Passing child reference to variable mother reference not possible ?

// conceptual unit code:
type
TBase = class // mother
public
constructor Create; virtual;
end;

type
TBaseClass = class of Tbase;

var
mClassType : TBaseClass;

procedure TestCreate( out ParaObject : TBase );
begin
ParaObject := mClassType.Create;
end;

// conceptual user code:

type
TChild = class(TBase) // child
public
constructor Create; override;
end;

procedure Test;
var
vObject : TChild;
begin
// trying to pass child reference to variable mother reference
TestCreate( vObject );
end;

TestCreate does not compile in Delphi 2007 with error message:

"
[Pascal Error] [Unit].pas(95): E2033 Types of actual and formal var
parameters must be identical"

The strange thing i: if TestCreate would be changed to:

procedure TestCreate( out ParaObject : TObject );

then I am pretty sure that it would compile fine...

So the question is:

"Why does Delphi allow (var Sender : Tobject) parameters but not (var Sender
: TSomethingElse) ???"

Is it a shortcoming in the Delphi 2007 compiler/language ?!?

It seems so...

Bye,
Skybuck.

Hmm I was wrong about it being allowed for (var Sender : TObject)
parameters... it is only allowed for (Sender : TObject) parameters...

This is unfortunate because I was hoping to get rid of these nasty and
boring typecasts which are now needed:

program TestProgram;

{$APPTYPE CONSOLE}

{

Test passing child reference to variable mother reference

version 0.01 created on 7 october 2010 by Skybuck Flying


}


uses
SysUtils;

type
TMother = class
public
mData : string;
end;

TChild = class(TMother)
public
mData2 : string;
end;

procedure TestCreate1( var Para : TObject );
begin
Para := TChild.Create;
end;

procedure TestCreate2( var Para : TMother );
begin
Para := TChild.Create;
end;

procedure Main;
var
vChild1 : TChild;
vChild2 : TChild;
begin
// TestCreate1( vChild1 ); // not allowed
// TestCreate2( vChild2 ); // not allowed

// these typecasts bore me to death.
TestCreate1( TObject(vChild1) );
TestCreate2( TMother(vChild2) );

if vChild1 is TChild then
begin
writeln('ok1');
end;

if vChild2 is TChild then
begin
writeln('ok2');
end;
end;

begin
try
Main;
except
on E:Exception do
Writeln(E.Classname, ': ', E.Message);
end;
ReadLn;
end.

Bye,
Skybuck.