Re: Weird V-table issue

James Kanze <>
Wed, 30 Jul 2008 00:55:29 -0700 (PDT)
On Jul 29, 1:52 am, wrote:

Thanks for all the replies. I do understand that the standard
way of doing things is to use virtual inheritance and dynamic
casting. However, I would like to clarify that I am working
on an embedded environment (and due to some other reasons
which I do not want to detail here), and I am restricted to
using non-virtual inheritance and no dynamic casting.

Further clarification on QueryInterface: I would like to
define a generic method which all the other interfaces
inherit. All the implementation objects implement the
QueryInterface() method to return the interfaces they they
implement. In such a scenario, the out- parameter of the
QueryInterface has to be a void **.

Why? In order to use the result, the user must know the type.

Maybe some sort of template function is what you're looking for:

    class Network ...
        template< typename Interface >
        bool queryInterface( Interface** result )
            *result = this ;
            return true ;
        // ...
    } ;

The problem with your code is that the QueryInterface throws
away information that the function needs in order to perform
correctly. Consider something like:

    INetworkA* pA ;
    INetworkB* pB ;

    netObj->queryInterface( (void**)pA ) ;
    netObj->queryInterface( (void**)pB ) ;

The actual addresses of the INetworkA and INetworkB subclasses
of the Network are different; how can queryInterface know which
one to return if you don't tell him which one is needed. (With
the template version, above, you wouldn't use the (void**) cast,
and two different functions would be called.)

James Kanze (GABI Software)
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