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May 12th, 2005

How to call constructor for descendant of TObject ?

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At runtime I obtain an instance of descendant of TObject (for example its name is TGoodForm).
The definition of TGoodForm is unknown at compile time (I know that it is TGoodForm from TObject::ClassName()).
The question: how to create new instance of TGoodForm ?
In another words, can I get address of constructor through reference to existing object at runtime?
(Assembler is permitted)

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admdvv

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May 12th, 2005

Re: How to call constructor for descendant of TObject ?

Quote originally posted by admdvv ...

At runtime I obtain an instance of descendant of TObject (for example its name is TGoodForm).
The definition of TGoodForm is unknown at compile time (I know that it is TGoodForm from TObject::ClassName()).
The question: how to create new instance of TGoodForm ?
In another words, can I get address of constructor through reference to existing object at runtime?
(Assembler is permitted)

Can you post code by chance?

subtronic

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May 12th, 2005

Re: How to call constructor for descendant of TObject ?

C++ Syntax (Toggle Plain Text)
for(int i=0, n=form->ComponentCount; i<n; ++i){
    TComponent* comp=form->Components[i];
    if(comp->ClassInfo()=="TGoodForm"){
        // here I want to create new instance of the same class
    }
}
for(int i=0, n=form->ComponentCount; i<n; ++i){
    TComponent* comp=form->Components[i];
    if(comp->ClassInfo()=="TGoodForm"){
        // here I want to create new instance of the same class
    }
}

<< moderator edit: added [code][/code] tags >>

admdvv

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Re: How to call constructor for descendant of TObject ?

Having address of constructor, I'll write:

C++ Syntax (Toggle Plain Text)
typedef TObject* __fastcall (*Ctor)(TComponent* owner);
TObject* newObject=(*(Ctor)address)(form);
typedef TObject* __fastcall (*Ctor)(TComponent* owner);
TObject* newObject=(*(Ctor)address)(form);

<< moderator edit: added [code][/code] tags >>

admdvv

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Re: How to call constructor for descendant of TObject ?

Maybe I'm not understanding you correctly..

Generated: Thu May 12 16:17:59 2005

(Toggle Plain Text)

[001]     for (int i = 0, n = form->ComponentCount; i < n; ++i) {
[002]             TComponent *comp = form->Components[i];
[003]             if (comp->ClassInfo() == "TGoodForm") {
[004]                     TGoodForm *tf = new TGoodForm();        // ????
[005]             }
[006]     }

subtronic

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May 14th, 2005

Re: How to call constructor for descendant of TObject ?

But I have no declaration of class TGoodForm (I get instance from runtime library), and compiler doesn't know about TGoodForm::TGoodForm.
But I found the solution:

The solution is to create delphi function which calls virtual constructor

// delphi unit ComponentCreator.pas

unit ComponentCreator;
interface
uses Classes;
function ComponentCreate(cls: TComponentClass; Owner: TComponent): TComponent;
implementation
function ComponentCreate(cls: TComponentClass; Owner: TComponent): TComponent;
begin
ComponentCreate:=cls.Create(Owner);
end;
end.

// C++ unit

#include "ComponentCreator.hpp"

TClass newClass=existingComponent->ClassType();
RegisterClass(newClass);
TComponent* newComponent=ComponentCreate(newClass, form);
UnRegisterClass(newClass);
newComponent->Name=newName;

// It was useful to analyze VCL source file "Classes.pas"

admdvv

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