### 15.4.5 Object-Oriented Programming

Here are the commands for formatting descriptions about abstract objects, such as are used in object-oriented programming. A class is a defined type of abstract object. An instance of a class is a particular object that has the type of the class. An instance variable is a variable that belongs to the class but for which each instance has its own value.

In a definition, if the name of a class is truly a name defined in the programming system for a class, then you should write an `@code` around it. Otherwise, it is printed in the usual text font.

`@defcv category class name`

  The `@defcv` command is the general definition command for variables associated with classes in object-oriented programming. The `@defcv` command is followed by three arguments: the category of thing being defined, the class to which it belongs, and its name. Thus,

      @defcv {Class Option} Window border-pattern
      …
      @end defcv

  illustrates how you would write the first line of a definition of the `border-pattern` class option of the class `Window`.

  The template is:

      @defcv category class name
      …
      @end defcv

  `@defcv` creates an entry in the index of variables.

`@defivar class name`

  The `@defivar` command is the definition command for instance variables in object-oriented programming. `@defivar` is equivalent to ‘`@defcv {Instance Variable} …`’

  The template is:

      @defivar class instance-variable-name
      body-of-definition
      @end defivar

  `@defivar` creates an entry in the index of variables.

`@deftypeivar class data-type name`

  The `@deftypeivar` command is the definition command for typed instance variables in object-oriented programming. It is similar to `@defivar` with the addition of the data-type parameter to specify the type of the instance variable. `@deftypeivar` creates an entry in the index of variables.

`@defop category class name arguments…`

  The `@defop` command is the general definition command for entities that may resemble methods in object-oriented programming. These entities take arguments, as functions do, but are associated with particular classes of objects.

  For example, some systems have constructs called wrappers that are associated with classes as methods are, but that act more like macros than like functions. You could use `@defop Wrapper` to describe one of these.

  Sometimes it is useful to distinguish methods and operations. You can think of an operation as the specification for a method. Thus, a window system might specify that all window classes have a method named `expose`; we would say that this window system defines an `expose` operation on windows in general. Typically, the operation has a name and also specifies the pattern of arguments; all methods that implement the operation must accept the same arguments, since applications that use the operation do so without knowing which method will implement it.

  Often it makes more sense to document operations than methods. For example, window application developers need to know about the `expose` operation, but need not be concerned with whether a given class of windows has its own method to implement this operation. To describe this operation, you would write:

      @defop Operation windows expose

  The `@defop` command is written at the beginning of a line and is followed on the same line by the overall name of the category of operation, the name of the class of the operation, the name of the operation, and its arguments, if any.

  The template is:

      @defop category class name arguments…
      body-of-definition
      @end defop

  `@defop` creates an entry, such as ‘`expose` on `windows`’, in the index of functions.

`@deftypeop category class data-type name arguments…`

  The `@deftypeop` command is the definition command for typed operations in object-oriented programming. It is similar to `@defop` with the addition of the data-type parameter to specify the return type of the method. `@deftypeop` creates an entry in the index of functions.

`@defmethod class name arguments…`

  The `@defmethod` command is the definition command for methods in object-oriented programming. A method is a kind of function that implements an operation for a particular class of objects and its subclasses.

  `@defmethod` is equivalent to ‘`@defop Method …`’. The command is written at the beginning of a line and is followed by the name of the class of the method, the name of the method, and its arguments, if any.

  For example:

      @defmethod bar-class bar-method argument
      …
      @end defmethod

  illustrates the definition for a method called `bar-method` of the class `bar-class`. The method takes an argument.

  The template is:

      @defmethod class method-name arguments…
      body-of-definition
      @end defmethod

  `@defmethod` creates an entry, such as ‘`bar-method` on `bar-class`’, in the index of functions.

`@deftypemethod class data-type name arguments…`

  The `@deftypemethod` command is the definition command for methods in object-oriented typed languages, such as C++ and Java. It is similar to the `@defmethod` command with the addition of the data-type parameter to specify the return type of the method.