[fpc-pascal] how to simulate abstract properties

[Graeme Geldenhuys]

Marc Santhoff wrote:
> 
> the compiler runs and throws no error. Fine, but this way every class

Because you did not instantiate any instances of that class. Here is the 
output after compiling my sample program with FPC 2.2.5

===============================
$ fpc test1.pas
Free Pascal Compiler version 2.2.5 [2009/05/28] for i386
Copyright (c) 1993-2008 by Florian Klaempfl
Target OS: Linux for i386
Compiling test1.pas
test1.pas(31,24) Warning: Constructing a class "TDerived2" with abstract 
methods
Linking test1
35 lines compiled, 0.1 sec
1 warning(s) issued
===============================

It clearly shows the error, which is correct. Just declaring a class 
with abstract methods doesn't mean the compiler must throw an error. 
TDerived2 might have another class deriving from it, which adds the getA 
implementation.

As far as I can see, the compiler is doing everything correctly.

===========[ complete sample application ]==================
program test1;

{$mode objfpc}{$H+}

type
    TBase = class(TObject)
    private
      function getA: integer; virtual; abstract;
    public
      property A:integer read getA;
    end;

    TDerived = class(TBase)
    private
      function getA: integer; override;
    end;

    TDerived2 = class(TBase)
    end;

function TDerived.getA: integer;
begin
   result := 99;
end;

var
   a: TDerived;
   b: TDerived2;
begin
   a := TDerived.Create;
   b := TDerived2.Create;

   a.Free;
   b.Free;
end.
==================[ end ]=======================


Regards,
   - Graeme -

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