Re: Inheritance Problem (MSVC 6)

Hans-Dieter Dreier wrote:

In order to make a class non-abstract all pure virtual functions it
inherits have to have final non-pure overriders. Overriding only
works *directly* in the hierarchy. You cannot expect your class'
uncle to override your class' father's pure functions.

    struct ABC {
        virtual void foo() = 0;
    };

    struct CC : ABC {
        void foo() {}
    };

    struct YAABC : ABC {
        // here you have one inherited virtual function
        // virtual void foo() = 0;
        // and it's pure
    };

    struct MYCC : YAABC, CC {
        // here you have two inherited functions, one
        // comes from YAABC and its pure and the other
        // comes from CC, and it's OK.
    };

The language cannot decide for you what MYCC::foo should do. You
actually have two virtual functions inherited, and one of them is
pure that does not have the final overrider. See subclause 10.3
for the actual rules involved. Paragraph 10 shows the situation
similar to yours (no unambiguous final overrider).

To disambiguate the behaviour you have to introduce the final
overrider into 'MYCC'. What it does is up to you, but the most
acceptable way is

    struct MYCC : YAABC, CC {
        void foo() { return CC::foo(); }
    };

That cannot be done automatically, it would generate more problems
than it would solve.