a wide range of generic collections:
936 * **Arrays** are generally used for a quick access to the item, however an array is a fixed sized aggregate.
937 * **Sequences** are variable-sized structures, they avoid the use of large and quasi-empty arrays. A sequence item is longer to access than an array item: only an exploration in sequence is effective (but sequences are not adapted for numerous explorations). Arrays and sequences are commonly used as data structures for more complex objects.
938 * On the other hand, **maps** are dynamic structures where the size is constantly adapted to the number of inserted items and the access time for an item is effective. Maps structures are commonly used in cases of numerous explorations: they are typically internal data structures for complex algorithms. **Sets** generate the same results as maps but computation time is considerable.
939 * **Lists, queues** and **stacks** are minor structures similar to sequences but with other exploration algorithms.
941 Most collections follow value semantics: their instances are the actual collections, not **handles** to a collection. Only arrays and sequences may also be manipulated by handle, and therefore shared.
943 @subsubsection occt_fcug_3_1_2 Generic general-purpose Aggregates
945 #### TCollection_Array1
947 These are unidimensional arrays similar to C arrays, i.e. of fixed size but dynamically dimensioned at construction time.
948 As with a C array, the access time for an *Array1* indexed item is constant and is independent of the array size. Arrays are commonly used as elementary data structures for more complex objects.
950 *Array1* is a generic class which depends on *Item*, the type of element in the array.
952 *Array1* indexes start and end at a user-defined position. Thus, when accessing an item, you must base the index on the lower and upper bounds of the array.
954 #### TCollection_Array2
956 These are bi-dimensional arrays of fixed size but dynamically dimensioned at construction time.
958 As with a C array, the access time for an *Array2* indexed item is constant and is independent of the array size. Arrays are commonly used as elementary data structures for more complex objects.
960 *Array2* is a generic class which depends on *Item*, the type of element in the array.
962 *Array2* indexes start and end at a user-defined position. Thus, when accessing an item, you must base the index on the lower and upper bounds of the array.
964 #### TCollection_HArray1
966 These are unidimensional arrays similar to C arrays, i.e. of fixed size but dynamically dimensioned at construction time.
967 As with a C array, the access time for an *HArray1* or *HArray2* indexed item is constant and is independent of the array size. Arrays are commonly used as elementary data structures for more complex objects.
969 *HArray1* objects are **handles** to arrays.
970 * *HArray1* arrays may be shared by several objects.
971 * You may use a *TCollection_Array1* structure to have the actual array.
973 *HArray1* is a generic class which depends on two parameters:
974 * **Item**, the type of element in the array,
975 * **Array**, the actual type of array handled by *HArray1*. This is an instantiation with **Item** of the *TCollection_Array1* generic class.
977 *HArray1* indexes start and end at a user-defined position. Thus, when accessing an item, you must base the index on the lower and upper bounds of the array.
979 #### TCollection_HArray2
981 These are bi-dimensional arrays of fixed size but dynamically dimensioned at construction time.
983 As with a C array, the access time for an *HArray2* indexed item is constant and is independent of the array size. Arrays are commonly used as elementary data structures for more complex objects.
985 *HArray2* objects are **handles** to arrays.
986 * *HArray2* arrays may be shared by several objects.
987 * You may use a *TCollection_Array2* structure to have the actual array.
989 *HArray2* is a generic class which depends on two parameters:
990 * *Item*, the type of element in the array,
991 * *Array*, the actual type of array handled by *HArray2*. This is an instantiation with *Item* of the *TCollection_Array2* generic class.
993 #### TCollection_HSequence
995 This is a sequence of items indexed by an integer.
997 Sequences have about the same goal as unidimensional arrays *TCollection_HArray1*: they are commonly used as elementary data structures for more complex objects. But a sequence is a structure of *variable size*: sequences avoid the use of large and quasi-empty arrays. Exploring a sequence data structure is effective when the exploration is done in sequence; elsewhere a sequence item is longer to read than an array item. Note also that sequences are not effective when they have to support numerous algorithmic explorations: a map is better for that.
999 *HSequence* objects are **handles** to sequences.
1000 * *HSequence* sequences may be shared by several objects.
1001 * You may use a *TCollection_Sequence* structure to have the actual sequence.
1003 *HSequence* is a generic class which depends on two parameters:
1004 * *Item*, the type of element in the sequence,
1005 * *Seq*, the actual type of sequence handled by *HSequence*. This is an instantiation with *Item* of the *TCollection_Sequence* generic class.
1007 #### TCollection_HSet
1009 This is a collection of non-ordered items without any duplicates. At each transaction, the system checks if there are no duplicates.
1010 *HSet* objects are *handles* to sets.
1011 *HSet* is a generic class which depends on two parameters:
1012 * *Item*, the type of element in the set,
1013 * *Set*, the actual type of set handled by *HSet*. This is an instantiation with *TCollection_Set* generic class.
1015 #### TCollection_List
1017 These are ordered lists of non-unique objects which can be accessed sequentially using an iterator.
1018 Item insertion in a list is very fast at any position. But searching for items by value may be slow if the list is long, because it requires a sequential search.
1020 *List* is a generic class, which depends on *Item*, the type of element in the structure.
1021 Use a *ListIterator* iterator to explore a *List* structure.
1023 An iterator class is automatically instantiated from the *TCollection_ListIterator* class at the time of instantiation of a *List* structure.
1025 A sequence is a better structure when searching for items by value.
1027 Queues and stacks are other kinds of list with a different access to data.
1029 #### TCollection_Queue
1031 This is a structure, where items are added at the end and removed from the front. The first item entered will be the first removed (**FIFO** structure: First In First Out). *Queue* is a generic class which depends on *Item*, the type of element in the structure.
1033 #### TCollection_Sequence
1035 This is a sequence of items indexed by an integer.
1036 Sequences have about the same goal as unidimensional arrays (*TCollection_Array1*): they are commonly used as elementary data structures for more complex objects. But a sequence is a structure of *variable size*: sequences avoid the use of large and quasi-empty arrays. Exploring a sequence data structure is effective when the exploration is done *in sequence*; elsewhere a sequence item is longer to read than an array item. Note also that sequences are not effective when they have to support numerous algorithmic explorations: a map is better for that.
1038 *Sequence* is a generic class which depends on *Item*, the type of element in the sequence.
1040 #### TCollection_Set
1042 This is a collection of non-ordered items without any duplicates. At each transaction, the system checks if there are no duplicates.
1044 A set generates the same result as a map. A map is more effective; so it is advisable to use maps instead of sets.
1046 *Set* is a generic class which depends on *Item*, the type of element in the set.
1047 Use *SetIterator* iterator to explore a *Set* structure.
1049 #### TCollection_Stack
1051 This is a structure where items are added and removed from the top. The last item entered will be the first removed.
1053 *Stack* is a generic class which depends on *Item*, the type of element in the structure.
1054 Use a *StackIterator* iterator to explore a *Stack* structure.
1056 @subsubsection occt_fcug_3_1_3 Generic Maps
1058 Maps are dynamically extended data structures where data is quickly accessed with a key. *TCollection_BasicMap* is a root class for maps.
1060 #### General properties of maps
1063 Map items may contain complex non-unitary data, thus it can be difficult to manage them with an array. The map allows a data structure to be indexed by complex data.
1065 The size of a map is dynamically extended. So a map may be first dimensioned for a little number of items. Maps avoid the use of large and quasi-empty arrays.
1067 The access time for a map item is much better than the one for a sequence, list, queue or stack item. It is comparable with the access time for an array item. It depends on the size of the map and on the quality of the user redefinable function (the *hashing function*) to find quickly where is the item.
1069 The performance of a map exploration may be better of an array exploration because the size of the map is adapted to the number of inserted items.
1071 That is why maps are commonly used as internal data structures for algorithms.
1075 A map is a data structure for which data are addressed by *keys*.
1077 Once inserted in the map, a map item is referenced as an *entry* of the map.
1079 Each entry of the map is addressed by a key. Two different keys address two different entries of the map.
1080 The position of an entry in the map is called a *bucket*.
1082 A map is dimensioned by its number of buckets, i.e. the maximum number of entries in the map. The performance of a map is conditioned by the number of buckets.
1084 The *hashing function* transforms a key into a bucket index. The number of values that can be computed by the hashing function is equal to the number of buckets of the map.
1086 Both the hashing function and the equality test between two keys are provided by a *hasher* object.
1088 A map may be explored by a *map iterator*. This exploration provides only inserted entries in the map (i.e. non empty buckets).
1090 #### Collections of generic maps
1092 The *Collections* component provides numerous generic derived maps.
1094 These maps include automatic management of the number of *buckets*: they are automatically resized when the number of *keys* exceeds the number of buckets. If you have a fair idea of the number of items in your map, you can save on automatic resizing by specifying a number of buckets at the time of construction, or by using a resizing function. This may be considered for crucial optimization issues.
1096 *Keys, items* and *hashers* are parameters of these generic derived maps.
1098 *TCollection_MapHasher* class describes the functions required by any *hasher*, which is to be used with a map instantiated from the **Collections** component.
1100 An iterator class is automatically instantiated at the time of instantiation of a map provided by the *Collections* component if this map is to be explored with an iterator. Note that some provided generic maps are not to be explored with an iterator but with indexes (*indexed maps*).
1102 ##### TCollection_DataMap
1104 This is a map used to store keys with associated items. An entry of **DataMap** is composed of both the key and the item.
1105 The *DataMap* can be seen as an extended array where the keys are the indexes.
1107 *DataMap* is a generic class which depends on three parameters:
1108 * *Key* is the type of key for an entry in the map,
1109 * *Item* is the type of element associated with a key in the map,
1110 * *Hasher* is the type of hasher on keys.
1112 Use a *DataMapIterator* iterator to explore a *DataMap* map.
1114 An iterator class is automatically instantiated from the *TCollection_DataMapIterator* generic class at the time of instantiation of a *DataMap* map.
1116 *TCollection_MapHasher* class describes the functions required for a *Hasher* object.
1118 ##### TCollection_DoubleMap
1120 This is a map used to bind pairs of keys (Key1,Key2) and retrieve them in linear time.
1122 *Key1* is referenced as the first key of the *DoubleMap* and *Key2* as the second key.
1124 An entry of a *DoubleMap* is composed of a pair of two keys: the first key and the second key.
1126 *DoubleMap* is a generic class which depends on four parameters:
1127 * *Key1* is the type of the first key for an entry in the map,
1128 * *Key2* is the type of the second key for an entry in the map,
1129 * *Hasher1* is the type of hasher on first keys,
1130 * *Hasher2* is the type of hasher on second keys.
1132 Use *DoubleMapIterator* to explore a *DoubleMap* map.
1134 An iterator class is automatically instantiated from the *TCollection_DoubleMapIterator* class at the time of instantiation of a *DoubleMap* map.
1136 *TCollection_MapHasher* class describes the functions required for a *Hasher1* or a *Hasher2* object.
1138 ##### TCollection_IndexedDataMap
1140 This is map to store keys with associated items and to bind an index to them.
1142 Each new key stored in the map is assigned an index. Indexes are incremented as keys (and items) stored in the map. A key can be found by the index, and an index can be found by the key. No key but the last can be removed, so the indexes are in the range 1...Upper, where *Upper* is the number of keys stored in the map. An item is stored with each key.
1144 An entry of an *IndexedDataMap* is composed of both the key, the item and the index. An *IndexedDataMap* is an ordered map, which allows a linear iteration on its contents. It combines the interest:
1145 * of an array because data may be accessed with an index,
1146 * and of a map because data may also be accessed with a key.
1148 *IndexedDataMap* is a generic class which depends on three parameters:
1149 * *Key* is the type of key for an entry in the map,
1150 * *Item* is the type of element associated with a key in the map,
1151 * *Hasher* is the type of hasher on keys.
1153 ##### TCollection_IndexedMap
1155 This is map used to store keys and to bind an index to them.
1157 Each new key stored in the map is assigned an index. Indexes are incremented as keys stored in the map. A key can be found by the index, and an index by the key. No key but the last can be removed, so the indexes are in the range 1...Upper where Upper is the number of keys stored in the map.
1159 An entry of an *IndexedMap* is composed of both the key and the index. An *IndexedMap* is an ordered map, which allows a linear iteration on its contents. But no data is attached to the key. An *IndexedMap* is typically used by an algorithm to know if some action is still performed on components of a complex data structure.
1161 *IndexedMap* is a generic class which depends on two parameters:
1162 * *Key* is the type of key for an entry in the map,
1163 * *Hasher* is the type of hasher on keys.
1165 ##### TCollection_Map
1167 This is a basic hashed map, used to store and retrieve keys in linear time.
1169 An entry of a *Map* is composed of the key only. No data is attached to the key. A *Map* is typically used by an algorithm to know if some action is still performed on components of a complex data structure.
1171 *Map* is a generic class which depends on two parameters:
1172 * *Key* is the type of key in the map,
1173 * *Hasher* is the type of hasher on keys.
1175 Use a *MapIterator* iterator to explore a *Map* map.
1177 ##### TCollection_MapHasher
1179 This is a hasher on the *keys* of a map instantiated from the *Collections* component.
1181 A hasher provides two functions:
1182 * *HashCode()* function transforms a key into a bucket index in the map. The number of values that can be computed by the hashing function is equal to the number of buckets in the map.
1183 * *IsEqual* is the equality test between two keys. Hashers are used as parameters in generic maps provided by the **Collections** component.
1185 *MapHasher* is a generic class which depends on the type of keys, providing that *Key* is a type from the *Standard* package. In such cases *MapHasher* may be directly instantiated with *Key*. Note that the package *TColStd* provides some of these instantiations.
1187 Elsewhere, if *Key* is not a type from the *Standard* package you must consider *MapHasher* as a template and build a class which includes its functions, in order to use it as a hasher in a map instantiated from the *Collections* component.
1189 Note that *TCollection_AsciiString* and *TCollection_ExtendedString* classes correspond to these specifications, in consequence they may be used as hashers: when *Key* is one of these two types you may just define the hasher as the same type at the time of instantiation of your map.
1191 @subsubsection occt_fcug_3_1_4 Iterators
1193 #### TCollection_BasicMapIterator
1195 This is a root class for map iterators. A map iterator provides a step by step exploration of all the entries of a map.
1197 #### TCollection_DataMapIterator
1199 These are functions used for iterating the contents of a *DataMap* map.
1201 A map is a non-ordered data structure. The order in which entries of a map are explored by the iterator depends on its contents and change when the map is edited. It is not recommended to modify the contents of a map during the iteration: the result is unpredictable.
1203 #### TCollection_DoubleMapIterator
1205 These are functions used for iterating the contents of a *DoubleMap* map.
1207 #### TCollection_ListIterator
1209 These are unctions used for iterating the contents of a *List* data structure.
1211 A *ListIterator* object can be used to go through a list sequentially, and as a bookmark to hold a position in a list. It is not an index, however. Each step of the iteration gives the current position of the iterator, to which corresponds the current item in the list. The current position is not defined if the list is empty, or when the exploration is finished.
1213 An iterator class is automatically instantiated from this generic class at the time of instantiation of a *List* data structure.
1215 #### TCollection_MapIterator
1217 These are functions used for iterating the contents of a *Map* map.
1218 An iterator class is automatically instantiated from this generic class at the time of instantiation of a *Map* map.
1220 #### TCollection_SetIterator
1222 These are functions used for iterating the contents of a *Set* data structure.
1223 An iterator class is automatically instantiated from this generic class at the time of instantiation of a *Set* structure.
1225 #### TCollection_StackIterator
1227 These are functions used for iterating the contents of a **Stack** data structure.
1229 An iterator class is automatically instantiated from this generic class at the time of instantiation of a *Stack* structure.
1231 @subsection occt_fcug_3_2 Collections of Standard Objects
1232 @subsubsection occt_fcug_3_2_1 Overview
1233 While generic classes of the *TCollection* package are the root classes that describe the generic purpose of every type of collection, classes effectively used are extracted from the *TColStd* package.
1234 The *TColStd* and *TShort* packages provide frequently used instantiations of generic classes with objects from the *Standard* package or strings from the *TCollection* package.
1236 @subsubsection occt_fcug_3_2_2 Description
1237 These instantiations are the following:
1238 * Unidimensional arrays: instantiations of the *TCollection_Array1* generic class with *Standard* Objects and *TCollection* strings.
1239 * Bidimensional arrays: instantiations of the *TCollection_Array2* generic class with *Standard* Objects.
1240 * Unidimensional arrays manipulated by handles: instantiations of the *TCollection_HArray1* generic class with *Standard* Objects and *TCollection* strings.
1241 * Bidimensional arrays manipulated by handles: instantiations of the *TCollection_HArray2* generic class with *Standard* Objects.
1242 * Sequences: instantiations of the *TCollection_Sequence* generic class with *Standard* objects and *TCollection* strings.
1243 * Sequences manipulated by handles: instantiations of the *TCollection_HSequence* generic class with *Standard* objects and *TCollection* strings.
1244 * Lists: instantiations of the *TCollection_List* generic class with *Standard* objects.
1245 * Queues: instantiations of the *TCollection_Queue* generic class with *Standard* objects.
1246 * Sets: instantiations of the *TCollection_Set* generic class with *Standard* objects.
1247 * Sets manipulated by handles: instantiations of the *TCollection_HSet* generic class with *Standard* objects.
1248 * Stacks: instantiations of the *TCollection_Stack* generic class with *Standard* objects.
1249 * Hashers on map keys: instantiations of the *TCollection_MapHasher* generic class with *Standard* objects.
1250 * Basic hashed maps: instantiations of the *TCollection_Map* generic class with *Standard* objects.
1251 * Hashed maps with an additional item: instantiations of the *TCollection_DataMap* generic class with *Standard* objects.
1252 * Basic indexed maps: instantiations of the *TCollection_IndexedMap* generic class with *Standard* objects.
1253 * Indexed maps with an additional item: instantiations of the *TCollection_IndexedDataMap* generic class with *Standard_Transient* objects.
1254 * Class *TColStd_PackedMapOfInteger* provides alternative implementation of map of integer numbers, optimized for both performance and memory usage (it uses bit flags to encode integers, which results in spending only 24 bytes per 32 integers stored in optimal case). This class also provides Boolean operations with maps as sets of integers (union, intersection, subtraction, difference, checks for equality and containment).
1256 @subsection occt_fcug_3_3 NCollections
1257 @subsubsection occt_fcug_3_3_1 Overview
1259 *NCollection* package allows to not use WOK development environment in projects. Though it is quite natural to develop a code based on OCCT in any environment accepted in the industry, there is still one limitation: the so-called OCCT generic classes provided in TCollection package require compilation of the definitions in the CDL language and therefore can only be instantiated in WOK development environment.
1261 The NCollection library provides a full replacement of all TCollection generic classes so that any OCCT collection could be instantiated via C++ template or macro definitions. It can be used in WOK as a package development unit, or in any other configuration, since it only uses the standard capabilities of C++ compiler.
1263 Macro definitions of these classes are stored in *NCollection_Define\*.hxx* files. These definitions are now obsolete though still can be used, particularly for compatibility with the existing code. On the contrary, template classes in *NCollection_\*.hxx* files are recommended, they are supported by OPEN CASCADE Company and further developed according to various needs.
1265 The technology used in this unit continues and complements the one offered in the header file *Standard_DefineHandle* – allowing to implement outside CDL the classes managed by Handle, also providing OCCT RTTI support.
1267 @subsubsection occt_fcug_3_3_2 Instantiation of collection classes
1269 Now we are going to implement the definitions from *NCollection* in the code, taking as an example a sequence of points (analogue of *TColgp_SequenceOfPnt*).
1271 #### Definition of a new collection class
1273 Let the header file be *MyPackage_SequenceOfPnt.hxx* :
1275 Template class instantiaton
1277 #include <NCollection_Sequence.hxx>
1278 #include <gp_Pnt.hxx>
1279 typedef NCollection_Sequence<gp_Pnt> MyPackage_SequenceOfPnt;
1284 #include <NCollection_DefineSequence.hxx>
1285 #include <gp_Pnt.hxx>
1288 The following line defines the class "base collection of points"
1290 DEFINE_BASECOLLECTION(MyPackage_BaseCollPnt, gp_Pnt)
1293 The following line defines the class *MyPackage_SequenceOfPnt*
1296 DEFINE_SEQUENCE (MyPackage_SequenceOfPnt, MyPackage_BaseCollPnt , gp_Pnt)
1299 #### Definition of a new collection class managed by Handle
1301 It is necessary to provide relevant statements both in the header ( .hxx file) and the C++ source ( .cxx file).
1303 Header file MyPackage_HSequenceOfPnt.hxx:
1306 #include <NCollection_DefineHSequence.hxx>
1307 #include <gp_Pnt.hxx>
1310 The following line defines the class "base collection of points"
1313 DEFINE_BASECOLLECTION(MyPackage_BaseCollPnt, gp_Pnt)
1316 The following line defines the class *MyPackage_SequenceOfPnt*
1319 DEFINE_SEQUENCE (MyPackage_SequenceOfPnt, MyPackage_BaseCollPnt, gp_Pnt)
1322 The following line defines the classes *MyPackage_HSequenceOfPnt* and *Handle(MyPackage_HSequenceOfPnt)*
1325 DEFINE_HSEQUENCE (MyPackage_HSequenceOfPnt, MyPackage_SequenceOfPnt)
1328 Source code file will be *MyPackage_HSequenceOfPnt.cxx* or any other .cxx file (once in the whole project):
1331 IMPLEMENT_HSEQUENCE (MyPackage_HSequenceOfPnt)
1334 @subsubsection occt_fcug_3_3_3 Class architecture
1337 To understand the basic architecture of the classes instantiated from *NCollection* macros, please refer to the documentation on *TCollection* package, particularly to CDL files. Almost all API described there is preserved in *NCollection*. Changes are described in corresponding *NCollection_Define\*.hxx* files.
1339 Nevertheless the internal structure of NCollection classes is more complex than that of *TCollection* ones, providing more capabilities. The advanced layer of architecture is described in the next chapter Features.
1341 There are two principal changes:
1342 * In *TCollection* some classes ( Stack, List, Set, Map, DataMap, DoubleMap ) define the Iterator type, the name of Iterator being like *MyPackage_DoubleMapIteratorOfDoubleMapOfIntegerReal*. In *NCollection* each Iterator is always defined as subtype of the collection *MyPackage_DoubleMapOfIntegerReal::Iterator*.
1343 * Hashed collections (of type Map\* ) require in *TCollection* that the special class *Map\*Hasher* is defined. In *NCollection* it is only required that the global functions *IsEqual* and *HashCode* are defined.
1345 #### Interface to classes defined in CDL
1347 The classes defined above can be used as types for fields, parameters of methods and return values in CDL definitions. In our example, if MyPackage is a CDL package, you will need to create the file *MyPackage_SequenceOfPnt.hxx* containing or including the above definitions, and then to add the line: imported *SequenceOfPnt* to file *MyPackage.cdl*;
1349 Then the new collection type can be used in any CDL definition under the name *SequenceOfPnt* from *MyPackage*.
1351 @subsubsection occt_fcug_3_3_4 New collection types
1353 There are 4 collection types provided as template classes:
1354 * *NCollection_Vector*
1355 * *NCollection_UBTree*
1356 * *NCollection_SparseArray*
1357 * *NCollection_CellFilter*
1361 This type is implemented internally as a list of arrays of the same size. Its properties:
1362 * Direct (constant-time) access to members like in Array1 type; data are allocated in compact blocks, this provides faster iteration.
1363 * Can grow without limits, like List, Stack or Queue types.
1364 * Once having the size LEN, it cannot be reduced to any size less than LEN – there is no operation of removal of items.
1366 Insertion in a Vector-type class is made by two methods:
1367 * _SetValue(ind, theValue)_ – array-type insertion, where ind is the index of the inserted item, can be any non-negative number. If it is greater than or equal to Length(), then the vector is enlarged (its Length() grows).
1368 * _Append(theValue)_ – list-type insertion equivalent to _myVec.SetValue(myVec.Length(), theValue)_ – incrementing the size of the collection.
1370 Other essential properties coming from List and Array1 type collections:
1371 * Like in *List*, the method *Clear()* destroys all contained objects and releases the allocated memory.
1372 * Like in *Array1*, the methods *Value()* and *ChangeValue()* return a contained object by index. Also, these methods have the form of overloaded operator ().
1376 The name of this type stands for “Unbalanced Binary Tree”. It stores the members in a binary tree of overlapped bounding objects (boxes or else).
1377 Once the tree of boxes of geometric objects is constructed, the algorithm is capable of fast geometric selection of objects. The tree can be easily updated by adding to it a new object with bounding box.
1378 The time of adding to the tree of one object is O(log(N)), where N is the total number of objects, so the time of building a tree of N objects is O(N(log(N)). The search time of one object is O(log(N)).
1380 Defining various classes inheriting *NCollection_UBTree::Selector* we can perform various kinds of selection over the same b-tree object.
1382 The object may be of any type allowing copying. Among the best suitable solutions there can be a pointer to an object, handled object or integer index of object inside some collection. The bounding object may have any dimension and geometry. The minimal interface of *TheBndType* (besides public empty and copy constructor and operator =) used in UBTree algorithm as follows:
1388 inline void Add (const MyBndType& other);
1389 // Updates me with other bounding type instance
1391 inline Standard_Boolean IsOut (const MyBndType& other) const;
1392 // Classifies other bounding type instance relatively me
1394 inline Standard_Real SquareExtent() const;
1395 // Computes the squared maximal linear extent of me (for a box it is the squared diagonal of the box).
1400 This interface is implemented in types of Bnd package: *Bnd_Box, Bnd_Box2d, Bnd_B2x, Bnd_B3x*.
1402 To select objects you need to define a class derived from *UBTree::Selector* that should redefine the necessary virtual methods to maintain the selection condition. Usually this class instance is also used to retrieve selected objects after search.
1403 The class *UBTreeFiller* is used to randomly populate a *UBTree* instance. The quality of a tree is better (considering the speed of searches) if objects are added to it in a random order trying to avoid the addition of a chain of nearby objects one following another.
1404 Instantiation of *UBTreeFiller* collects objects to be added, and then adds them at once to the given UBTree instance in a random order using the Fisher-Yates algorithm.
1405 Below is the sample code that creates an instance of *NCollection_UBTree* indexed by 2D boxes (Bnd_B2f), then a selection is performed returning the objects whose bounding boxes contain the given 2D point.
1408 typedef NCollection_UBTree<MyData, Bnd_B2f> UBTree;
1409 typedef NCollection_List<MyData> ListOfSelected;
1410 //! Tree Selector type
1411 class MyTreeSelector : public UBTree::Selector
1414 // This constructor initializes the selection criterion (e.g., a point)
1416 MyTreeSelector (const gp_XY& thePnt) : myPnt(thePnt) {}
1417 // Get the list of selected objects
1419 const ListOfSelected& ListAccepted () const
1421 // Bounding box rejection - definition of virtual method. @return True if theBox is outside the selection criterion.
1423 Standard_Boolean Reject (const Bnd_B2f& theBox) const
1424 { return theBox.IsOut(myPnt); }
1425 // Redefined from the base class. Called when the bounding of theData conforms to the selection criterion. This method updates myList.
1427 Standard_Boolean Accept (const MyData& theData)
1428 { myList.Append(theData); }
1431 ListOfSelected myList;
1434 // Create a UBTree instance and fill it with data, each data item having the corresponding 2D box.
1437 NCollection_UBTreeFiller <MyData, Bnd_B2f> aTreeFiller(aTree);
1439 const MyData& aData = …;
1440 const Bnd_B2d& aBox = aData.GetBox();
1441 aTreeFiller.Add(aData, aBox);
1445 // Perform selection based on ‘aPoint2d’
1446 MyTreeSelector aSel(aPoint2d);
1448 const ListOfSelected = aSel.ListAccepted();
1455 This type has almost the same features as Vector but it allows to store items having scattered indices. In Vector, if you set an item with index 1000000, the container will allocate memory for all items with indices in the range 0-1000000. In SparseArray, only one small block of items will be reserved that contains the item with index 1000000.
1457 This class can be also seen as equivalence of *DataMap<int,TheItemType>* with the only one practical difference: it can be much less memory-expensive if items are small (e.g. Integer or Handle).
1459 This type has both interfaces of DataMap and Vector to access items.
1463 This class represents a data structure for sorting geometric objects in n-dimensional space into cells, with associated algorithm for fast checking of coincidence (overlapping, intersection, etc.) with other objects. It can be considered as a functional alternative to UBTree, as in the best case it provides the direct access to an object like in an n-dimensional array, while search with UBTree provides logarithmic law access time.
1465 @subsubsection occt_fcug_3_3_5 Features
1467 NCollection defines some specific features, in addition to the public API inherited from TCollection classes.
1471 Every collection defines its Iterator class capable of iterating the members in some predefined order. Every Iterator is defined as a subtype of the particular collection type (e.g., MyPackage_StackOfPnt::Iterator ). The order of iteration is defined by a particular collection type. The methods of Iterator are:
1473 * _void Init (const MyCollection&)_ - initializes the iterator on the collection object;
1474 * _Standard_Boolean More () const_ - makes a query if there is another non-iterated member;
1475 * _void Next ()_ - increments the iterator;
1476 * _const ItemType& Value () const_ - returns the current member;
1477 * _ItemType& ChangeValue () const_ - returns the mutable current member
1480 typedef Ncollection_Sequence<gp_Pnt>
1481 MyPackage_SequenceOfPnt
1482 void Perform (const MyPackage_SequenceOfPnt& theSequence)
1484 MyPackage_SequenceOfPnt::Iterator anIter (theSequence);
1485 for (; anIter.More(); anIter.Next()) {
1486 const gp_Pnt aPnt& = anIter.Value();
1492 This feature is present only for some classes in *TCollection (Stack, List, Set, Map, DataMap, DoubleMap)*. In *NCollection* it is generalized.
1495 #### Class BaseCollection
1497 There is a common abstract base class for all collections for a given item type (e.g., gp_Pnt). Developer X can arbitrarily name this base class like *MyPackage_BaseCollPnt* in the examples above. This name is further used in the declarations of any (non-abstract) collection class to designate the C++ inheritance.
1499 This base class has the following public API:
1500 * abstract class Iterator as the base class for all Iterators descried above;
1501 * _Iterator& CreateIterator () const_ - creates and returns the Iterator on this collection;
1502 * _Standard_Integer Size () const_ - returns the number of items in this collection;
1503 * *void Assign (const NCollection_BaseCollection& theOther)* - copies the contents of the Other to this collection object;
1505 These members enable accessing any collection without knowing its exact type. In particular, it makes possible to implement methods receiving objects of the abstract collection type:
1508 #include <NColection_Map.hxx>
1509 typedef NCollection_Map<gp_Pnt> MyPackage_MapOfPnt;
1510 typedef NCollection_BaseCollection<gp_Pnt> MyPackage_BaseCollPnt;
1511 MyPackage_MapOfPnt aMapPnt;
1513 gp_Pnt aResult = COG (aMapPnt);
1515 gp_Pnt COG(const MyPackage_BaseCollPnt& theColl)
1517 gp_XYZ aCentreOfGravity(0., 0., 0.);
1518 // create type-independent iterator (it is abstract type instance)
1519 MyPackage_BaseCollString::Iterator& anIter = theColl.CreateIterator();
1520 for (; anIter.More(); anIter.Next()) {
1521 aCentreOfGravity += anIter.Value().XYZ();
1523 return aCentreOfGravity / theColl.Size();
1527 Note that there are fundamental differences between the shown type-independent iterator and the iterator belonging to a particular non-abstract collection:
1528 * Type-independent iterator can only be obtained via the call CreateIterator(); the typed iterator - only via the explicit construction.
1529 * Type-independent iterator is an abstract class, so it is impossible to copy it or to assign it to another collection object; the typed iterators can be copied and reassigned using the method Init() .
1530 * Type-independent iterator is actually destroyed when its collection object is destroyed; the typed iterator is destroyed as any other C++ object in the corresponding C++ scope.
1532 The common point between them is that it is possible to create any number of both types of iterators on the same collection object.
1534 #### Heterogeneous Assign
1536 The semantics of the method *Assign()* has been changed in comparison to *TCollection*. In *NCollection* classes the method *Assign()* is virtual and it receives the object of the abstract *BaseCollection* class (see the previous section). Therefore this method can be used to assign any collection type to any other if only these collections are instantiated on the same *ItemType*.
1538 For example, conversion of *Map* into *Array1* is performed like this:
1541 #include <NCollection_Map.hxx>
1542 #include <NCollection_Array1.hxx>
1543 typedef NCollection_Map<gp_Pnt> MyPackage_MapOfPnt;
1544 typedef NCollection_Array1<gp_Pnt> MyPackage_Array1OfPnt;
1546 MyPackage_MapOfPnt aMapPnt;
1548 MyPackage_Array1OfPnt anArr1Pnt (1, aMapPnt.Size());
1549 anArr1Pnt.Assign (aMapPnt); // heterogeneous assignment
1552 There are some aspects to mention:
1553 * Unlike in *TCollection*, in *NCollection* the methods *Assign* and operator= do not coincide. The former is a virtual method defined in the *BaseCollection* class. The latter is always defined in instance classes as a non-virtual inline method and it corresponds exactly to the method *Assign* in *TCollection* classes. Therefore it is always profitable to use operator= instead of *Assign* wherever the types on both sides of assignment are known.
1554 * If the method *Assign* copies to *Array1* or *Array2* structure, it first checks if the size of the array is equal to the number of items in the copied collection object. If the sizes differ, an exception is thrown, as in *TCollection_Array1.gxx*.
1555 * Copying to *Map, IndexedMap, DataMap* and *IndexedDataMap* can bring about a loss of data: when two or more copied data items have the same key value, only one item is copied and the others are discarded. It can lead to an error in the code like the following:
1558 MyPackage_Array1OfPnt anArr1Pnt (1, 100);
1559 MyPackage_MapOfPnt aMapPnt;
1561 aMapPnt.Assign(anArr1Pnt);
1562 anArr1Pnt.Assign(aMapPnt);
1565 Objects of classes parameterised with two types (*DoubleMap, DataMap* and *IndexedDataMap*) cannot be assigned. Their method *Assign* throws the exception *Standard_TypeMismatch* (because it is impossible to check if the passed *BaseCollection* parameter belongs to the same collection type).
1569 All constructors of *NCollection* classes receive the *Allocator* Object as the last parameter. This is an object of a type managed by Handle, inheriting *NCollection_BaseAllocator*, with the following (mandatory) methods redefined:
1572 Standard_EXPORT virtual void* Allocate (const size_t size);
1573 Standard_EXPORT virtual void Free (void * anAddress);
1576 It is used internally every time when the collection allocates memory for its item(s) and releases this memory. The default value of this parameter (empty *Handle*) designates the use of *NCollection_BaseAllocator* X where the functions *Standard::Allocate* and *Standard::Free* are called. Therefore if the user of *NCollection* does not specify any allocator as a parameter to the constructor of his collection, the memory management will be identical to the one in *TCollection* and other Open CASCADE Technology classes.
1578 Nevertheless, the it is possible to define a custom *Allocator* type to manage the memory in the most optimal or convenient way for his algorithms.
1580 As one possible choice, the class *NCollection_IncAllocator* is included. Unlike *BaseAllocator*, it owns all memory it allocates from the system. Memory is allocated in big blocks (about 20kB) and the allocator keeps track of the amount of occupied memory. The method *Allocate* just increments the pointer to non-occupied memory and returns its previous value. Memory is only released in the destructor of *IncAllocator*, the method *Free* is empty. If used efficiently, this Allocator can greatly improve the performance of OCCT collections.
1584 @subsection occt_fcug_3_4 Strings
1586 The **Strings** component provides services to manipulate character strings.
1587 **Strings** are classes that handle dynamically sized sequences of characters based on both ASCII (normal 8-bit character type) and Unicode (16-bit character type). They provide editing operations with built-in memory management which make the relative objects easier to use than ordinary character arrays.
1588 *Strings* may also be manipulated by *handle*, and therefore shared.
1590 @subsubsection occt_fcug_3_4_1 Examples
1592 #### TCollection_AsciiString
1594 A variable-length sequence of ASCII characters (normal 8-bit character type). It provides editing operations with built-in memory management to make *AsciiString* objects easier to use than ordinary character arrays.
1595 *AsciiString* objects follow value semantics;, that is, they are the actual strings, not handles to strings, and are copied through assignment. You may use *HAsciiString* objects to get handles to strings.
1597 #### TCollection_ExtendedString
1599 A variable-length sequence of "extended" (UNICODE) characters (16-bit character type). It provides editing operations with built-in memory management to make *ExtendedString* objects easier to use than ordinary extended character arrays.
1601 *ExtendedString* objects follow value semantics;, that is, they are the actual strings, not handles to strings, and are copied through assignment. You may use *HExtendedString* objects to get handles to strings.
1603 #### TCollection_HAsciiString
1605 A variable-length sequence of ASCII characters (normal 8-bit character type). It provides editing operations with built-in memory management to make *HAsciiString* objects easier to use than ordinary character arrays.
1606 *HAsciiString* objects are *handles* to strings.
1607 * *HAsciiString* strings may be shared by several objects.
1608 * You may use an *AsciiString* object to get the actual string.
1609 *HAsciiString* objects use an *AsciiString* string as a field.
1611 #### TCollection_HExtendedString
1613 A variable-length sequence of extended; (UNICODE) characters (16-bit character type). It provides editing operations with built-in memory management to make *ExtendedString* objects easier to use than ordinary extended character arrays.
1614 *HExtendedString* objects are *handles* to strings.
1615 * *HExtendedString* strings may be shared by several objects.
1616 * You may use an *ExtendedString* object to get the actual string.
1617 *HExtendedString* objects use an *ExtendedString* string as a field.
1619 @subsubsection occt_fcug_3_4_2 Conversion
1621 *Resource_Unicode* provides functions to convert a non-ASCII *C string* given in ANSI, EUC, GB or SJIS format, to a Unicode string of extended characters, and vice versa.
1623 @subsection occt_fcug_3_5 Unit Conversion
1625 The *UnitsAPI* global functions are used to convert a value from any unit into another unit. Conversion is executed among three unit systems:
1626 * the **SI System**,
1627 * the user’s **Local System**,
1628 * the user’s **Current System**.
1629 The **SI System** is the standard international unit system. It is indicated by *SI* in the signatures of the *UnitsAPI* functions.
1631 The OCCT (former MDTV) System corresponds to the SI international standard but the length unit and all its derivatives use the millimeter instead of the meter.
1633 Both systems are proposed by Open CASCADE Technology; the SI System is the standard option. By selecting one of these two systems, you define your **Local System** through the *SetLocalSystem* function. The **Local System** is indicated by *LS* in the signatures of the *UnitsAPI* functions.
1634 The Local System units can be modified in the working environment. You define your **Current System** by modifying its units through the *SetCurrentUnit* function. The Current System is indicated by *Current* in the signatures of the *UnitsAPI* functions.
1635 A physical quantity is defined by a string (example: LENGTH).
1638 @section occt_occt_fcug_4 Math Primitives and Algorithms
1639 @subsection occt_occt_fcug_4_1 Overview
1640 Math primitives and algorithms available in Open CASCADE Technology include:
1641 * Vectors and matrices
1642 * Geometric primitives
1645 @subsection occt_occt_fcug_4_2 Vectors and Matrices
1646 The Vectors and Matrices component provides a C++ implementation of the fundamental types Matrix and Vector, currently used to define more complex data structures. The Vector and Matrix classes support vectors and matrices of real values with standard operations such as addition, multiplication, transposition, inversion etc.
1647 Vectors and matrices have arbitrary ranges which must be defined at declaration time and cannot be changed after declaration.
1650 math_Vector v(1, 3);
1651 // a vector of dimension 3 with range (1..3)
1652 math_Matrix m(0, 2, 0, 2);
1653 // a matrix of dimension 3x3 with range (0..2, 0..2)
1654 math_Vector v(N1, N2);
1655 // a vector of dimension N2-N1+1 with range (N1..N2)
1658 Vector and Matrix objects use value semantics. In other words, they cannot be shared and are copied through assignment.
1661 math_Vector v1(1, 3), v2(0, 2);
1663 // v1 is copied into v2. a modification of v1 does not affect v2
1666 Vector and Matrix values may be initialized and obtained using indexes which must lie within the range definition of the vector or the matrix.
1669 math_Vector v(1, 3);
1670 math_Matrix m(1, 3, 1, 3);
1671 Standard_Real value;
1679 Some operations on Vector and Matrix objects may not be legal. In this case an exception is raised. Two standard exceptions are used:
1680 * *Standard_DimensionError* exception is raised when two matrices or vectors involved in an operation are of incompatible dimensions.
1681 * *Standard_RangeError* exception is raised if an access outside the range definition of a vector or of a matrix is attempted.
1684 math_Vector v1(1, 3), v2(1, 2), v3(0, 2);
1686 // error: Standard_DimensionError is raised
1689 // OK: ranges are not equal but dimensions are
1693 // error: Standard_RangeError is raised
1696 @subsection occt_occt_fcug_4_3 Primitive Geometric Types
1697 Before creating a geometric object, you must decide whether you are in a 2d or in a 3d context and how you want to handle the object.
1698 The *gp* package offers classes for both 2d and 3d objects which are handled by value rather than by reference. When this sort of object is copied, it is copied entirely. Changes in one instance will not be reflected in another.
1699 The *gp* package defines the basic non-persistent geometric entities used for algebraic calculation and basic analytical geometry in 2d & 3d space. It also provides basic transformations such as identity, rotation, translation, mirroring, scale transformations, combinations of transformations, etc. Entities are handled by value.
1700 The available geometric entities are:
1701 * 2d & 3d Cartesian coordinates (x, y, z)
1713 * Infinite cylindrical surface
1718 @subsection occt_occt_fcug_4_4 Collections of Primitive Geometric Types
1720 Before creating a geometric object, you must decide whether you are in a 2d or in a 3d context and how you want to handle the object.
1721 If you do not need a single instance of a geometric primitive but a set of them then the package which deals with collections of this sort of object, *TColgp*, will provide the necessary functionality.
1722 In particular, this package provides standard and frequently used instantiations of generic classes with geometric objects, i.e. *XY*, *XYZ*, *Pnt*, *Pnt2d*, *Vec*, *Vec2d*, *Lin*, *Lin2d*, *Circ*, *Circ2d.*
1723 These are non-persistent classes.
1725 @subsection occt_occt_fcug_4_5 Basic Geometric Libraries
1726 There are various library packages available which offer a range of basic computations on curves and surfaces.
1727 If you are dealing with objects created from the *gp* package, the useful algorithms are in the elementary curves and surfaces libraries - the *ElCLib* and *ElSLib* packages.
1728 * *EICLib* provides methods for analytic curves. This is a library of simple computations on curves from the *gp* package (Lines, Circles and Conics). It is possible to compute points with a given parameter or to compute the parameter for a point.
1729 * *EISLib* provides methods for analytic surfaces. This is a library of simple computations on surfaces from the package *gp* (Planes, Cylinders, Spheres, Cones, Tori). It is possible to compute points with a given pair of parameters or to compute the parameter for a point. There is a library for calculating normals on curves and surfaces.
1731 Additionally, *Bnd* package provides a set of classes and tools to operate with bounding boxes of geometric objects in 2d and 3d space.
1733 @subsection occt_occt_fcug_4_6 Common Math Algorithms
1734 The common math algorithms library provides a C++ implementation of the most frequently used mathematical algorithms. These include:
1735 * Algorithms to solve a set of linear algebraic equations,
1736 * Algorithms to find the minimum of a function of one or more independent variables,
1737 * Algorithms to find roots of one, or of a set, of non-linear equations,
1738 * An algorithm to find the eigenvalues and eigenvectors of a square matrix.
1740 All mathematical algorithms are implemented using the same principles. They contain:
1741 A constructor performing all, or most of, the calculation, given the appropriate arguments. All relevant information is stored inside the resulting object, so that all subsequent calculations or interrogations will be solved in the most efficient way.
1743 A function *IsDone* returning the boolean true if the calculation was successful.
1744 A set of functions, specific to each algorithm, enabling all the various results to be obtained.
1745 Calling these functions is legal only if the function *IsDone* answers **true**, otherwise the exception *StdFail_NotDone* is raised.
1747 The example below demonstrates the use of the Gauss class, which implements the Gauss solution for a set of linear equations.The following definition is an extract from the header file of the class *math_Gauss*:
1752 Gauss (const math_Matrix& A);
1753 Standard_Boolean IsDone() const;
1754 void Solve (const math_Vector& B,
1755 math_Vector& X) const;
1759 Now the main program uses the Gauss class to solve the equations a*x1=b1 and a*x2=b2:
1762 #include <math_Vector.hxx>
1763 #include <math_Matrix.hxx>
1766 math_Vector a(1, 3, 1, 3);
1767 math_Vector b1(1, 3), b2(1, 3);
1768 math_Vector x1(1, 3), x2(1, 3);
1769 // a, b1 and b2 are set here to the appropriate values
1770 math_Gauss sol(a); // computation of the
1771 // LU decomposition of A
1772 if(sol.IsDone()) { // is it OK ?
1773 sol.Solve(b1, x1); // yes, so compute x1
1774 sol.Solve(b2, x2); // then x2
1777 else { // it is not OK:
1779 sol.Solve(b1, x1); // error:
1780 // StdFail_NotDone is raised
1785 The next example demonstrates the use of the *BissecNewton* class, which implements a combination of the Newton and Bissection algorithms to find the root of a function known to lie between two bounds. The definition is an extract from the header file of the class *math_BissecNewton*:
1788 class BissecNewton {
1790 BissecNewton (math_FunctionWithDerivative& f,
1791 const Standard_Real bound1,
1792 const Standard_Real bound2,
1793 const Standard_Real tolx);
1794 Standard_Boolean IsDone() const;
1795 Standard_Real Root();
1799 The abstract class *math_FunctionWithDerivative* describes the services which have to be implemented for the function f which is to be used by a *BissecNewton* algorithm. The following definition corresponds to the header file of the abstract class *math_FunctionWithDerivative*:
1802 class math_FunctionWithDerivative {
1804 virtual Standard_Boolean Value
1805 (const Standard_Real x, Standard_Real& f) = 0;
1806 virtual Standard_Boolean Derivative
1807 (const Standard_Real x, Standard_Real& d) = 0;
1808 virtual Standard_Boolean Values
1809 (const Standard_Real x,
1811 Standard_Real& d) = 0;
1815 Now the test sample uses the *BissecNewton* class to find the root of the equation *f(x)=x**2-4* in the interval [1.5, 2.5]: the function to solve is implemented in the class *myFunction* which inherits from the class *math_FunctionWithDerivative*, then the main program finds the required root.
1818 #include <math_BissecNewton.hxx>
1819 #include <math_FunctionWithDerivative.hxx>
1820 class myFunction : public math_FunctionWithDerivative
1822 Standard_Real coefa, coefb, coefc;
1825 myFunction (const Standard_Real a, const Standard_Real b,
1826 const Standard_Real c) :
1827 coefa(a), coefb(b), coefc(c)
1830 virtual Standard_Boolean Value (const Standard_Real x,
1833 f = coefa * x * x + coefb * x + coefc;
1836 virtual Standard_Boolean Derivative (const Standard_Real x,
1839 d = coefa * x * 2.0 + coefb;
1842 virtual Standard_Boolean Values (const Standard_Real x,
1843 Standard_Real& f, Standard_Real& d)
1845 f = coefa * x * x + coefb * x + coefc;
1846 d = coefa * x * 2.0 + coefb;
1852 myFunction f(1.0, 0.0, 4.0);
1853 math_BissecNewton sol(F, 1.5, 2.5, 0.000001);
1854 if(Sol.IsDone()) { // is it OK ?
1855 Standard_Real x = sol.Root(); // yes.
1861 @subsection occt_occt_fcug_4_7 Precision
1863 On the OCCT platform, each object stored in the database should carry its own precision value. This is important when dealing with systems where objects are imported from other systems as well as with various associated precision values.
1865 The *Precision* package addresses the daily problem of the geometric algorithm developer: what precision setting to use to compare two numbers. Real number equivalence is clearly a poor choice. The difference between the numbers should be compared to a given precision setting.
1867 Do not write _if (X1 == X2),_ instead write _if (Abs(X1-X2) < Precision)._
1869 Also, to order real numbers, keep in mind that _if (X1 < X2 - Precision)_ is incorrect.
1870 _if (X2 - X1 > Precision)_ is far better when *X1* and *X2* are high numbers.
1872 This package proposes a set of methods providing precision settings for the most commonly encountered situations.
1874 In Open CASCADE Technology, precision is usually not implicit; low-level geometric algorithms accept precision settings as arguments. Usually these should not refer directly to this package.
1876 High-level modeling algorithms have to provide a precision setting to the low level geometric algorithms they call. One way is to use the settings provided by this package. The high-level modeling algorithms can also have their own strategy for managing precision. As an example the Topology Data Structure stores precision values which are later used by algorithms. When a new topology is created, it takes the stored value.
1877 Different precision settings offered by this package cover the most common needs of geometric algorithms such as *Intersection* and *Approximation*.
1878 The choice of a precision value depends both on the algorithm and on the geometric space. The geometric space may be either:
1879 * a real space, 3d or 2d where the lengths are measured in meters, micron, inches, etc.
1880 * a parametric space, 1d on a curve or 2d on a surface where numbers have no dimension.
1881 The choice of precision value for parametric space depends not only on the accuracy of the machine, but also on the dimensions of the curve or the surface.
1882 This is because it is desirable to link parametric precision and real precision. If you are on a curve defined by the equation *P(t)*, you would want to have equivalence between the following:
1885 Abs(t1-t2) < ParametricPrecision
1886 Distance (P(t1),P(t2)) < RealPrecision.
1889 @subsubsection occt_occt_fcug_4_7_1 The Precision package
1890 The *Precision* package offers a number of package methods and default precisions for use in dealing with angles, distances, intersections, approximations, and parametric space.
1891 It provides values to use in comparisons to test for real number equalities.
1892 * Angular precision compares angles.
1893 * Confusion precision compares distances.
1894 * Intersection precision is used by intersection algorithms.
1895 * Approximation precision is used by approximation algorithms.
1896 * Parametric precision gets a parametric space precision from a 3D precision.
1897 * *Infinite* returns a high number that can be considered to be infinite. Use <i>-Infinite</i> for a high negative number.
1899 @subsubsection occt_occt_fcug_4_7_2 Standard Precision values
1900 This package provides a set of real space precision values for algorithms. The real space precisions are designed for precision to *0.1* nanometers. The only unit available is the millimeter.
1901 The parametric precisions are derived from the real precisions by the *Parametric* function. This applies a scaling factor which is the length of a tangent to the curve or the surface. You, the user, provide this length. There is a default value for a curve with <i>[0,1]</i> parameter space and a length less than 100 meters.
1902 The geometric packages provide Parametric precisions for the different types of curves.
1903 The *Precision* package provides methods to test whether a real number can be considered to be infinite.
1905 #### Precision::Angular
1907 This method is used to compare two angles. Its current value is *Epsilon(2 * PI)* i.e. the smallest number *x* such that *2*PI + x* is different of *2\*PI*.
1909 It can be used to check confusion of two angles as follows:
1910 _Abs(Angle1 - Angle2) < Precision::Angular()_
1912 It is also possible to check parallelism of two vectors (_Vec_ from _gp_) as follows _V1.IsParallel(V2,Precision::Angular())_
1914 Note that *Precision::Angular()* can be used on both dot and cross products because for small angles the *Sine* and the *Angle* are equivalent. So to test if two directions of type *gp_Dir* are perpendicular, it is legal to use the following code:
1915 _Abs(D1 * D2) < Precision::Angular()_
1917 #### Precision::Confusion
1919 This method is used to test 3D distances. The current value is *1.e-7*, in other words, 1/10 micron if the unit used is the millimeter.
1921 It can be used to check confusion of two points (_Pnt_ from _gp_) as follows:
1922 _P1.IsEqual(P2,Precision::Confusion())_
1924 It is also possible to find a vector of null length (_Vec_ from _gp_) :
1925 _V.Magnitude() < Precision::Confusion()_
1927 #### Precision::Intersection
1929 This is reasonable precision to pass to an Intersection process as a limit of refinement of Intersection Points. *Intersection* is high enough for the process to converge quickly. *Intersection* is lower than *Confusion* so that you still get a point on the intersected geometries. The current value is *Confusion() / 100*.
1931 #### Precision::Approximation
1933 This is a reasonable precision to pass to an approximation process as a limit of refinement of fitting. The approximation is greater than the other precisions because it is designed to be used when the time is at a premium. It has been provided as a reasonable compromise by the designers of the Approximation algorithm. The current value is *Confusion() * 10*.
1934 Note that Approximation is greater than Confusion, so care must be taken when using Confusion in an approximation process.
1936 @section occt_fcug_5 Data Storage
1937 @subsection occt_fcug_5_1 Saving and Opening Files
1939 @image html /user_guides/foundation_classes/images/foundation_classes_image007.png "Example of Saving-Opening workflow"
1940 @image latex /user_guides/foundation_classes/images/foundation_classes_image007.png "Example of Saving-Opening workflow"
1942 In the example, the roots of the transferable transient objects *TopoDS_Shape, Geom_Geometry* and *Geom2d_Geometry* are used in algorithms, they contain data and temporary results.
1943 The associated objects in the persistent domain are *PTopoDS_HShape, PGeom_Geometry* and *PGeom2d_Geometry*. They contain a real data structure which is stored in a file.
1944 Note that when an object is stored, if it contains another stored object, the references to the contained object are also managed.
1945 @image html /user_guides/foundation_classes/images/foundation_classes_image008.png "Saving-Opening mechanism"
1946 @image latex /user_guides/foundation_classes/images/foundation_classes_image008.png "Saving-Opening mechanism"
1949 @subsection occt_fcug_5_2 Basic Storage Procedures
1951 @subsubsection occt_fcug_5_2_1 Saving
1953 The storage procedure of a transient object follows five main steps.
1954 1. Create an I/O driver for files. For example, *FSD_File f()*;
1955 2. Instance the data schema, which will process your persistent information. The schema is used for read/write operations. If ShapeSchema is the name of your schema:
1957 Handle(ShapeSchema) s = new ShapeSchema;
1959 3. Create a persistent shape from a transient shape.
1961 TopoDS_Shape aShape;
1962 PTColStd_TransientPersistentMap aMap;
1963 Handle(PTopoDS_HShape) aPShape = MgtBRep::Translate
1964 (aShape, aMap, MgtBRep_WithoutTriangle);
1966 4. Create a new container and fill it using the *AddRoot()* method.
1968 Handle(Storage_Data) d = new Storage_Data;
1969 d -> AddRoot (“ObjectName”, aPShape);
1971 You may add as many objects as you want in this container.
1972 5. Save to the archive.
1977 @subsubsection occt_fcug_5_2_2 Opening
1978 The retrieval mechanism is the opposite of the storage mechanism. The procedure for retrieving an object is as follows:
1980 1. Create an I/O driver and instance a data schema (if not done).
1981 2. Read the persistent object from the archive and get the list of objects using *Roots()* method.
1983 Handle(Storage_Data) d = s -> Read(f);
1984 Handle(Storage_HSeqOfRoot) roots = d-> Roots();
1986 3. Loop on root objects to get *Standard_Persistent* objects (the following sequence only gets the first root).
1988 Handle(Standard_Persistent) p;
1989 Handle(Standard_Root) r;
1990 if(roots -> Length() >= 1) {
1991 r = roots -> Value(1);
1995 4. DownCast the persistent object to a *PTopoDS_Hshape*.
1997 Handle(PTopoDS_HShape) aPShape;
1998 aPShape = Handle(PTopoDS_HShape)::DownCast(p);
2000 5. Create the *TopoDS_Shape*.
2002 TopoDS_Shape aShape;
2003 PTColStd_PersistentTransientMap aMap;
2004 MgtBRep::Translate (aPShape, aMap, aShape, MgtBRep_WithoutTriangle);
projects / occt.git / blob