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:
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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. 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.