5.1 Declaration

Free Pascal supports object oriented programming. In fact, most of the compiler is written using objects. Here we present some technical questions regarding object oriented programming in Free Pascal. Objects should be treated as a special kind of record. The record contains all the fields that are declared in the objects definition, and pointers to the methods that are associated to the objects’ type.

An object is declared just as a record would be declared; except that now, procedures and functions can be declared as if they were part of the record. Objects can ”inherit” fields and methods from ”parent” objects. This means that these fields and methods can be used as if they were included in the objects declared as a ”child” object.

Furthermore, a concept of visibility is introduced: fields, procedures and functions can be declared as public or private. By default, fields and methods are public, and are exported outside the current unit. Fields or methods that are declared private are only accessible in the current unit: their scope is limited to the implementation of the current unit.

The prototype declaration of an object is as follows:

_________________________________________________________________________________________________________ object types
------------object-----------------------------------end--------
  -packed -|        heritage| -|----component list------|
                             - object visibility specifier-

--       - -                 -  --------------------------------
  heritage  (  object type identifier )

--            --------------------------------------------------
  component list ---           --| --              ---|
                 -field definition-|   --method-definition-|

--field definition- identifier list- :- type- ;------------------------------

--method definition-|--function header----;-method directives------------
                 |-procedure header--|
                 -constructor header|
                   desctuctor header

--method directives-|--------------------------|--------------------
                 - virtual- ;--|------------| -call modifiers ;--
                 |           -abstract -;-- |
                 ------ reintroduce- ;------

--object visibility specifier-|-private-----------------------------------
                      |protected--|
                      --public ---
___________________________________________________________________

As can be seen, as many private and public blocks as needed can be declared.

Method definitions are normal function or procedure declarations. Contrary to TP and Delphi, fields can be declared after methods in the same block, i.e. the following will generate an error when compiling with Delphi or Turbo Pascal, but not with FPC:

Type  
  MyObj = Object  
    Procedure Doit;  
    Field : Longint;  
  end;

Remark: Free Pascal also supports the packed object. This is the same as an object, only the elements (fields) of the object are byte-aligned, just as in the packed record. The declaration of a packed object is similar to the declaration of a packed record :

Type  
  TObj = packed object;  
   Constructor init;  
   ...  
   end;  
  Pobj = ˆTObj;  
Var PP : Pobj;

Similarly, the {$PackRecords } directive acts on objects as well.