BBC BASIC Programmers' Reference

Calling object methods using structures

by Richard Russell, March 2007

Calling COM, OLE or ActiveX object methods generally involves the use of a rather arcane syntax, such as this example taken from the BB4W manual under Displaying GIF and JPEG images:

        SYS !(!gpp%+24), gpp%, ^hmw% : REM. IPicture::get_Width
        SYS !(!gpp%+28), gpp%, ^hmh% : REM. IPicture::get_Height

Here it is not at all clear what methods are being called, and how, hence the need for comments.

It is possible to improve the clarity considerably by using a structure to represent the object. You can then call the object's methods as follows:

        SYS IPicture.get_Width%, gpp%, ^hmw%
        SYS IPicture.get_Height%, gpp%, ^hmh%

To make this work you need to declare a structure containing all the object's methods in the correct order. In the case of the IPicture interface the structure needs to be declared as follows:

        DIM IPicture{QueryInterface%, AddRef%, Release%, get_Handle%, get_hPal%, \
        \            get_Type%, get_Width%, get_Height%, Render%, set_hPal%, get_CurDC%, \
        \            SelectPicture%, get_KeepOriginalFormat%, put_KeepOriginalFormat%, \
        \            PictureChanged%, SaveAsFile%, get_Attributes%}

To complete the process we must point the structure at the object's dispatch table:

        PTR(IPicture{}) = !gpp%

The memory originally allocated when the structure was created is wasted, but this isn't a problem if it is declared as a LOCAL structure.

As a practical illustration of the use of this technique here is a replacement for the PROCdisplay routine listed in the BB4W manual:

        DEF PROCdisplay(picture$, xpos%, ypos%, xsize%, ysize%)
        LOCAL iid{}, IPicture{}, oleaut32%, olpp%, gpp%, hmw%, hmh%, picture%, res%
        DIM iid{a%,b%,c%,d%}, picture% LOCAL 513
        DIM IPicture{QueryInterface%, AddRef%, Release%, get_Handle%, get_hPal%, \
        \            get_Type%, get_Width%, get_Height%, Render%, set_hPal%, get_CurDC%, \
        \            SelectPicture%, get_KeepOriginalFormat%, put_KeepOriginalFormat%, \
        \            PictureChanged%, SaveAsFile%, get_Attributes%}
 
        SYS "LoadLibrary", "OLEAUT32.DLL" TO oleaut32%
        SYS "GetProcAddress", oleaut32%, "OleLoadPicturePath" TO olpp%
        IF olpp%=0 ERROR 100, "Could not get address of OleLoadPicturePath"
 
        SYS "MultiByteToWideChar", 0, 0, picture$, -1, picture%, 256
 
        iid.a% = &7BF80980
        iid.b% = &101ABF32
        iid.c% = &AA00BB8B
        iid.d% = &AB0C3000
 
        SYS olpp%, picture%, 0, 0, 0, iid{}, ^gpp%
        IF gpp% = 0 ERROR 100, "OleLoadPicturePath failed"
 
        PTR(IPicture{}) = !gpp%
 
        SYS IPicture.get_Width%, gpp%, ^hmw%
        SYS IPicture.get_Height%, gpp%, ^hmh%
        SYS IPicture.Render%, gpp%, @memhdc%, xpos%, ypos%, xsize%, ysize%, 0, \
        \                      hmh%, hmw%, -hmh%, 0 TO res%
        IF res% ERROR 100, "IPicture::Render failed"
 
        SYS IPicture.Release%, gpp%
        SYS "InvalidateRect", @hwnd%, 0, 0
        SYS "UpdateWindow", @hwnd%
        ENDPROC