Extending TImage

BlitzMax Forums/BlitzMax Programming/Extending TImage

d-bug

(Posted 2006) [#1]

Hello,
could someone please tell me why TImage always return Null, if i try to cast it:

Type TMyImage Extends TImage

        'result = Null
	Function Open:TMyImage (url:Object)
		Local image:TMyImage = TMyImage(TImage.Load(url,MASKEDIMAGE,255,0,255))
		Return image
	End Function

        'result = null
        Function Open2:TMyImage (url:Object)
                Local image:TImage = TImage.Load(url,MASKEDIMAGE,255,0,255)
                If image Return TMyImage(image)
         End Function

End Type

I've tested several ways, but casting TImage doesn't seem to work.

Dreamora

(Posted 2006) [#2]

If you check your class you will find out that TMyImage actually isn't handled as a fully TImage extend.

Add a function create for example that returns :TMyImage
On a correct extend, the compiler would already throw an error that function declaration differs (as TImage.Create returns :TImage) but it does not in that case.

rdodson41

(Posted 2006) [#3]

Basically in TImage.Load you are creating a TImage, but not a TMyImage. So since image is really a TImage casting it to a TMyImage will return null. Its the old saying of how a square is a rectangle, but a rectangle is not a square.

Fabian.

(Posted 2006) [#4]

TMyImage(TImage.Load(url,MASKEDIMAGE,255,0,255))

TImage.Load returns a TImage, so casting to TMyImage will return Null.

You could write an image wrapper:

Type TMyImage Extends TImage
  Field src:TImage

  Method DoThisForAllMethods ( Parameters )
    Return src.DoThisForAllMethods ( Parameters )
  EndMethod
EndType