Is it possible ?

In details:

Suppose:

program Test;
type
  TClassA = class;
  TClassB = class(TClassA);
  TClassC = class(TClassA);
  procedure P( x: TClassA);
  begin
    ...
  end;
begin
  P(TClassB.Create)
end.

If I watch x inside P in the IDE, i see it is of type TClassA.

But I want to know it's runtime type, that is if it is TClassB or TClassC.

Olle

Do you want to see it in the debugger? Then you need to try to examine tobject properties or methods (like Anton said).

If you want to know it in a program, use the IS operator.

program Test;
type
  TClassA = class
	    end;
  TClassB = class(TClassA);
  TClassC = class(TClassA);
  procedure P( x: TClassA);
  begin
    if x IS TClassB then
     writeln('TClassB');
    if x IS TClassA then
     writeln('TClassA');
    if x IS TClassC then
     writeln('TClassC');
  end;
begin
  P(TClassB.Create)
end.

will print class B AND A (because it is also a TClassA)

Yes, because "is" also returns true if the instance is of a type that inherits from tclassb. So the "is" operator is translated into a call to the following helper routine

     function fpc_do_is(aclass : tclass;aobject : tobject) :
boolean;[public,alia
s: 'FPC_DO_IS']; {$ifdef hascompilerproc} compilerproc; {$endif}
       begin
          fpc_do_is:=assigned(aobject) and assigned(aclass) and
            aobject.inheritsfrom(aclass);
       end;

Jonas

Hello, Olle!

One possibility is:

type
  TClassA = class(TObject);

Your classes then inherit all methods of TObject, including TObject.ClassType and TObject.InheritsFrom. So it's possible to write things like:

procedure P(x: TClassA);
begin
  if x.ClassType = TClassA then begin
    bla bla
  end else if x.ClassType = TClassB then begin
    bla bla
  end;
  if x.InheritsFrom(TClassC) then begin
    // true for TClassC and descendants
    bla bla
  end;
end;

hth,

Anton.

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"Adas Methode war, wie sich zeigen wird, Tagträume in offenbar korrekte Berechnungen einzuweben."

Doris Langley Moore: Ada, Countess of Lovelace (London 1977).

----------

Anton Tichawa
Volkertstrasse 19 / 20
A-1020 Wien
mobil: +43 664 52 07 907
email:

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Check the function
x.classname

It returns the class name.

Michael.

Hello, Marco!

if x IS TClassB then

What object code is generated for this? I assume that, informally speaking,

Ord(TClassB) = Ord(@VMT)

and that

if x IS TClassB

yields somthing like

if @(x.VMT) = TClassB?

Am I wrong here?

Thanks,

Anton.

----------

"Adas Methode war, wie sich zeigen wird, Tagträume in offenbar korrekte Berechnungen einzuweben."

Doris Langley Moore: Ada, Countess of Lovelace (London 1977).

----------

Anton Tichawa
Volkertstrasse 19 / 20
A-1020 Wien
mobil: +43 664 52 07 907
email:

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