Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Marco van de Voort via fpc-pascal 


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



TWindow(myClass).CreateNewWindow;


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



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 ...');


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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Adriaan van Os via fpc-pascal 

Olivier Sannier via fpc-pascal wrote:


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;


OK. A typecast with TWindowClass works.

Thanks,

Adriaan van Os

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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Olivier Sannier via fpc-pascal 

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 :

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

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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Adriaan van Os via fpc-pascal 

Martin Frb via fpc-pascal wrote:


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

It was mentioned before

On 01/05/2024 19:43, Jean SUZINEAU via fpc-pascal wrote:

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( );


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
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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Martin Frb via fpc-pascal 

On 02/05/2024 08:32, Adriaan van Os via fpc-pascal wrote:

Martin Frb via fpc-pascal 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.


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

It was mentioned before

On 01/05/2024 19:43, Jean SUZINEAU via fpc-pascal wrote:

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( );


It must be cast to another "class of"
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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Adriaan van Os via fpc-pascal 

Martin Frb via fpc-pascal 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

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Re: [fpc-pascal] Type-casting a class variable

2024-05-02 Thread Adriaan van Os via fpc-pascal 



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

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Re: [fpc-pascal] Type-casting a class variable

2024-05-01 Thread Sven Barth via fpc-pascal 
Adriaan van Os via fpc-pascal  schrieb am
Mi., 1. Mai 2024, 17:07:

> 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 ?
>

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
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Re: [fpc-pascal] Type-casting a class variable

2024-05-01 Thread Martin Frb via fpc-pascal 

On 01/05/2024 16:28, Adriaan van Os via fpc-pascal wrote:

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 ?




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.

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Re: [fpc-pascal] Type-casting a class variable

2024-05-01 Thread Jean SUZINEAU via fpc-pascal 

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( );

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