class of - instance created is wrong

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class of - instance created is wrong

Graeme Geldenhuys-2
Hi,

I've used the Class Of language feature a lot in the past. eg: tiOPF
uses it a lot, I use it to generate various reports in our company
projects etc.

So I tried to implement a "lazy mans" factory method in my project, so
it can generate a class instance based on what the developer chooses.

I have the Class Of defined as follows...


  TfpgCanvasBase = class(TObject)
     ...
  end;

  TfpgCanvasBaseClass = class of TfpgCanvasBase;


I have a few Canvas class hierarchies. Here are two of them, which I'm
trying to use in this example:


  TfpgCanvas
   |
   +-- TfpgX11Canvas
        |
        +-- TfpgCanvasBase


  TAgg2D
   |
   +-- TfpgCanvasBase



I then have a global variable which will point to the actual class
type to instantiate.

  DefaultCanvasClass: TfpgCanvasBaseClass = nil;


Then somewhere early on in your project you can define which Canvas
class to use by simply doing the following...

   DefaultCanvasClass := TfpgCanvas;
or
   DefaultCanvasClass := TAgg2D;



Here is the simple factory method that the Window class uses to
instantiate a new Canvas object.
Excuse the Writeln() statements and comment - they are for debugging purposes.

function TfpgWindow.CreateCanvas: TfpgCanvasBase;
begin
  writeln('>> TfpgWindow.CreateCanvas');
  writeln('     DefaultCanvasClass = ' + DefaultCanvasClass.ClassName);
//  result := TAgg2D.Create(self);
//  result := TfpgCanvas.Create(self);
  Result := DefaultCanvasClass.Create(self);
  writeln('<< TfpgWindow.CreateCanvas');
end;


I then have Writeln() statements in each Canvas constructor outputting
the name of that class. So when a TfpgCanvas instance is created, the
output should  be similar to the class hierarchy listed above.


Now the problem...


Using the DefaultCanvasClass to create an instance, it seems to leave
out the latest class in the hierarchy. So if DefaultCanvasClass =
TfpgCanvas, I see console output as follows;

  creating... TfpgX11Canvas
  creating... TfpgCanvasBase

NOTE the missing TfpgCanvas. Needless to say my program crashes instantly.

The same happens when DefaultCanvasClass = TAgg2D. The console output is then.

  creating... TfpgCanvasBase

Again, the actual TAgg2D class constructor is by-passed somehow?? Yet
the writeln() statement inside the CreateCanvas() method, which asks
for the ClassName of DefaultCanvasClass, DOES output the correct class
name. TfpgCanvas or TAgg2D.


Now if I change CreateCanvas() to create a instance of a Canvas class
directly - without the usage of DefaultCanvasClass... One of the two
commented out lines of code, then everything works fine. But
obviously, I don't want to hard-code which canvas class to use.


Can anybody see what I am doing wrong? I've tested with FPC 2.6.0,
2.6.1 and 2.7.1 under 64-bit Linux. All compilers give the same
results. Like I've said, I have used the Factory Method pattern many
times (though a bit more advanced than what I did here - normally
using a mapping class), and it always works well.

BTW:
I took this global variable idea from the fptimer implementation,
which is part of FPC. I assume the fptimer implementation works
correctly.



--
Regards,
  - Graeme -


_______________________________________________
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http://fpgui.sourceforge.net
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Re: class of - instance created is wrong

Mattias Gaertner


Graeme Geldenhuys <[hidden email]> hat am 15. März 2012 um 10:07 geschrieben:

>[...]
>   TfpgCanvas
>    |
>    +-- TfpgX11Canvas
>         |
>         +-- TfpgCanvasBase
>
>
>   TAgg2D
>    |
>    +-- TfpgCanvasBase
>[...]
>   DefaultCanvasClass: TfpgCanvasBaseClass = nil;
> [...]
> function TfpgWindow.CreateCanvas: TfpgCanvasBase;
> begin
>   writeln('>> TfpgWindow.CreateCanvas');
>   writeln('     DefaultCanvasClass = ' + DefaultCanvasClass.ClassName);
> //  result := TAgg2D.Create(self);
> //  result := TfpgCanvas.Create(self);
>   Result := DefaultCanvasClass.Create(self);
>   writeln('<< TfpgWindow.CreateCanvas');
> end;
> [...]
>   creating... TfpgX11Canvas
>   creating... TfpgCanvasBase
>
> NOTE the missing TfpgCanvas. Needless to say my program crashes instantly.
>[...]
> Can anybody see what I am doing wrong?

 

Is the constructor virtual? 

 

 

> I've tested with FPC 2.6.0, 2.6.1 and 2.7.1 under 64-bit Linux. All compilers give the same
> results.

>[...]

 

Mattias

 


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Re: class of - instance created is wrong

Graeme Geldenhuys-2
On 15 March 2012 11:36, Mattias Gaertner  wrote:
>
> Is the constructor virtual?


I owe you a beer!  :-)

The original constructor in TfpgCanvasBase was virtual, but some
descendant classes did a 'reintroduce' of the constructor which wasn't
virtual. Looking at the code now, the reintroduce isn't needed any
more and thus removed.

It works perfectly now, thanks!!  fpGUI developers can now easily
switch between X11, GDI and AggPas canvas's. :-D


--
Regards,
  - Graeme -


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http://fpgui.sourceforge.net
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Re: class of - instance created is wrong

Leonardo M. Ramé
>________________________________
> From: Graeme Geldenhuys <[hidden email]>
>To: FPC-Pascal users discussions <[hidden email]>
>Sent: Thursday, March 15, 2012 7:11 AM
>Subject: Re: [fpc-pascal] class of - instance created is wrong
>
>On 15 March 2012 11:36, Mattias Gaertner  wrote:
>>
>> Is the constructor virtual?
>
>
>It works perfectly now, thanks!!  fpGUI developers can now easily
>switch between X11, GDI and AggPas canvas's. :-D
>


Hi Graeme, this is completely off topic. Does AggPas works in 64bits Linux? I remember it didn't compile last year.

Regards,
Leonardo M. Ramé
http://leonardorame.blogspot.com

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Re: class of - instance created is wrong

Graeme Geldenhuys-2
On 15 March 2012 12:31, Leonardo M. Ramé  wrote:
>
>
> Hi Graeme, this is completely off topic. Does AggPas works in 64bits Linux? I remember it didn't compile last year.
>

I does now! ;-)

I haven't publish my copy of AggPas yet (which contains the 64-bit
fixes). I'll probably push those changes to the fpGUI repository later
today or tomorrow.

I'll also supply some of the vital patches to the Lazarus community,
so they can fix their AggPas copy.



--
Regards,
  - Graeme -


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http://fpgui.sourceforge.net
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Re: class of - instance created is wrong

zaher dirkey
On Thu, Mar 15, 2012 at 12:52 PM, Graeme Geldenhuys <[hidden email]> wrote:
On 15 March 2012 12:31, Leonardo M. Ramé  wrote:
>
>
> Hi Graeme, this is completely off topic. Does AggPas works in 64bits Linux? I remember it didn't compile last year.
>

I does now! ;-)

I haven't publish my copy of AggPas yet (which contains the 64-bit
fixes). I'll probably push those changes to the fpGUI repository later
today or tomorrow.

Would u push it to new repository (under ur account).

I'll also supply some of the vital patches to the Lazarus community,
so they can fix their AggPas copy.

And why there is many of AggPas, what happened to the original one?
 
Best Regards
Zaher Dirkey


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Re: class of - instance created is wrong

Graeme Geldenhuys-2
On 18 March 2012 08:03, Zaher Dirkey wrote:
>>
> Would u push it to new repository (under ur account).


I'm planning to publish it to the fpGUI repository today.


> And why there is many of AggPas, what happened to the original one?

The original AggPas author only released AggPas as a zip / tarball
archive.  There never was any public source code repository. The
original author also moved on, and created a commercial venture from
AggPas, now called CrossGL (Cross platform Graphics Library), so he
doesn't work on the original AggPas version any more.

Also, fpGUI can now use AggPas as the graphics library (instead of
XLib or GDI) - see the screenshot in the fpgui.development newsgroup.
Once fully tested, it will become the default graphics library for
fpGUI, so a copy of AggPas must be included with fpGUI to compile.

As for Lazarus including a copy of AggPas - I have no idea what was
the plans behind that. They will never use AggPas as a graphics
library for LCL, because it is not "native". Lazarus's AggPas package
is also not a default installed package in Lazarus IDE as far as I
know. So it seems the thinking was to supply an optional and more
powerful Canvas class, compared to the default LCL Canvas, for those
developers that need it in specialised projects.

--
Regards,
  - Graeme -


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http://fpgui.sourceforge.net
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Re: class of - instance created is wrong

zaher dirkey

On Sun, Mar 18, 2012 at 9:22 AM, Graeme Geldenhuys <[hidden email]> wrote:
I'm planning to publish it to the fpGUI repository today.

No, I meant not under fpGUI, thanks.

Best Regards
Zaher Dirkey


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Re: class of - instance created is wrong

Graeme Geldenhuys-2
In reply to this post by Graeme Geldenhuys-2
On 18 March 2012 09:51, Zaher Dirkey  wrote:
>
> But that mean you force us to use fpGUI :P


No, the AggPas code is still kept in a separate folder. The only
mixing between fpGUI & AppPas is localised to a single unit, and that
unit is not needed for any of AggPas specific code or demos.


--
Regards,
  - Graeme -


_______________________________________________
fpGUI - a cross-platform Free Pascal GUI toolkit
http://fpgui.sourceforge.net
_______________________________________________
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