Suppose I have a

var myClass: TClass

and (for example) a

	class function TWindow.CreateNewWindow( ....)

Now I want to call CreateNewWindow for var myClass. Of course, depending on the class, CreateNewWindow will behave different. Type-casting myClass to TWindow crashes (in my setup). And even if it worked, wouldn't it loose the class information ?

Of course, by adding an extra parameter of type TClass to CreateNewWindow, the issue can be solved. But that's far from elegant.

How to solve this ? Anything I missed ?

Regards,

Adriaan van Os

Hello,

You should cast to TWindowClass as TWindow is used to cast an instance, not a class reference.

Note that you may not have TWindowClass, in which case you need to declare it like this:

type
     TWindowClass = class of TWindow;

Note that CreateNewWindow would have to be virtual for derived clases methods to be called.

Le 01/05/2024 à 16꞉28, Adriaan van Os via fpc-pascal a écrit :

I didn't tested but I imagine it could be done with something like this ?

type
        TWindow_Class= class of TWindow;
begin
           ...
           (myClass as TWindow_Class).CreateNewWindow( ....);

Silly question, but did you assign the value to the variable?

myClass := TWindow;
TWindow(myClass).CreateNewWindow;

The cast is not always needed.

If you have

var myClass: TMyClass;

with
  TMyClass = class
   class  procedure  CreateNewWindow; virtual
  end;

And any inherited class correctly overrides this, then you don't need the typecast.

Adriaan van Os via fpc-pascal schrieb am
Mi., 1. Mai 2024, 17:07:

In addition to what Martin said: as long as you have a non-static class method the value of the variable you call the class method on (e.g. myClass in your example) will be passed as Self parameter. So no need for extra parameters.

Regards,
Sven

In addition to what Martin said: as long as you have a non-static class method the value of the variable you call the class method on (e.g. myClass in your example) will be passed as Self parameter. So no need for extra parameters.

But how can myClass be passed ?

myClass.CreateNewWindow

is not accepted by the compiler, because CreateNewWindow is not a method of TClass, whereas

TWindow.CreateNewWindow

doesn't pass myClass.

Regards,

Adriaan van Os

Martin Frb wrote

Silly question, but did you assign the value to the variable? myClass := TWindow;

That's what I did.

TWindow(myClass).CreateNewWindow;

And this is what crashes. I can report this, if the type-cast is supposed to work.

Regards,

Adriaan van Os

My example (2nd part of it) was actually wrong.

It was mentioned before

It must be cast to another "class of"

When I do that, the compiler emits

Class or COM interface type expected, but got "TClass"

I am using

	{$mode macpas}
	{$modeswitch class}
	{$interfaces corba}

Regards,

Adriaan van Os

OK. A typecast with TWindowClass works.

Thanks,

Adriaan van Os

Op 2-5-2024 om 08:32 schreef Adriaan van Os via fpc-pascal:

Known gotcha. Is and as are no good for method variables, use:

if myclass.inheritsfrom(twindow) then

    twindow(myclass).createnewwindow

else

   raise exception.create('something''s rotten in the state of ...');