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 delcared 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. The prototype declaration of an object is as follows:
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object types
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As can be seen, as many private and public blocks as needed can be declared. Method definitions are normal function or procedure declarations. Fields cannot be declared after methods in the same block, i.e. the following will generate an error when compiling:
Type MyObj = Object Procedure Doit; Field : Longint; end; |
Type MyObj = Object Public Procedure Doit; Private 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 = ^TObj; Var PP : Pobj; |