Possible to inherit implemenation of the same interface in a new class?

[Vincent van der Leun]

Hi there,

Let's hope I can make myself clear:

I have a single interface that I want to implement in different classes. The implementations of the interface in the classes are almost the same, only a few of the METHODs differ.

What I would like to do is the following:

#COMPILE EXE
#DIM ALL

'I usually "publish" Interfaces that I can use outside the current file in the program's header.
'(this way I know which interfaces are supposed to be implemented by the
'program and which interfaces I only created for internal use)
INTERFACE IncredibleInterface
    INHERIT CUSTOM
    METHOD AAA()
    METHOD BBB()
    METHOD CCC()
END INTERFACE


'Here I create class CTest1 that has the "default implementation" of above's IncredibleInterface interface.
CLASS CTest1
    INTERFACE IncredibleInterface
        INHERIT CUSTOM

        METHOD AAA()
            ? "AAA1"
        END METHOD

        METHOD BBB()
            ? "BBB1"
        END METHOD

        METHOD CCC()
            ? "CCC1"
        END METHOD
    END INTERFACE
END CLASS


'Now I want to create class CTest2, which implements the IncredibleInterface just like CTest1.
'I want to override 1 of the methods though, this is the only Method which has its own implementation.
CLASS CTest2
    INTERFACE IncredibleInterface
        INHERIT CTest1,IncredibleInterface
        OVERRIDE METHOD BBB()
            ? "BBB2"
        END METHOD
    END INTERFACE
END CLASS


FUNCTION PBMAIN () AS LONG

    DIM CT1 AS IncredibleInterface
    DIM CT2 AS IncredibleInterface
    CT1 = CLASS "CTest1"
    CT1.AAA
    CT1.BBB
    CT1.CCC

    CT2 = CLASS "CTest2"
    CT2.AAA
    CT2.BBB
    CT2.CCC
    
    CT1 = NOTHING
    CT2 = NOTHING
    
    WAITKEY$
END FUNCTION

When I try this in PB/CC 5.01, I get the following error though:

PowerBASIC Console Compiler
PB/CC Version 5.01
Copyright (c) 1998-2009 PowerBasic Inc.
Venice, Florida USA
All Rights Reserved

Error 816 in C:\VDLTech\Berra\Tests\InheritTest2.bas(10:005): Internal system error
Line 10: END INTERFACE

Do I understand correctly that inheritance of an implementation is only possible when we create a new interface? That's not what I want (in this case).

The only solution I came up with is the following, but it's not really pretty IMHO, not to mention too much typing :

#COMPILE EXE
#DIM ALL

'I usually publish Interfaces that I use outside this file outside the class, like this:
INTERFACE IncredibleInterface
    INHERIT CUSTOM
    METHOD AAA()
    METHOD BBB()
    METHOD CCC()
END INTERFACE


'Here I create class CTest1 that has the "default implementation" of above's IncredibleInterface interface.
CLASS CTest1
    INTERFACE IncredibleInterface
        INHERIT CUSTOM

        METHOD AAA()
            ? "AAA1"
        END METHOD

        METHOD BBB()
            ? "BBB1"
        END METHOD

        METHOD CCC()
            ? "CCC1"
        END METHOD
    END INTERFACE
END CLASS


'Nowe I want to create class CTest2, which implements the IncredibleInterface just like CTest1.
'I want to override 1 of the methods though, this is the only Method which its own implementation.
CLASS CTest2
    INSTANCE m_ct1 AS IncredibleInterface

    CLASS METHOD Create
        m_ct1 = CLASS "CTest1"
    END METHOD

    CLASS METHOD Destroy
        'Yes, I know it's not neccessary, but I like to clean up my mess.
        m_ct1 = NOTHING
    END METHOD

    INTERFACE IncredibleInterface
        INHERIT CUSTOM
        METHOD AAA()
            m_ct1.AAA
        END METHOD

        METHOD BBB()
            ? "BBB2"
        END METHOD

        METHOD CCC()
            m_ct1.CCC
        END METHOD

    END INTERFACE
END CLASS


FUNCTION PBMAIN () AS LONG

    DIM CT1 AS IncredibleInterface
    DIM CT2 AS IncredibleInterface
    CT1 = CLASS "CTest1"
    CT1.AAA
    CT1.BBB
    CT1.CCC

    CT2 = CLASS "CTest2"
    CT2.AAA
    CT2.BBB
    CT2.CCC

    CT1 = NOTHING
    CT2 = NOTHING

    WAITKEY$
END FUNCTION

Does anyone have any suggestions or know of a different solution that I missed?

Best regards, Vincent

[José Roca]

You can, but changing the name of the second interface.

'SED_PBCC
#COMPILE EXE
#DIM ALL

'I usually "publish" Interfaces that I can use outside the current file in the program's header.
'(this way I know which interfaces are supposed to be implemented by the
'program and which interfaces I only created for internal use)
INTERFACE IncredibleInterface
    INHERIT CUSTOM
    METHOD AAA()
    METHOD BBB()
    METHOD CCC()
END INTERFACE


'Here I create class CTest1 that has the "default implementation" of above's IncredibleInterface interface.
CLASS CTest1
    INTERFACE IncredibleInterface
        INHERIT CUSTOM

        METHOD AAA()
            ? "AAA1"
        END METHOD

        METHOD BBB()
            ? "BBB1"
        END METHOD

        METHOD CCC()
            ? "CCC1"
        END METHOD
    END INTERFACE
END CLASS


'Now I want to create class CTest2, which implements the IncredibleInterface just like CTest1.
'I want to override 1 of the methods though, this is the only Method which has its own implementation.
CLASS CTest2
    INTERFACE IncredibleInterface2
        INHERIT CTest1,IncredibleInterface
        OVERRIDE METHOD BBB()
            ? "BBB2"
        END METHOD
    END INTERFACE
END CLASS


FUNCTION PBMAIN () AS LONG

    DIM CT1 AS IncredibleInterface
    DIM CT2 AS IncredibleInterface2
    CT1 = CLASS "CTest1"
    CT1.AAA
    CT1.BBB
    CT1.CCC

    CT2 = CLASS "CTest2"
    CT2.AAA
    CT2.BBB
    CT2.CCC
    
    CT1 = NOTHING
    CT2 = NOTHING
    
    WAITKEY$
END FUNCTION

[Michael Mattias]

Error 816 in C:\VDLTech\Berra\Tests\InheritTest2.bas(10:005): Internal system error
Line 10: END INTERFACE

Slightly O/T, but that is an automatic "report to PB Support."

[Bob Zale]

The issue here is that a child interface can inherit from a parent interface. A child interface cannot inherit from itself. As noted previously, the derived interface must be unique and therefore have a different name.

The next update to PowerBASIC will generate a more descriptive exception report in this case.

Best regards,

Bob Zale
PowerBASIC Inc.