[occt.git] / dox / dev_guides / cdl / cdl.md

 Component Definition Language (CDL)  {#dev_guides__cdl}
==============================

@tableofcontents

@section occt_cdl_0 DEPRECATION WARNING

Please note that CDL is considered as obsolete and is to be removed in one of future releases of OCCT.

@section occt_cdl_1 CDL and Application Architecture

CDL is the component  definition language of the Open CASCADE Technology (**OCCT**) programming  platform. Some components, which CDL allows you to create, are specific to OCCT  application architecture. These and other components, which you can define  using CDL include the following: 

  * Class (including enumeration,  exception)
  * Package
  * Schema
  * Executable
  * Client.
  
A **class** is the  fundamental software component in object-oriented development. Because of a  very large number of resources used in large-scale applications, the **class** itself  is too small to be used as a basic management unit. 

So, while the class is  the basic data component defined in CDL, this language also provides a way to  group classes, **enumerations**, and **exceptions** together – the **package**.  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. In  practice, a class name is prefixed with the name of its package e.g.  *Geom_Circle*. 

Using the services  described in the **packages**, you can construct an **executable**. You  can also group together services provided by **packages**.  

To save data in a file,  you need to define persistent classes. Then, you group these classes in a  schema, which provides the necessary read/write tools. 


@image html /dev_guides/cdl/images/cdl_image003.png "Building  an Open CASCADE Technology application" 
@image latex /dev_guides/cdl/images/cdl_image003.png "Building  an Open CASCADE Technology application" 
    
@section occt_cdl_2 Introduction to  CDL
@subsection occt_cdl_2_1  Purposes of the Language

You can use CDL to **define** **data** in the Open CASCADE Technology environment. CDL allows you to define  various kinds of data types supporting the application architecture and  development methodology, which you envision. CDL is neither an analysis  formalism (e.g. Booch methodology) nor a data manipulation language (e.g. C++). 

You use CDL in the **design** **phase** of a development process to define a set of software components which  best model the concepts stated in the application specification. 

@image html /dev_guides/cdl/images/cdl_image004.png "The Development Process" 
@image latex /dev_guides/cdl/images/cdl_image004.png "The Development Process" 


From a structural point  of view, CDL is an object-oriented language. It is centered on the notion of  the **class** - a data type, which represents an elementary concept. CDL  offers various means of organizing classes, mostly under the fundamental form  of **packages**. A package contains a set of classes, which share some  semantic relationship. This greatly simplifies your task of managing individual  classes when confronted with a very large number of them. 

Once you have defined  the classes and packages using CDL, you can implement their **methods** -  i.e., their functionality - in one of the data manipulation languages supported  by the OCCT environment (currently C++). 

Even though you can describe classes directly in C++  and save them as header files (.hxx), to do so would forfeit all the advantages  of using CDL. These are: 

  * Precise, complete, and  easy-to-read description of the software components.
  * Creation of a link with the  database; object persistence forms part of the predefined environment of the  language.
  * Multi-language access to the  services of an application engine – a specific architectural form created using  the CDL tools, which serves as the motor of an application.
  
@subsection occt_cdl_2_2   Overview of CDL

CDL is an object-oriented  language. In other words, it structures a system around data types rather than  around the actions carried out on them. In this context, an **object** is an  **instance** of a data type, and its definition determines how you can use  it. Each data type is implemented by one or more classes, which make up the  basic elements of the system. 

@subsubsection occt_cdl_2_2_1    Classes

A class is an  implementation of a **data type**. It defines its **behavior** and its **representation**. 

The behavior of a class  is its programming interface - the services offered by its **methods**. The  representation of a class is its data structure - the **fields**, which  store its data. 

Every object is an **instance** of its class. For example, the object *p* of the data type *Point* is  an instance of the class *Point*. 

The class Point could be  defined as in the example below: 

@code
class Point from  GeomPack
    ---Purpose: represents a point in 3D space.
   is
    Create returns Point;
fields
    x, y, z : Real;
end Point; 
@endcode

The definition of this class comprises two sections: 

  * one starting with the  keywords **is**
  * one starting with the keyword  **fields**.

The first section  contains a list of methods available to the clients of the class. The second  section defines the way in which instances are represented. Once this class has  been compiled you could **instantiate** its data type in a C++ test program  as in the example below: 



@code
GeomPack_Point p;
@endcode


@subsubsection occt_cdl_2_2_2     Categories of Types

You declare the  variables of a **data manipulation language** as being of certain data  types. These fall into two categories: 

  * Data types manipulated by  handle (or reference)
  * Data types manipulated by  value
  
    @image html /dev_guides/cdl/images/cdl_image005.png "Manipulation of data types" 
    @image latex /dev_guides/cdl/images/cdl_image005.png  "Manipulation of data types" 

As seen above, you  implement data types using classes. However, classes not only define their data  representation and methods available for their instances, but they also define  how the instances will be manipulated: 
  * A data type manipulated by  value contains the instance itself.
  * A data type manipulated by  handle contains a reference to the instance.

The most obvious  examples of data types manipulated by value are the predefined **primitive  types**: Boolean, Character, Integer, Real... 

A variable of a data  type manipulated by handle, which is not attached to an object, is said to be **null**.  To reference an object, you need to instantiate the class with one of its  constructors. This is done in C++ as in the following syntax: 

~~~~~
Handle(myClass) m = new myClass;
~~~~~


@subsubsection occt_cdl_2_2_3     Persistence

An object is called **persistent** if it can be permanently stored. In other words, you can use the object  again at a later date, both in the application, which created it, and in  another application. 

In order to make an  object persistent, you need to declare it in CDL as inheriting from the **Persistent**  class, or to have one of its parent classes inheriting from the *Persistent* class. 

Note that the classes  inheriting from the *Persistent* class are handled by reference. 

**Example** 
~~~~~
class Watch inherits Persistent
~~~~~

In this example,  building the application, you add the *Watch* class to the corresponding schema  of data types. 
If, running the  application, you instantiate an object of the *Watch* class, you have the  possibility of storing it in a file. 
You cannot store objects  instantiated from classes, which inherit from the *Storable* class.  However, you can store them as fields of an object, which inherits from  *Persistent*. 

Note that the objects  inheriting from Storable are handled by value. 

**Example** 
~~~~~
If 
class WatchSpring  inherits Storable 
//then this could be  stored as a field of a Watch 
//object: 
class Watch inherits  Persistent 
is...... 
fields 
name :  ConstructorName; 
powersource :  WatchSpring; 
end; 
~~~~~

@subsubsection occt_cdl_2_2_4    Packages

In large-scale long-term  development the task of marshalling potentially thousands of classes is likely  to quickly prove unmanageable. CDL introduces the notion of **package** of  classes containing a set of classes, which have some semantic or syntactic  relationship. For example, all classes representing a particular set of  electronic components might make up a package called Diode. 

As the package name  prefixes the class name when implementing such class (in C++ for example),  classes belonging to different packages can have the same name. For example,  two packages, one dealing with finance and the other dealing with aircraft  maneuvers, might both contain a class called *Bank*, without any possibility of  confusion. 

**Example** 
~~~~~
Finance_Bank 
Attitude_Bank 
~~~~~


@subsubsection occt_cdl_2_2_5     Inheritance

The purpose of  inheritance is to reduce development workload. The inheritance mechanisms allow  you to declare a new class as already containing the characteristics of an  existing class. This new class can then be rapidly specialized for a task at  hand. This eliminates the necessity of developing each component “from  scratch”. 

For example, having  already developed a class *BankAccount*, you can quickly specialize new classes -  *SavingsAccount, LongTermDepositAccount, MoneyMarketAccount,  RevolvingCreditAccount*, etc.. 

As a consequence, when  two or more classes inherit from a parent (or ancestor) class, all these  classes surely inherit the behavior of their parent (or ancestor). For example,  if the parent class *BankAccount* contains the method *Print* that tells it to  print itself out, then all its descendent classes offer the same service. 

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 inherited methods are not implemented. This forces you to  create a new class used to redefine the methods. In this way, you guarantee a  certain minimum common behavior among descendent classes. 

**Example** 
~~~~~
deferred class BankAccount inherits Persistent 
is 
....... 
fields 
name :  AccountHolderName; 
balance : CreditBalance; 
end; 
~~~~~

@subsubsection occt_cdl_2_2_6     Genericity

You will often wish to  model a certain type of behavior as a class. For example, you will need a list  modeled as a class. 

In order to be able to  list different objects, the class *List* must be able to accept different  data types as parameters. This is where genericity comes in: you first declare  a list declared as the generic class *List*, willing to accept any data  type (or only a particular set of acceptable data types). Then, when you want  to make a list of a certain type of object, you instantiate the class *List* with  the appropriate data type. 

**Example** 

~~~~~
generic class NewList (Item) 
inherits OldList 
is 
..... 
end ; 
~~~~~

Items may be of any  type, an Integer or a Real for example. 

When defining the  package, add the following line: 
**Example** 
~~~~~
class NewListOfInteger instantiates 
NewList (Integer); 
~~~~~

@subsubsection occt_cdl_2_2_7     Exceptions

The behavior of any  object is implemented by methods, which you define in its class declaration.  The definition of these methods includes not only their signature (their  programming interface) but also their domain of validity. 

In CDL, this domain is  expressed by **exceptions**. Exceptions are raised under various error  conditions. This mechanism is a safeguard of software quality. 

@subsubsection occt_cdl_2_2_8     Completeness

You use CDL to define  data types. Such definitions are not considered complete unless they contain  the required amount of structured commentary. 

The compiler does not  enforce this required degree of completeness, so it is the responsibility of  the developer to ensure that all CDL codes are properly annotated. 

Completeness is regarded  as an essential component of long-term viability of a software component. 


@subsection occt_cdl_2_3   Lexical Conventions
@subsubsection occt_cdl_2_3_1    Syntax  notation

In this manual, CDL  declarations are described using a simple variant of the Backus-Naur formalism.  Note the following: 

  * Italicized words, which may  also be hyphenated, denote syntactical categories, for example *declaration-of-a-non-generic-class* ;
  * Keywords appear in bold type: **class** ;
  * Brackets enclose optional  elements: 
~~~~~
  identifier [from package-name] 
~~~~~
  * Curly braces enclose repeated  elements. The element may appear zero or many times: 
~~~~~  
  integer ::=  digit{digit} 
~~~~~
  * Vertical bars separate  alternatives:
~~~~~
passing-method ::=  <b>[in] | out | in out </b> 
~~~~~
  * Two apostrophes enclose a  character or a string of characters, which must appear:
~~~~~
exponent ::=  ’E’[’+’]integer | ’E-’ integer 
~~~~~
**NOTE** To introduce the ideas progressively, the  examples presented in this manual may be incomplete, and thus not compilable by  the CDL compiler. 


@subsubsection occt_cdl_2_3_2    Lexical  elements

A CDL source is composed  of text from one or more compiled units. The text of each compiled unit is a  string of separate lexical elements: **identifiers**, **keywords**, **constants**,  and **separators**. The separators (blank spaces, end of line, format  characters) are ignored by the CDL compiler, but these are often necessary for  separating identifiers, keywords, and constants. 


@subsubsection occt_cdl_2_3_3     Comments

With CDL, you cannot use  the expression of all useful information about a development unit. In  particular, certain information is more easily expressed in natural language.  You can add such information to the CDL description of a data type. 

Rubrics and free  comments are to be differentiated: 

**Free comments** are preceded by the characters “--” (two  hyphens), and they terminate at the end of the line in which they appear. 
**Example** 
~~~~~
--This is a comment 
~~~~~

Unlike rubrics, free  comments can appear before or after any lexical element. The first written  character of the comment itself *must not* be a hyphen. If a hyphen is  necessary make sure it is preceded by a blank. 
**Example** 
~~~~~
-- -List item 
~~~~~
**Rubrics** are various types of comments attached to CDL components.  A rubric is a comment made up of three hyphens, name of the rubric (without any  intermediary space) and then a colon and a space. It is terminated by the  beginning of the following rubric, or by the end of the commentary. 

**Example** 
~~~~~
---Purpose:This is an example of a 
--rubric composed of a 
--comment which extends to 
--four lines. 
~~~~~

The different categories  of rubrics and the form of their content do not depend on the Component  Description Language, but on the tool for which it is intended.  

The use of commentary is  generally governed by the internal programming standards of an enterprise. You  are encouraged to use various well-defined rubrics, such as Purpose, Warning,  Example, References, Keywords, etc. 

These rubrics can be  attached to: 

  * Packages
  * Classes
  * Methods
  * Schemas
  * Executables
  * Clients

@subsubsection occt_cdl_2_3_4     Identifiers

An identifier is an  arbitrary chain of characters, either letters or digits, but it must begin with  a letter. 

The underscore “_” is  considered to be a letter as long as it doesn’t appear at the beginning or the  end of an identifier. 

Capital and small  letters are not equivalent (i.e. AB, Ab, aB, ab are four different  identifiers). 


@subsubsection occt_cdl_2_3_5     Keywords

The following is a list  of keywords. 

* alias 
* any                    
* as                     
* asynchronous 
* class                     
* client                 
* deferred            
* end 
* enumeration           
* exception           
* executable        
* external 
* fields                     
* friends               
* from                 
* generic 
* immutable              
* imported            
* inherits              
* instantiates 
* is                          
* library                
* like                   
* me 
* mutable                 
* myclass             
* out                   
* package 
* pointer                   
* primitive             
* private              
* protected 
* raises                    
* redefined           
* returns              
* schema 
* static                     
* to                      
* uses                 
* virtual 

In a CDL file, the  following characters are used as punctuation: 
; : , = ( ) [ ] ‘ “ 

@subsubsection occt_cdl_2_3_6     Constants

There are three  categories of constants: 

  * Numeric
  * Literal
  * Named

#### Numeric Constants

There are two types of  numeric constants: integer and real. 

An **integer** constant  consists of a string of digits, which may or may not be preceded by a sign.  Integer constants express whole numbers. 

**Examples** 
~~~~~
1995         0            -273         +78 
~~~~~
A **real** constant  may or may not be preceded by a sign, and consists of an integral part followed  by a decimal point and a fractional part (either one or both parts may be null,  but both parts must always be present). It may also be followed by the letter E  to indicate that the following figures represent the exponent (also optionally  signed). 

**Examples** 
~~~~~
5.0        0.0           -0.8E+3          5.67E-12 
~~~~~
#### Literal Constants

Literal constants  include individual characters and strings of characters. 

An **individual  character** constant is a single printable character enclosed by two  apostrophes. (See the definition of the class Character in the Standard  Package). 

**Examples** 
~~~~~
 ‘B’       ‘y’      ‘&amp;’      ‘*’      ‘’’ ‘‘ 
~~~~~
A **string** constant  is composed of printable characters enclosed by quotation marks. 

**Examples** 
~~~~~
’’G’’     ’’jjjj’’      ’’This is a character string, isn’t it?’’ 
~~~~~
The **quotation mark** can  itself appear within a character string as long as it is preceded by a  backslash. 

**Examples** 
~~~~~
’’This film was  originally called \’’Gone with the Tide\’’.’’ 
~~~~~

#### Named Constants

Named constants are  sub-divided into two categories: Booleans and enumerations. 

**Booleans** can be of two types: True or False. 

An **enumeration** constant  is an identifier, which appears in the description of an enumeration. 

@section occt_cdl_3 Software  Components

@subsection occt_cdl_3_1   Predefined Resources
@subsubsection occt_cdl_3_1_1     Primitive types

Primitive types are  predefined in the language and they are **manipulated by value**. 

Four of these primitives  are known to the schema of the database because they inherit from the class **Storable**.  In other words, they can be used in the implementation of persistent objects,  either when contained in entities declared within the methods of the object, or  when they form part of the internal representation of the object. 

The primitives inheriting  from **Storable** are the following: 

* **Boolean** Is used to represent logical data. It has only  two values: *True* and *False*. 
* **Byte** 8-bit number. 
* **Character** Designates any ASCII character. 
* **ExtCharacter** Is an extended character. 
* **Integer** Is an integer number. 
* **Real** Denotes a real number (i.e. one with a whole and  a fractional part, either of which may be null). 
* **ShortReal** Real with a smaller choice of values and memory  size. 

There are also  non-storable primitives. They are: 

* **CString** Is used for literal constants. 
* **ExtString** Is an extended string. 
* **Address** Represents a byte address of undetermined size. 

The services offered by  each of these types are described in the Standard Package. 


@subsubsection occt_cdl_3_1_2     Manipulating types by reference (by handle)

Two types are  manipulated by handle: 

  * Types defined using classes  inheriting from the **Persistent** class are storable in a file.
  * Types defined using classes  inheriting from the **Transient** class.
  
These types are not storable as such in a file. 

@image html /dev_guides/cdl/images/cdl_image006.png "Manipulation of a data type by reference"
@image latex /dev_guides/cdl/images/cdl_image006.png "Manipulation of a data type by reference"


@subsubsection occt_cdl_3_1_3     Manipulating types by value

Types, which are  manipulated by value, behave in a more direct fashion than those manipulated by  handle. As a consequence, they can be expected to perform operations faster,  but they cannot be stored independently in a file. 

You can store types  known to the schema (i.e. either primitives or inheriting from Storable) and  manipulated by value inside a persistent object as part of the representation.  This is the only way for you to store objects “manipulated by value” in a file. 

@image html /dev_guides/cdl/images/cdl_image007.png "Manipulation of a data type by value" 
@image latex /dev_guides/cdl/images/cdl_image007.png "Manipulation of a data type by value" 

Three types are  manipulated by value: 

  * Primitive types
  * Enumerated types
  * Types defined by classes not  inheriting from Persistent or Transient, whether directly or not

@subsubsection occt_cdl_3_1_4   Summary  of properties


Here is a summary of how various data types are handled and their  storability:
 
| | Manipulated by handle | Manipulated by value |
| :---- | :---- | :---- |
| storable | Persistent | Primitive, Storable (storable if nested in a persistent class) |
| temporary | Transient | Other | 




@subsection occt_cdl_3_2   Classes

@subsubsection occt_cdl_3_2_1    Class  declaration

The class is the main  system for creating data types under CDL. By analyzing any CDL-based software,  you find that classes are the modular units that make up packages. When you  describe a new class, you introduce a new data type. 

Whatever the category of  the described type (manipulated by value, Storable or not, manipulated by  handle, Persistent or not) the structure of the class definition remains the  same. The syntax below illustrates it: 

~~~~~
-- declaration-of-a-simple-class ::= 
class class-name from package-name 
[uses data-type {  ’,’ data-type } ] 
[raises  exception-name { ’,’ exception-name} ] 
is class-definition 
end [ class-name ]  ’;’ 
data-type ::=  enumeration-name | class-name | 
exception-name | primitive-type 
package-name ::=  identifier 
class-name ::=  identifier 
class-definition ::= 
[{member-method}] 
[declaration-of-fields] 
[declaration-of-friends] 
~~~~~
Class name becomes a new  data type, which you can use inside its own definition. Other types appearing  in the definition must either be primitive types, previously declared classes,  exceptions, or enumerations. 

Apart from the types  defined in the Standard Package, which are **implicitly visible** everywhere,  you need to declare the data types after the keyword **uses**. This concerns  both the class behavior and its internal representation. 

**Exceptions** are declared after the word **raises**. 

**Example** 
~~~~~
class Line from  GeomPack 
usesPoint,  Direction, Transformation 
raisesNullDirection,  IdenticalPoints 
is-- class  definition follows here 
-- using Point,  Direction and 
-- Transformation  objects,and the 
-- NullDirection and  Identical- 
-- -Points  exceptions. 
end Line; 
~~~~~

The elements, which make  up the definition of a class, are divided into four parts: 
  * the behavior
  * the invariants
  * the internal representation
  * the friend methods and friend  classes.

    @image html /dev_guides/cdl/images/cdl_image009.png "Contents of a class"
    @image latex /dev_guides/cdl/images/cdl_image009.png "Contents of a class"

@subsubsection occt_cdl_3_2_2     Categories of classes

Classes fall into three categories: 
  * Ordinary classes
  * Deferred classes
  * Generic classes

#### Deferred classes

The principal  characteristic of a **deferred class** is that you cannot instantiate it.  Its purpose is to provide a given behavior shared by a hierarchy of  classes and dependent on the implementation of the descendents. This allows guaranteeing a certain base of inherited behavior common to all classes based on a particular deferred class. Deferred classes are declared as in the  following syntax: 

~~~~~
-- declaration-of-a-deferred-class ::= deferred class  class-name 
[inherits class-name  {’,’ class-name}] 
[uses data-type {’,’  data-type}] 
[raises exception-name  {’,’ exception-name}] 
        is class-definition 
        end [class-name]’;’ 
~~~~~
Please, note that a deferred class does not have to contain a  constructor

<h4>Generic classes</h4>

The principal  characteristic of a **generic class** is that it offers a set of  functional behavior to manipulate other data types. To instantiate  a generic class you need to pass a data type in argument. Generic classes are  declared as in the following syntax: 

~~~~~
-- declaration-of-a-generic-class ::= [deferred] generic  class class-name ’(’generic-type {’,’generic-type}’)’ 
[inheritsclass-name {’,’ class-name}] 
[usesdata-type {’,’  data-type}] 
[raisesexception-name  {’,’ exception-name}] 
[{[visibility]  declaration-of-a-class}] 
        is class-definition 
        end [class-name]’;’ 
generic-type ::=  identifier as type-constraint 
identifier ::=  letter{[underscore]alphanumeric} 
type-constraint ::= any | class-name [’(’data-type {’,’ data-type}’)’] 
~~~~~


@subsection occt_cdl_3_3   Packages

@subsubsection occt_cdl_3_3_1    Package  declaration

**Packages** are  used to group   classes, which have some logical coherence. For example, the  Standard Package groups together all the predefined resources of the language.  In its simplest form, a package contains the declaration of all data types,  which it introduces. You may also use a package to offer public methods and  hide its internal classes by declaring them private. 

**Example** 

~~~~~
-- package-declaration ::= package package-name 
        [uses package-name {’,’ package-name}] 
        is package-definition 
        end [package-name]’;’ 
-- package-name ::= identifier 
-- package-definition ::= 
        [{type-declaration}] 
        [{package-method}] 
-- type-declaration ::= 
        [private] declaration-of-an-enumeration |       [private] declaration-of-a-class |      declaration-of-an-exception 
-- package-method ::= identifier [simple-formal-part][returned-type -declaration] 
[error-declaration] 
[is private]’;’ 
~~~~~

The data types described  in a package *may* include one or more of the following data types: 
  * Enumerations
  * Object classes
  * Exceptions
  * Pointers to other object  classes.

Inside a package, two  data types *cannot* have the same name. 

You declare data types  before using them in the definition of other data types. 

When two classes are **mutually  recursive**, one of the two must be first declared in an incomplete  fashion. 

Grouped behind the  keyword **uses** are the names of all the packages containing definitions of  classes of which the newly introduced data types are clients. 

The methods you declare  in a package do not belong to any particular class. **Package methods** must carry a name different from the data types contained in the package. Like  any other method, they can be overloaded. With the exception of the keyword **me** and the visibility (a package method can *only* be either public or  private) package methods are described in the same way as **instance methods**. 

@image html /dev_guides/cdl/images/cdl_image010.png "Contents of a package" 
@image latex /dev_guides/cdl/images/cdl_image010.png "Contents of a package" 


The example of the package below includes some of the basic data structures: 

~~~~~
package Collection 
        uses 
        Standard 
        is 
exception NoSuchObject inherits Failure; 
exception NoMoreObject inherits Failure; 
generic class SingleList; 
generic class Set; 
end Collection; 
~~~~~

Note that the class *Set* is declared after the declarations of the *NoSuchObject* and *NoMoreObject*  exceptions and the *SingleList* class of which Set is a client. In the same way, the classes *Failure*, *Persistent*, and the exception *NoSuchObject* are defined before they are used. They are  defined in the *Standard* package, which appears after the keyword **uses**. 

@subsubsection occt_cdl_3_3_2    Name space

The **name space** or  **scope** of a class extends from the beginning of its declaration up to the  end of the package in which it appears. 

Sometimes, two classes,  which come from separate packages, are both visible to a third package and  carry the same name. For example, there might be two different classes both  called “Window” in a screen generator package and in an architectural package.  As a client of a data type, you can find yourself in the position of having to  remove the ambiguity over the origin of this type; you do this by means of the  keyword **from**.
 

~~~~~
-- class-name ::= identifier [from package-name] 
-- exception-name ::= identifier [from package-name] 
-- enumeration-name ::= identifier [from package-name] 
~~~~~

You can use the keyword **from** everywhere the name of a class, exception, or enumeration appears. As a  consequence, as a client of the class “Window” you could write wherever  necessary: 

~~~~~
Window from ScreenGenerator 
-- or 
Window from ArchitecturalFeatures 
~~~~~

**Note** that within the description of a package the keyword **from** must be used when referencing any data type  that is not defined in this package.

Here is a further  example: 

~~~~~
class Line from Geom 
uses 
        Point from Geom2d, 
        Point from Geom3d 
is 
        -- class definition  using Point from AppropriatePackage wherever Point  appears 
end; 
~~~~~

@subsubsection occt_cdl_3_3_3     Declaration of classes

You cannot describe a  package in one single file. You need to describe it in different units and send  them separately to the CDL compiler. Each compilation unit can contain the  declaration of a class or of a package. When you describe a class in a unit  different than that, which describes its package, you need to specify which  package the class belongs to. You do this using the keyword **from**. 

If the **from** clause  appears in the **uses** clause of the package, it does not need to be  repeated elsewhere. 

The following example  takes the package “Collection” which was presented above, but this time it is  divided into three compilation units. 

~~~~~
-- First compilation unit, the package “Collection” : 
package Collection 
        uses 
        Standard 
        is 
exception  NoMoreObject inherits Failure from Standard; 
exception NoSuchObject inherits Failure from Standard; 
generic class SingleList; 
generic class Set, Node, Iterator; 
end Collection; 
-- Second compilation unit, the class “SingleList” : 
generic class SingleList from Collection (Item as 
Storable) 
        inherits 
                Persistent from Standard 
                raises 
                NoSuchObject from  Collection 
                is 
                -- definition of the  SingleList class 
        end SingleList; 
-- Third compilation unit, the class “Set” : 
generic class Set from Collection (Item as Storable) 
        inherits 
                Persistent from Standard; 
        raises 
                NoSuchObject from Collection, 
                NoMoreObject from  Collection 
                private class Node  instantiates SingleList 
                from Collection  (Item); 
 end Set; 
~~~~~

NOTE 
It is not explicitly stated that the *Node* class  belongs to the *Collection* package. In fact any nested class necessarily  belongs to the package of the class, which encompasses it. 

Note that a package can  hide certain classes (just as it can hide methods) by declaring them **private**.  To make a class private, you prefix its description with the keyword **private**.  In the example of the *Collection* package, the *SingleList* class serves only  to implement the *Set* class. It is recommended to make it private. You write  this as in the following syntax: 


**Example** 
~~~~~
package Collection 
        uses 
        Standard 
        is 
generic class Set,  Node, Iterator; 
private generic class SingleList; 
exception NoMoreObject inherits Failure from Standard; 
end Collection; 
~~~~~



@subsection occt_cdl_3_4   Other Data Types

The other data types are: 
  * Enumerations
  * Imports
  * Aliases
  * Exceptions
  * Pointers

@subsubsection occt_cdl_3_4_1     Enumerations

The **enumerated types** are  the second type, which is manipulated by value. Unlike the primitive types they  are extensible because they are defined by the user under the form of  enumerations. An enumeration is an ordered sequence of named whole constant  values called enumeration constants. 

**Example** 
~~~~~
declaration-of-an-enumeration ::= 
enumeration enumeration-name 
is identifier {’,’ identifier} 
[end [enumeration-name]]’;’ 
enumeration-name ::= identifier 
~~~~~
The declaration of an  enumeration creates an enumerated type. An object of this type can successively  take the value of any one of the constants cited in the list. 

**Example** 
~~~~~
enumeration MagnitudeSign is Negative, Null, Positive; 
~~~~~

Inside a package, two  enumeration constants cannot have the same name, even if they belong to  different enumerated types.
 
**Example** 
~~~~~
enumeration Cars is 
        Honda, 
        Ford, 
        Volkswagen, 
        Renault 
end; 
enumeration AmericanPresidents is 
        Nixon, 
        Reagan, 
        Ford, -- Error: ‘Ford’ already defined 
        Carter 
end; 
~~~~~

@subsubsection occt_cdl_3_4_2    Imports

An **imported type** is  one of which which has not been defined in CDL. It is up to the supplier of  this data type to ensure compatibility with the CDL language by providing  services which allow CDL to recognize the imported data type. 

The CDL syntax for  declaring an imported type is: 
~~~~~
declaration-of-an-imported-type::=[private] imported  typename ; 
~~~~~

Let us try to define an imported type:

* In the *MyPack.cdl* file, you declare the imported type:
~~~~~ 
package MyPack 
        .... 
        imported MyImport; 
        .... 
        end Mypack; 
~~~~~   
* In the *MyPack_MyImport.hxx* file, you write the following C++ code: 
~~~~~
#ifndef _MyPack_MyImport_HeaderFile 
#define _MyPack_MyImport_HeaderFile 
#include Standard_Type.hxx 
typedef unsigned long MyPack_MyImport; 
extern const Handle(Standard_Type)&amp; TYPE 
(MyPack_MyImport); 
~~~~~
* In the *MyPack_MyImport.cxx* file, you write the following C++ code: 
~~~~~
#ifndef _MyPack_MyImport_HeaderFile 
#include MyPack_MyImport.hxx 
#endif 
const Handle(Standard_Type)&amp; TYPE (MyPack_MyImport) 
{ 
        static Handle(Standard_Type) _aType = 
                new Standard_Type  (“MyPack_MyImport”,sizeof 
                (MyPack_MyImport)) 
                 return _aType; 
        } 
~~~~~

Then, add the names of  these two files <i>(MyPack_MyImport.hxx, MyPack_MyImport.cxx)</i> to a file called  FILES in the src subdirectory of the package. If the file does not exist you  must create it. 


@subsubsection occt_cdl_3_4_3    Aliases

An **alias** is an  extra name for a type, which is already known. It is declared as in the  following syntax: 

~~~~~
declaration-of-an-alias::= [private] alias type1 is type2  [from apackage] ; 
~~~~~

**Example** 
~~~~~
alias Mass is Real; 
---Purpose: 
-- Defined as a quantity of matter. 
-- Gives rise to the  inertial and 
-- gravitational  properties of a body. 
-- It is measured in  kilograms. 
~~~~~

Having defined *Mass* as  a type of *Real*, you can use either *Mass* or *Real* to type an  argument when defining a method. 


@subsubsection occt_cdl_3_4_4     Exceptions

In the model recommended  by CDL, the principal characteristic of errors is that they are treated in a  different place from the place where they appear. In other words, the methods  recovering and those raising a given exception are written independently from  each other. 

Subsequently this poses  the problem of communication between the two programs. The principle adopted  consists in viewing the exception as both a class and an object. The exception  class (by means of one of its instances) is used to take control of an  exception, which has been raised. 

Consequently, error conditions  are defined by means of **classes of exceptions**. Exception classes are  arranged hierarchically so as to be able to recover them in groups. They are  all descendents of a single root class called *Failure*, and it is at the level  of this class that the behavior linked to the raising of exceptions is  implemented. 
~~~~~
declaration-of-an-exception ::=exception exception-name inherits exception-name 
~~~~~

All exceptions share identical behavior, that of the  class *Failure*. Here are some examples of exception classes: 
~~~~~
exception NumericError inherits Failure; 
exception Overflow inherits NumericError; 
exception Underflow inherits NumericError; 
~~~~~

The use of exceptions as  a means to interrupt the normal execution of one program and then take control  of another one depends on the programming language used to implement the  methods. See the following chapter <a href="#occt_cdl_4">“Defining the Software Components”</a>  on page 32. 


@subsection occt_cdl_3_5   Schemas

The purpose of a **schema** is to list persistent data types, which will be stored in files by the  application. A schema groups together persistent packages. A persistent package  is one, which contains at least one persistent class. 

~~~~~
declaration-of-a-schema ::= 
schema SchemaName 
is 
{package PackageName;} 
{class ClassName;} 
end;
~~~~~
 
For example 
~~~~~
schema Bicycle 
---Purpose: Defines the Bicycle schema. 
is 
package  FrameComponents; 
package WheelComponents; 
end; 
~~~~~


**Note** that it is  unnecessary to specify all the dependencies of the packages. It is sufficient  to specify the highest level ones. The others on which they depend are  automatically supplied. 

@subsection occt_cdl_3_6   Executables

The purpose of an **executable** is to make an executable program without a front-end. It can be used to  test more than one package at a time. An executable is written in a .cdl file  as a set of packages. 
**Example** 
~~~~~
definition-of-an-executable ::= 
        executable ExecutableName 
        is 
{ 
executable ExecutablePart 
        [uses  [Identifier as external] 
        [{’,’  Identifier as external}] 
        [UnitName as  library] 
        [{’,’  UnitName as library}] 
        is 
        {FileName  [as C++|c|fortran|object];} 
        end; 
        } 
end; 
~~~~~

**Example** 
~~~~~
executable MyExecUnit 
        ---Purpose: 
        -- Describes the  executable MyExecUnit 
        is 
        executable myexec 
        -- the binary file 
        uses 
        Tcl_Lib as external 
        is 
        myexec; 
        -- the C++ file 
        end; 
        -- several binaries can be specified in one .cdl file. 
        executable myex2 
        is 
        myex2; 
end; 
end; 
~~~~~

@section occt_cdl_4 Defining the Software Components

@subsection occt_cdl_4_1   Behavior

The behavior of an  object class is defined by a list of **methods**, which 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: 

* **Object constructor** Creates an instance of the described class. A  class will have one or more object constructors with various arguments or none. 
* **Instance method** Operates on the instance which owns it. 
* **Class method** Does not work on individual instances, only on the class itself. 

@subsubsection occt_cdl_4_11    Object  Constructors

A constructor is a  function, which allows the **creation of instances** of the class it  describes. 

~~~~~
constructor-declaration ::= 
Create [ simple-formal-part ] declaration-ofconstructed-type 
[ exception-declarations ] 
simple-formal-part ::= 
’(’  initialization-parameter {’;’ initialization parameter}’)’ 
initialization-parameter ::= 
identifier {’,’ identifier} ’:’ parameter-access  datatype 
[ ’=’ initial-value ] 
parameter-access ::= 
mutable | [ immutable ] 
initial_value ::= 
numeric-constant | literal-constant | named-constant 
declaration-of-constructed-type ::= 
returns [ mutable ] class-name 
~~~~~

The name of the  constructors is fixed: “Create”. The object returned by a constructor is  always of the type of the described class. If that type is manipulated by a  handle, you *must* declare it as **mutable**, in which case the content  of the instance it references is accessible for further modification. 

For example, the  constructor of the class “Point” 
~~~~~
Create (X, Y, Z : Real) 
returns mutable  Point; 
~~~~~

With the exception of  the types predefined by the language, all types of initialization parameters *must* appear in the **uses** clause of the class of which the constructor is a  member. 

When an initialization  parameter is of a type which is manipulated by a handle, an access right *must* be associated with it so as to express if the internal representation of  the referenced object is modifiable (**mutable**) or not (**immutable**).  The default option is **immutable**. Let, for example, take the constructor of the  persistent class “Line”. 

~~~~~
Create (P : mutable Point; D : mutable Direction) 
returns mutable  Line; 
~~~~~

In the above example “P”  and “D” must be mutable because the constructor stores them in the internal  representation of the created line, which is mutable itself. An alternative  would be to accept immutable initialization parameters and then copy them into  the constructor in a mutable form. 

The parameters of a  native type can have a default value: this is expressed by assigning a constant  of the same type to the parameter concerned. Parameters, which have a default  value, may not be present when the call to the constructor is made, in which  case they take the value specified in the declaration. For this reason, they  must all be grouped at the end of the list. Let, for example, take the constructor of the  persistent class “Vector”. 

~~~~~
Create (D : mutable Direction; M : Real = 1.0) 
returns mutable  Vector; 
~~~~~

A class can have many  constructors (in this case, you say they are **overloaded**) provided that  they differ in their syntax and that the presence of parameters having default  values does not create ambiguities. 

The restrictions on  their use are expressed by a list of **exceptions** against which each  constructor is protected. 

Each class must have at  least one constructor to be able to create objects of its type. 

@subsubsection occt_cdl_4_1_2     Instance Methods

An instance method is a  function or procedure, which applies to any instance of the class, which  describes it. 

**Example** 
~~~~~
declaration-of-an-instance-method ::= identifier formal-part-of-instance-method 
[  declaration-of-returned-type ] 
[  exception-declaration ] 
formal-part-of-instance-method  ::=  ’(’ me [’:’  passing-mode parameter-access ] {’;’ parameter}’)’ 
parameter ::= identifier {’,’  identifier} ’:’ passing-mode 
parameter-access 
data-type [ ’=’ initial-value ] 
passing-mode ::= [ in ] | out | in  out 
parameter-access ::= mutable |  [immutable] 
declaration-of-returned-type  ::= returns  return-access data-type 
return-access ::= mutable |[ immutable  ]| any 
~~~~~

The name **me** denotes  the object to which the method is applied: you call this the “principal object”  of the method. The passing mode expresses whether the direct content of the  principal object or a parameter is either: 

  * read
  * created and returned
  * read then updated and  returned by the method.

Remember that the direct  content of an argument of a type which is manipulated by value contains the  internal representation of the object itself. Thus, when the argument is of  this type, **out** and **in out** mean that the content of the object will  undergo a modification. When the method is a function (as is the case for  constructors), all the arguments must be **in** (read). This is the default  mode. 

In case of an argument  of a type manipulated by a handle, the direct content being an object identifier,  the passing mode addresses itself to the handle, and no longer to the internal  representation of the object, the modification of which is controlled by the  access right. An argument of this type declared **mutable** may see its  internal representation modified. If declared **immutable**, it is  protected. When a parameter is both **in out** and **mutable**, the  identifiers passed and returned denote two distinct modifiable objects. 

When the returned object  is manipulated by a handle it can be declared modifiable or not, or  indeterminate (**any**). To return an object with an indeterminate access  right means that the method transmits the identifier without changing its state  and that the method has no right to alter the access right. This functionality  is particularly useful in the case of collections; temporarily storing an  object in a structure and unable to modify its state. 

With the exception of  the types predefined by the language, all types of parameters and returned  objects, whether manipulated by a handle or by value, *must* appear in the  **uses** clause of the class of which the method is a member. 
As is the case for  constructors, some parameters can have a default value, provided that they are  of primitive or enumerated type. They are passed in the **in** mode, and  they are found at the end of the list of arguments. 

Overloading of instance  methods and use of exceptions and post-conditions is allowed and respects the  same rules than constructors. 

Note the overloading of  “Coord” in the following example of instance methods associated with the  persistent class “Point”: 

~~~~~
Coord (me; X, Y, Z : out Real); 
---Purpose: returns the coordinates of me 

Coord (me; i : Integer) returns Real; 
---Purpose: returns the abscissa (i=1), the 
-- ordinate (i=2) or the value (i=3) of  me 

SetCoord (me : mutable; X, Y, Z : Real); 
---Purpose: modifies the coordinates of me 

Distance (me; P : Point) returns Real 
---Purpose: returns the distance to a point 
~~~~~

In all these cases, **me** is implicitly an object of type *Point*. Only “SetCoord” is able to modify  the internal representation of a point. 

@subsubsection occt_cdl_4_1_3    Class  Methods

A class method is a  function or procedure relative to the class it describes, but does not apply to  a particular instance of the class. 

~~~~~
declaration-of-a-class-method ::= identifier formal-part-of-class-method 
[ declaration-of-returned-type ] 
[ exception-declaration ] 
formal-part-of-class-method ::= ’(’ myclass {’;’ parameter}’)’ 
~~~~~

The first parameter **myclass** indicates that the method does not apply to a previously created instance,  but to the class itself. The rest of the syntax is identical to that of the  instance methods. In particular, access rights (**mutable**, **immutable**,  **any**) and the argument passing mode (**in**, **out**, **in out**)  must remain unchanged. With the exception of the types predefined by the  language, all types of parameters must appear in the **uses** clause of the  class of which the method is a member. Overloading of class methods and the use  of exceptions and post-conditions is allowed, and it follows the same rules as  for constructors and instance methods. 

Examples of class  methods associated with the class “Real”: 

~~~~~
First (myclass) returns Real; 
---Purpose: returns lower limit of reals 

Last (myclass) returns Real; 
---Purpose: returns upper limit of reals 
~~~~~

@subsubsection occt_cdl_4_1_4    Package  Methods

Package methods are  methods which are members of a package. They are frequently used for library or  application initialization, or for offering an application programming  interface to the sources to the package. They are sometimes methods used for  development purposes but which are not made available to final end-users of the  package. 

~~~~~
package-method ::= identifier  [simple-formal-part][returned-type-declaration] 
[exception-declaration] 
[is private]’;’ 
~~~~~

@subsubsection occt_cdl_4_1_5     Sensitivity to Overloading

When there is more than  one method of a class, several methods share the same name but have different  syntax, you say the method is overloaded. 

In order that the  methods can be considered distinct, they must differ either in the number of  parameters, or one of their parameters must be of a different type. In  particular, you *cannot* overload a method if you merely modify it as  follows: 

  * The type of the returned  object when the method behaves as a function
  * The name or the mode of  passing a parameter (**in**, **out**, or **in out**) 
  * The mutability of passed  objects (**mutable**, **immutable**, **any**) 
  * Default value of a parameter.
  
@subsection occt_cdl_4_2   Internal Representation

Each object contains its  own state in a private space in the memory. This state consists of a set of **fields**, which include or reference other objects. 

**Example** 
~~~~~
declaration-of-the-internal-representation-of-a-class ::= fields field {field} 
field ::= identifier {’,’ identifier} ’:’ data-type [’[’integer {’,’integer}’]’]’;’ 
~~~~~

A copy of all the  defined fields exists locally in each instance of the class. This group of  fields will be initialized by the class constructors when the object is  instantiated. 

Fields *must not* have  the same name as any method of the class in which they appear. When the field  type is followed by a list of integer constants between square brackets, the  data will take the form of a multi-dimensional array containing objects of this  type. 

The following example  shows two equivalent ways of describing three fields of the “Real” type: 

**Example** 
~~~~~
fields 
x, y, z: Real; 
coord: Real[3]; 
~~~~~


Depending on their type,  Object fields have one of the two forms. When the field is of the “manipulated  by handle” type, it corresponds to an identifier. In this case, the contents of  the object can be shared by other objects or by a handle in a program. When the  field is of a “manipulated by value” type, it contains the value of the object.  In this case you say the object is **embedded**. 

@subsection occt_cdl_4_3   Exceptions
  
Exceptions describe  exceptional situations, which can arise during the execution of a method. With  the raising of an exception, the normal course of program execution is  interrupted. The actions carried out in response to this situation are called   treatment of exception. 
~~~~~
exception-treatment ::= raises exception-name  {’,’ exception-name} 
~~~~~
Each exception name  corresponds to a class of exceptions previously defined as being susceptible to  being raised by the method under which it appears. Exception classes must all  appear in the **raises** clause of the class of which the method is a member.  The class of exceptions is analogous to the class of objects described in this  manual. 

Take for example the  method which returns the x, y, or z coordinate of a point. 

~~~~~
Coord (me; i : Integer) returns Real 
---Purpose: 
-- Returns the abscissa (i=1) 
-- the ordinate (i=2) 
-- or the value (i=3) 
-- of me. 
raises OutOfRange; 
-- if i is not equal to 1, 2, or 3. 
~~~~~


Instance methods are  likely to raise certain exceptions called **systematic exceptions** which do  not have to appear. They are: 

* *NullObject* - raised when the principal object does not exist. 
* *ImmutableObject* - raised when a method tries to modify an immutable  principal object. 
* *TypeMismatch* - raised if an argument typed by association is of  an unsuitable type. 

These exceptions are described  in the Standard Package (System Toolkits). 


@subsection occt_cdl_4_4   Inheritance

@subsubsection occt_cdl_4_4_1     Overview

The notion of  inheritance comes from a development strategy according to which you begin by  modeling data in the most general fashion. Then you specialize it more and more  so as to correspond to more and more precise cases. 

For example, to develop  a basic geometry, you can first of all consider the group of geometric objects,  and then differentiate the points, vectors, and curves. You can specialize the  latter into conic sections, and then decompose them into circles, ellipses, and  hyperbolas. Then, the class of conics is considered as a sub-class of curves,  and a super-class of circles. 

A sub-class has at least  the behavior of its super-classes. Thus, a circle could be viewed as a conic, a  curve, or even as a geometric object. In each case, the applicable methods  belong to the level where you view the class. In this case, you say that the  sub-class inherits the behavior from its super-classes. 


**Example** 
~~~~~
declaration-of-a-sub-class ::= class class-name 
inherits class-name 
[uses data-type {’,’ data-type}] 
[raises exception-name {’,’ exception-name}] 
is class-definition 
end [class-name]’;’ 
~~~~~

A class cannot inherit  one of its descendent classes; nor can it inherit a native type. All the  classes of a system can be described in a non-cyclic diagram called the **inheritance  graph**. 

The definition of a  sub-class is identical to that of a simple class. Note that a super-class must  not appear in the **uses** clause of the sub-class, even if it appears  in the definition of the sub-class. The behavior of a sub-class includes as a  minimum all  instance methods and protected methods of its super-classes. 

**Note** that constructors and class methods are never  inherited. 

@subsubsection occt_cdl_4_4_2     Redefining methods

Certain inherited  methods can be redefined. 

**Example** 

~~~~~
declaration-of-a-redefined-method ::= identifier formal-part-of-instance-method [returnedtype- declaration] 
[declaration-of-exceptions] 
is redefined [visibility]’;’ 
~~~~~

A redefined method must conform  to the syntax described in the super-class where it appears. The exceptions  contained in the super-class can be renewed, and others may be added as long as  they inherit from an ancestor class. 

The redefined attribute  can be applied neither to a constructor, nor to a class method, since neither  of them can be inherited. If the redefined method is private or protected, the  visibility must be exactly repeated in the redefinition. For further details on  visibility, refer to <a href="#occt_cdl_4_6"> Visibility </a> section. 


**Example** 
~~~~~
SquareDistance (me; P : Point) returns Real 
is redefined private; 
~~~~~

With regards to the  internal representation, all fields defined in the super-classes are, by  default, inherited, but they can also be redefined. 

@subsubsection occt_cdl_4_4_3 Non-redefinable methods

Instance methods, which  are declared virtual are redefinable in descendent classes, and you can force  this redefinition by making a method **deferred**. For more details, see the  next section.
 
**Example** 

~~~~~
declaration-of-a-non-redefinable-method ::= identifier formal-part-of-instance-method [returnedtype- declaration] 
[declaration-of-exceptions] 
 is virtual [visibility]’;’ 
~~~~~

All methods are static  by default. To enable redefinition in all the child classes, add **is virtual** when declaring the method. 

You must also be able to  forbid redefinition. A redefinable method can become non-redefinable if you  declare: **is redefined static**. 


@subsubsection occt_cdl_4_4_4 Deferred Classes and Methods

The presence of certain  classes in the inheritance graph can be justified purely by their ability to  force certain behavior on other classes, in other words, to make other classes  provide various services. 

The CDL language allows  you to describe a class, which introduces methods without implementing them, so  as to force its descendent classes to define them. These are called **deferred  classes**; the non-implemented methods are also termed **deferred methods**. 


**Example** 
~~~~~
declaration-of-a-deferred-class ::= deferred class class-name 
[inherits class-name [uses data-type {’,’ data-type}] 
[raises exception-name {’,’ exception-name}] 
is class-definition 
end [class-name]’;’ 
declaration-of-a-deferred-method ::= identifier formal-part-of-instance-method [returnedtype- declaration] 
[declaration-of-exceptions] 
is deferred [visibility]’;’ 
~~~~~

Only instance methods  can be deferred. 

It is sufficient for a class to contain one deferred  method for it to be a deferred class. It can contain any number of deferred  methods (or none). 

A deferred class may  still have an internal representation but one or more **non-protected** constructors  would be necessary to initialize them. The constructors must be visible in the  sub-classes. 

The constructors of a  deferred class are called **Initialize** (not **Create**). They are **protected** by default, and do not return any object. You cannot create an object of a  deferred class type. 
For example, consider  the class *Point*, and its declaration as deferred. 

**Example** 
~~~~~
deferred class Point inherits Geometry is 
Initialize; 
---Purpose: Initializes the point. 
Coord (me; X, Y, Z : out Real) 
---Purpose: Returns the coordinates 
is deferred; 
SetCoord (me : mutable; X, Y, Z : Real) 
---Purpose: Modifies the coordinates 
is deferred; 
Distance (me; P : Point) returns Real; 
---Purpose: Returns the distance from the point P 
end Point; 
~~~~~

Notice that the function  *Distance* is not deferred. Although this class contains no representation,  this method is programmable by calling *Coord*. 

In a sub-class of a  deferred class, all deferred methods, which have been inherited, must be  implemented, then redeclared (the attribute **redefined** is useless for  this purpose), unless the sub-class is itself deferred. 

A non-deferred method  can be redefined as a deferred one, in which case it will be declared as  follows: **is redefined deferred**. 

The notion of deferred  class is very useful. The advantage of introducing it, as was previously shown  in the deferred class *Point*, is that the corresponding resources will be  available even before being implemented. Later, you can add different  representations to Point (for example, spherical or Cartesian coordinates)  without having to modify client programs. 

Thanks to the  possibility of redefining methods, this approach does not have any negative  impact on performance: a method implemented at the level of a deferred class  can be reprogrammed in one of its sub-classes while taking into account the  data representation. 

@subsubsection occt_cdl_4_4_5     Declaration by Association

At the heart of a class  hierarchy, object identifiers are compatible in the ascendant sense. Since the  *Conic* class is descended from the *Curve* class, an identifier of type *Curve* can  reference an object of type *Conic* (remember that the behavior of *Curve* is  applicable to *Conic*). In other words, you can assign a reference to a *Conic* to an identifier of type *Curve*, but not vice versa. 

For example, once the  classes have been compiled you could write a C++ test program in which you  instantiate a Conic but reference it with a handle to a Curve: 
~~~~~
Handle(Curve) c = new Conic 
~~~~~
This same rule applies  to parameters of methods; that is to say, you can call a method with  identifiers corresponding to a sub-type of that specified in its declaration.  To illustrate this, let us go back to the “Distance” method of the “Point” class: 

~~~~~
Distance (me; P : point) returns Real; 
~~~~~
Conforming to the rule  of type compatibility, you could make a call to the method “Distance” with  reference to an object from a class descended from “Point”. Consequently, if  “SphericPoint” is a sub-class of “Point” and therefore inherits this method, it  will be possible to calculate the distance between two “SphericPoint”, or  between a “SphericPoint” and a “Point”, without having to redefine the method. 

On the other hand,  sometimes you may want to force two parameters to be exactly of the same type,  and thus not apply the rule of type compatibility. To do this, you need to  associate the type of the concerned parameters in the method declaration. 

~~~~~
association-typing ::= like associated-parameter 
associated-parameter ::= me | identifier 
~~~~~

Note that identifier is the name of a parameter, which appears first in the formal part of the declaration of the method.


You can use this  technique, which consists in declaring by association, to declare a method that  will exchange the content of two objects, or a method, which copies another  object: 

~~~~~
Swap (me : mutable; With : mutable like me); 
DeepCopy (me) returns mutable like me; 
~~~~~

Make sure not to  write the Swap method as in the syntax below: 


~~~~~
Swap (me : mutable; With : mutable Point); 
~~~~~

In this case **me** may  be a CartesianPoint or a SphericalPoint, while *With* can only be a Point. 

@subsubsection occt_cdl_4_4_6     Redefinition of Fields

The creation of a  hierarchy of classes should be viewed as a means to specialize their behavior,  (e.g. a circle is more specialized than a conic section). The more you  specialize the object classes, the more it is justified to call into question  the inherited fields in order to obtain greater optimization. So, in the  description of the internal representation of a sub-class, it is possible not  to inherit all of the fields of the super-classes. You then say the fields have  been redefined. 

~~~~~
redefinition-of-the-representation-of-a-class ::= redefined redefinition-of-a-field {’,’ redefinition-of-a- 
field}’,’ 
redefinition-of-a-field ::= [field-name] from [class] class-name 
~~~~~

Redefinition of fields  can only be done in classes manipulated by a handle. 

This declaration appears  at the beginning of the definition of the internal representation of the  sub-class, which breaks the field inheritance. The non-inherited fields are all  those which come from the class specified behind the rubric **from**. 


@subsection occt_cdl_4_5   Genericity

@subsubsection occt_cdl_4_5_1     Overview

Inheritance is a  powerful mechanism for extending a system, but it does not always allow you to  avoid code duplication, particularly in the case where two classes differ only  in the type of objects they manipulate (you certainly encounter this phenomenon  in all basic structures). In such cases, it is convenient to send arbitrary  parameters representing types to a class. Such a class is called a **generic  class**. Its parameters are the generic types of the class. 

Generic classes are  implemented in two steps. You first declare the generic class to establish the  model, and then instantiate this class by giving information about the generic  types. 

@subsubsection occt_cdl_4_5_2     Declaration of a Generic Class

The syntax is as  follows: 

~~~~~
declaration-of-a-generic-class ::= [deferred] generic class class-name  ’(’generic-type {’,’generic-type}’)’ 
[inherits class-name 
[uses data-type {’,’ data-type}] 
[raises exception-name {’,’ exception-name}] 
        is class-definition 
        end [class-name]’;’ 
generic-type ::= identifier as type-constraint 
type-constraint ::= any | class-name [’(’data-type {’,’data-type}’)’] 
~~~~~

The names of generic  types become new types, which are usable in the definition of a class, both in  its behavior (methods) and its representation (fields). The generic type is  only visible inside the generic class introducing it. As a result, it is  possible to have another generic class using the same generic type within the  same package. 

When you specify the  type constraint under the form of a class name, you impose a minimum set of  behavior on the manipulated object.  

This shows that the  generic type has as a minimum the services defined in the class. This can be  any kind of a previously defined class, including another generic class, in  which case you state exactly with what types they are instantiated. 

When the generic type is  constrained by the attribute **any**, the generic class is intended to be  used for any type at all, and thus corresponds to classes whether manipulated  by a handle or by value. 

No class can inherit  from a generic class. 

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. 

Below is a partial  example of a generic class: a persistent singly linked list.
 
~~~~~
generic class SingleList (Item as Storable) 
        inherits Persistent 
        raises NoSuchObject 
        is 
        Create returns mutable SingleList; 
    ---Purpose: Creates an empty list 
        IsEmpty (me) returns  Boolean; 
                ---Purpose: Returns true if the list me is  empty 
        SwapTail (me :  mutable; S : in out mutable 
        SingleList) 
                ---Purpose: Exchanges the tail of list me  with S 
        -- Exception  NoSuchObject raised when me is empty 
        raises NoSuchObject; 
           Value (me) returns Item 
           ---Purpose: Returns first element of the list  me 
        -- Exception NoSuchObject  raised when me is empty 
        raises NoSuchObject; 
           Tail (me) returns mutable SingleList 
        ---Purpose: Returns  the tail of the list me 
        -- Exception  NoSuchObject raised when me is empty 
        raises NoSuchObject; 
           fields 
                Data : Item; 
           Next : SingleList; 
           end SingleList; 
~~~~~      

Even though no object of  the type “SingleList” IS created, the class contains a constructor. This class  constitutes a model, which will be recopied at instantiation time to create a  new class which will generate objects. The constructor will then be required. 

**Example** 
~~~~~
generic class Sequence(Item as any, Node as 
SingleList(Item)) 
inherits Object 
. . . 
end Sequence 
~~~~~

In the above example,  there are two generic types: *Item* and *Node*. The first imposes no restriction.  The second must at least have available the services of the class *SingleList* instantiated with the type with which *Sequence* will itself be instantiated. 

In the incomplete  declaration of a generic class, the keyword **generic** must appear. 

**Example** 
~~~~~
generic class SingleList; 
generic class Sequence; 
~~~~~

@subsubsection occt_cdl_4_5_3     Instantiation of a Generic Class

The syntax is as  follows: 

~~~~~
instantiation-of-a-generic-class ::= [deferred] class  class-name 
     instantiates class-name ’(’data-type {’,’ data-type}’);’ 
~~~~~

Instantiation is said to  be **static**. In other words, it must take place before any use can be made  of the type of the instantiated class. Each data type is associated term by  term with those declared at the definition of the generic class. These latter  ones, when they are not of the type **any**, restrict instantiation to those  classes, which have a behavior at least equal to that of the class specified in  the type constraint, including constructors. Note that this is not guaranteed  by inheritance itself. 

For example, let’s  instantiate the class *Sequence* for the type *Point*: 

~~~~~
class SingleListOfPoint instantiates SingleList(Point); 
class Sequence instantiates 
        Sequence(Point,SingleListOfPoint); 
~~~~~

The instantiation of a  generic deferred class is a deferred class (the **deferred** attribute must  be present during instantiation). An instantiated class cannot be declared in  an incomplete fashion. 

@subsubsection occt_cdl_4_5_4    Nested  Generic Classes

It often happens that  many classes are linked by a common generic type. This is the case when a base  structure provides an iterator, for example, in the class *Graph*. A graph is  made up of arcs, which join together the nodes, which reference objects of any  type. This type is generic both for the graph and for the node. 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. So as to group the  instantiation, CDL allows the declaration of certain classes to be nested. 

**Example** 

~~~~~
declaration-of-a-generic-class ::=  [deferred] generic class class-name  ’(’generic-type{’,’generic-type}’)’ 
   [inherits class-name {’,’ class-name}] 
   [uses data-type {’,’ data-type}] 
   [raises exception-name {’,’ exception-name}] 
   [{[visibility] class-declaration}] 
   is class-definition 
end [class-name]’;’ 
   class-declaration ::= incomplete-declaration-of-a-class | declaration-of-a-non-generic-class | instantiation-of-a-generic-class 
~~~~~

**Nested classes**, even though they are described as non-generic  classes, are generic by construction, being inside the class of which they are  a part. As a consequence, the generic types introduced by the **encompassing  class** can be used in the definition of the nested class. This is true even  if the generic type is only used in a nested class. The generic types still must appear as an argument of the encompassing class. All other types used by a  nested class must appear in its **uses** or **raises** clauses,  just as if it were an independent class. 

Nested classes are, by  default, **public**. In other words, they can be used by the clients of the  encompassing class. On the other hand, when one of the nested classes is  declared **private** or **protected**, this class must not appear  in any of the public methods of the other classes. It cannot be used in a  protected field because then it could be used in a sub-class, which implies it  would not be private. 

The following example  shows how to write the Set class with its iterator. 

~~~~~
generic class Set (Item as Storable) 
        inherits Persistent 
        private class Node  instantiates SingleList (Item); 
        class Iterator 
                   uses Set, Node 
                   raises  NoSuchObject, NoMoreObject 
                   is 
                   Create (S : Set)  returns mutable Iterator; 
                ---Purpose: Creates  an iterator on the group S 
                   More (me) returns  Boolean; 
                ---Purpose: Returns  true if there are still elements 
                   -- to explore 
                   Next (me) raises  NoMoreObject; 
                ---Purpose: Passes  to the following element 
                   Value (me)  returns any Item raises NoSuchObject; 
                ---Purpose: Returns  the current element 
                   fields 
                   Current : Node; 
                end Iterator; 
                is 
                   Create returns  mutable Set; 
                ---Purpose: Creates  an empty group 
                   IsEmpty (me)  returns Boolean; 
                ---Purpose: Returns  true if the group is empty 
                   Add (me :  mutable; T : Item); 
                ---Purpose: Adds an  item to the group me 
                   Remove (me :  mutable; T : item) raises 
                NoSuchObject; 
                ---Purpose: Removes  an item from the group me 
                   etc. 
                   fields 
                   Head : Node; 
        end Set; 
~~~~~

Note that in their  fields, both “Set” and “Iterator” are clients of another class, “Node”. This  last can be effectively declared **private** for it only appears in fields  which are themselves private. 

The instantiation of a  generic class containing nested classes remains unchanged. The same declaration  is used to instantiate the encompassing class and the nested classes. These  latter will have their name suffixed by the name supplied at instantiation,  separated by “Of”. For example, you instantiate the class “Set” described above  for the type “Point” as follows: 

~~~~~
class SetOfPoint instantiates Set(Point); 
~~~~~
In doing so, you  implicitly describe the classes “NodeOfSetOfPoint” and “IteratorOfSetOfPoint”,  which are respectively the result of the concatenation of “Node” and “Iterator”  with “Of” then “SetOfPoint”. 

Note that in the  incomplete declaration of an encompassing class, all the names of the nested  classes *must* appear behind that of the encompassing class. 

~~~~~
incomplete-declaration-of-a-generic-class ::= [deferred] generic class-name {’,’  class-name}; 
~~~~~

For example, an incomplete declaration of the above  class “Set” would be as in the example below: 

~~~~~
generic class Set, Node, Iterator; 
~~~~~

Only the encompassing  class can be deferred. In the above example only the class “Set” can be  deferred. 



@subsection occt_cdl_4_6   Visibility

@subsubsection occt_cdl_4_6_1     Overview

A field, method, class,  or package method is only available for use if it is **visible**. 
Each of these components  has a default visibility, which can be explicitly modified during class or  package declaration. The three possible states of visibility are: 
  * Public
  * Private
  * Protected

@subsubsection occt_cdl_4_6_2     Visibility of Fields

A field is **private**.  It can never be public - this would destroy the whole concept of data  encapsulation. The attribute **private** is redundant when it is applied to  a field. This means that a field is only visible to methods within its own  class. 
A field can be declared **protected**, which means that it becomes visible in subclasses of its own class. Its  contents can be modified by methods in subclasses. 

~~~~~
field ::= identifier {’,’ identifier} ’:’ data-type 
[’[’integer{’,’integer}’]’] 
[is protected]’;’ 
~~~~~

**Example** 

~~~~~
fields 
   Phi, Delta, Gamma : AngularMomenta [3] 
   is protected ; 
~~~~~

@subsubsection occt_cdl_4_6_3     Visibility of Methods

Methods act on fields.  Only methods belonging to a class can act on the fields of the class; this  stems from the principle of object encapsulation. Methods can be characterized  in three ways: by default, methods are **public**. Methods can be declared **private** or **protected** to restrict their usage. 

* **Public** methods are the default and generally the most common. They describe the behavior of a class or a package, and they are  callable by any part of a program. 
* **Private** methods  exist only for the internal structuring of their  class or their package. Private class methods can only be called by methods  belonging to the same class. Private package methods can only be called by all  methods belonging to the same package and its classes. 
* **Protected**  methods are private methods, which are also callable from  the interior of descendent classes.  

If you want to restrict  the usage of a method, you associate with it a visibility as follows : 
~~~~~
-- declaration-of-the-visibility ::= is visibility 
visibility ::= private | protected 
~~~~~

The declaration of the  visibility of a method appears at the end of its definition, before the final  semi-colon. The attribute **private** indicates that the method will only be  visible to the behavior of the class of which the method is a member; **protected** will propagate the visibility among the sub-classes of this class. 

For example, add to the  class “Line” an internal method allowing the calculation of the perpendicular  distance to the power of two, from the line to a point. 

~~~~~
SquareDistance (me; P : Point) returns Real 
is private;
~~~~~

@subsubsection occt_cdl_4_6_4     Visibility of Classes, Exceptions and Enumerations

The visibility of a  class is the facility to be able to use this class in the definition of another  class. The visibility of a class extends from the beginning of its declaration  up to the end of the package in which it appears. You have seen that the  keyword **uses** allows extension of this visibility to other packages. 

As was explained in the  section on “<a href="#occt_cdl_3_3_2">Name Space</a>”, any ambiguity, which arises from having two classes  with the same name coming from different packages, is dealt with by the use of  the keyword **from**. 

A class declared **private** is only available within its own package. 

@subsubsection occt_cdl_4_6_5    Friend  Classes and Methods

In certain cases,  methods need to have direct access to the private or protected parts of classes  of which they are clients. Such a method is called a **friend** of the  class, which is accessed. For example, you declare a method to be a friend when  a service can only be obtained via the use of another non-descendent class, or  perhaps when this will help to improve performance. 

Classes can also be  declared friends of other classes. In this case all the methods of the friend  class will have access to the fields and methods of the host class. The right  is **not reciprocal**. 

Friend classes or  methods are declared inside the class, which reveals its private and protected  data or methods to them. This helps in managing the continuing evolution of a  class, helping to recognize and to avoid the creation of side effects. 

**Example** 
~~~~~
declaration-of-friends ::= friends friend {’,’friend} 
   friend ::=    identifier from [class] class-name  [formal-part] | 
-- Defining the Software Components 67 
identifier from [package] package-name  [formal-part] | class] class-name 
   formal-part ::= simple-formal-part | formal-part-of-instance-method | formal-part-of-class-method 
~~~~~

The formal part must be present if the method contains one; thus this can be overloaded without  necessarily propagating the friend relationship among its homonyms. The keyword  **class** allows you to avoid certain ambiguities. For example, it removes  any confusion between “method M from class C” and “method M from package P”. 

As an example, take a  method, which calculates the perpendicular distance between a line and a point.  Suppose this method needs to access the fields of the point. In the class  “Point” you would write: 

~~~~~
friends Distance from Line (me; P : Point) 
~~~~~

A method can be a friend  to many classes. The class to which the method belongs does not need to  appear in the **uses** clause of other classes of which it is a friend. 

When the methods of a class are all friends  of another class, you can establish the friendship at the level of the class. 


| | Public | Private | Protected |
| :---- | :---- | :---- | :----- |
| Field | Does not exist | **Default** - Visible to methods in its own class and in friend classes | Visible to methods in its own class, sub-classes and friend classes |
| Method | **Default** - Callable anywhere | Callable by methods in its own class and in friend classes | Callable by methods in its own class, sub-classes and friend classes | 
| Class | **Default**  - Visible everywhere with the use of **from** rubric | Visible to classes in its own package | Does not exist | 
| Package method | **Default** - Callable everywhere with the use of **from** rubric | Visible to classes in its own package | Does not exist | 
| Nested Class | **Default** -  Visible to the clients of the encompassing class | Visible to the encompassing class and other classes nested in the encompassing class | Does not exist | 


@section occt_cdl_5 Appendix A. Syntax  Summary


This summary of the CDL  syntax will aid in the comprehension of the language, but does *not* constitute  an exact definition thereof. In particular, the grammar described here accepts  a super-set of CDL constructors semantically validated. 

(1) capital ::= ’A’ | ’B’ | ’C’ | ’D’ | ’E’ | ’F’ | ’G’ | ’H’ | ’I’ | ’J’ | ’K’ | ’L’ | ’M’ | ’N’ | 
’O’ | ’P’ | ’Q’ | ’R’ | ’S’ | ’T’ | ’U’ | ’V’ | ’W’ | ’X’ | ’Y’ | ’Z’ 

(2) non-capital ::= ’a’ | ’b’ | ’c’ | ’d’ | ’e’ | ’f’ | ’g’ | ’h’ | ’i’ | ’j’ | ’k’ | ’l’ | ’m’ | ’n’ | 
’o’ | ’p’ | ’q’ | ’r’ | ’s’ | ’t’ | ’u’ | ’v’ | ’w’ | ’x’ | ’y’ | ’z’ 

(3) digit ::= ’0’ | ’1’ | ’2’ | ’3’ | ’4’ | ’5’ | ’6’ | ’7’ | ’8’ | ’9’ 

(4) underscore ::= ’_’ 

(5) special character  ::= ’ ’ | ’!’ | ’”’ | ’#’ | ’$’ | ’%’ | ’&amp;’ | ’’’ | ’(’ | ’)’ | ’*’ | ’+’ | ’,’ | ’-’ | ’.’ | ’/’ | ’:’ | ’;’ | ’’ | ’=’ | ’’ | ’?’ | ’@’ | ’[’ | ’\’ | ’]’ | ’^’ | ’‘’ | ’{’ | ’|’ | ’}’ | ’~’ 

(6) printable  character::= capitals | non-capitals | digits | underscore | special characters 

(7) letter ::= capital |  non-capital 

(8) alphanumeric ::= letter | digit 

(9) identifier ::= letter{[underscore]alphanumeric} 

(10) integer ::= digit{digit} 

(11) exponent ::= ’E’[’+’]integer |  ’E-’integer 

(12) numeric-constant  ::= [’+’]integer ’.’ integer[exponent] | ’-’integer ’.’ integer[exponent] 


(13) literal-constant  ::= ’’’printable character’’’ | ’~’{printable 
character}’~’ 

(14) package-name ::= identifier 

(15) enumeration-name  ::= identifier [**from** package-name] 

(16) class-name ::= identifier [**from** package-name] 

(17) exception-name ::= identifier [**from** package-name] 

(18) constructor-name  ::= ’Create’ |  ’Initialize’ 

(19) primitive-type ::= ’Boolean’ |  ’Character’ | ’Integer’ | ’Real’ 

(20) data-type ::= enumeration-name | class-name | exception-name | primitive-type 

(21) passed-type ::= data-type | **like me** | **like** identifier 

(22) passing-mode ::= [**in**] | **out** | **in out** 

(23) parameter-access ::= **mutable** | [**immutable**] 

(23A) return-access ::= **mutable** | [**immutable**]| **any** 

(24) value ::= numeric-constant | literal-constant | identifier 

(25) parameter ::= identifier {’,’ identifier} ’:’ passing-mode access-right passed-type [’=’ value] 

(26) simple-formal-part  ::= ’(’parameter {’;’  parameter}’)’ 

(27)  formal-part-of-instance-method ::= ’(’ **me** [’:’ passing-mode access-right] {’;’ parameter}’)’ 

(28)  formal-part-of-class-method ::= ’(’ **myclass** {’;’ parameter}’)’ 

(29) visibility ::= **private** | **protected** 

(30) redefinition ::= **static** | **deferred** 

(31) definition-level ::= redefinition | **redefined** [redefinition] 

(32)  declaration-of-constructed-type ::= **returns** [**mutable**] class-name 

(33)  declaration-of-returned-type ::= **returns** return-access  passed-type 

(34)  declaration-of-errors ::= **raises** exception-name {’,’  exception-name} 

(35)  declaration-of-visibility ::= **is** visibility 

(36)  declaration-of-attributes-of-instance-method ::= **is** visibility | **is** definition-of-level [visibility] 

(37) constructor ::= constructor-name [simple-formal-part] 
[declaration-of-constructed-type] 
[declaration-of-errors] 
[declaration-of-visibility]’;’ 

(38) instance-method ::= identifier formal-part-of-instance-method 
[declaration of returned type] 
[declaration-of-errors] 
[declaration-of-attributes-of-instancemethod]’;’ 

(39) class-method ::= identifier formal-part-of-the-class-method 
[declaration of returned type] 
[declaration-of-errors] 
[declaration-of-visibility]’;’ 

(40) package-method ::= identifier [simple-formal-part] 
[declaration-of-returned-type] 
[declaration-of-errors] 
[**is private**]’;’ 

(41) member-method ::= constructor |  instance-method | class-method 

(42) formal-part ::= simple-formal-part | formal-part-of-instance-method| formal-part-of-class-method 

(43) friend ::= identifier **from** [**class**] class-name  [formal-part] 
| identifier **from** [**package**] package-name [formal-part] | 
[**class**] class-name 

(44) field ::= identifier {’,’ identifier} ’:’ data-type 
[’[’integer {’,’ integer}’]’] 
[**is protected**]’;’ 

45)  redefinition-of-field ::= [field-name] **from** [**class**] class-name 

(46)  declaration-of-fields ::= **fields** [**redefined** redefinition-of-field  {’,’ redefinition-of-field}’;’] 
field {field} 

(47)  declaration-of-an-alias::= [**private**] **alias** class-name1 **is** class-name2  [**from** package-name] 

(48)  declaration-of-friends ::= **friends** friend {’,’ friend} 

(49) class-definition  ::= [{member-method}] 
[declaration-of-fields] 
[declaration-of-friends] 

(50)  declaration-of-an-exception ::= **exception** exception-name **inherits** exception-name 

(51) declaration-of-an-enumeration  ::= **enumeration** enumeration-name 
**is** identifier {’,’  identifier} 
[**end** [enumeration-name]]’;’ 

(52)  incomplete-declaration-of-a-non-generic-class ::= 
[**deferred**] **class** class-name’;’ 

(53)  incomplete-declaration-of-a-generic-class ::= 
[**deferred**] **generic class** class-name {’,’ class-name}’;’ 

(54)  declaration-of-a-non-generic-class ::= 
[**deferred**] **class** class-name 
[**inherits** class-name 
[**uses** data-type {’,’ data-type}] 
[**raises** exception-name {’,’ exception-name}] 
**is** definition-of-a-class 
**end** [class-name]’;’ 

(55) type-constraint ::= **any** | class-name  [’(’data-type {’,’ data-type}’)’] 

(56) generic-type ::= identifier **as** type-constraint 

(57) declaration-of-a-generic-class ::=
[**deferred**] **generic class** class-name ’(’generic-type
{’,’ generic-type}’)’
[**inherits** class-name
[**uses** data-type {’,’ data-type}]
[**raises** exception-name {’,’ exception-name}]
[{[visibility] declaration-of-a-class}]
**is** class-definition
**end** [class-name]’;’

(58) instantiation-of-a-generic-class::=
[**deferred**] **class** class-name
**instantiates** class-name ’(’data-type
{’,’ data-type}’);’

(59) declaration-of-a-class::=
incomplete-declaration-of-a-non-generic-class
|
incomplete-declaration-of-a-generic-class |
declaration-of-a-non-generic-class |
declaration-of-a-generic-class |
instantiation-of-a-generic-class

(60) type-declaration ::=
[private] declaration-of-an-enumeration | [**private**] class-declaration | declaration-of-an-exception

(61) package-definition ::=
[{type-declaration}]
[{package-method}]

(62) package-declaration ::= **package** package-name
[**uses** package-name {’,’ package-name}]
  **is** package-definition
**end** [package-name]’;’

(63) executable-declaration ::=
             **executable** executable-name
                            **is**
            {
             **executable** executable-part
[**uses** [identifier **as external**]
     [{’,’ identifier **as external**}]
     [unit-name **as library**]
     [{’,’ unit-name **as library**}]
                            **is**
                     {file-name [as C++|c|fortran|object];}
                               **end** ’;’
                 }
                **end** ’;’

(64) schema-declaration ::=
 **schema** schema-name
  **is**
[{**package** package-name ’;’ }]
[{**class** class-name ’;’ }]
**end** ’;’





@section occt_cdl_6 Appendix B Comparison of CDL and C++ 

## Syntax for Data Types manipulated by Handle and by Value in CDL

|  | Handle | Value |
| :---- | :---- | :---- | 
| Permanent | Persistent | Storable |
| Temporary | Transient | Any |
| Reading | Immutable | In |
| Writing | Mutable | Out |
| Read/Write | Mutable | In out | 
| Return | Not specified : any | Without copy: --C++ return const& |

## Syntax for Data Types manipulated by Handle and by Value in C++

| | Handle | Value |
| :---- | :---- | :--- |
| C++ Declaration | Handle(PGeom_Point) p1; | gp_Pnt p2; |
| C++ Constructor | p1 = newPGeom_Point(p2); | p2(0.,0.,0.); |
| C++ Method | x=p1 -> XCoord(); | x=p2.XCoord(); |