Parallel class hirarchy c++ ends up in abstract class

Question

Why does this simple class hierarchy end up in a abstract class B? I don't know a buzz word to search for in this case. (Maybe Diamond-Problem?)

class I_A
{
    virtual void foo() = 0;
};

class I_B : public I_A
{
    virtual void bar() = 0;
};

class A : public I_A
{
    void foo() override {}
};

class B: public A, public I_B
{
    void bar() override {}
};

B b;  ///< Compiler says it is abstract

The buzzword would be "diamond problem". Related/dupe: [How does virtual inheritance solve the “diamond” (multiple inheritance) ambiguity?](https://stackoverflow.com/questions/2659116/how-does-virtual-inheritance-solve-the-diamond-multiple-inheritance-ambiguit). In general, you inherit `I_A` twice and you have a definition of `A::I_A::foo()`, but not a definition of `I_B::I_A::foo()`. — Yksisarvinen, Aug 12 '20 at 08:18
@Yksisarvinen You are right diamond describes the problem I run into. On my paper I drew it slightly different so I didn't realise. — Jonny Schubert, Aug 12 '20 at 08:20

score 0 · Accepted Answer · answered Aug 12 '20 at 08:21

Looks like B inherits from A, which inherits from I_B, which inherits from I_A. B also inherits from I_B, which inherits from I_A.

The diamond inheritance problem is the keyword to search, since you're inheriting the same thing two different ways. Stroustrup mentions this along with examples for what to do.

Parallel class hirarchy c++ ends up in abstract class

1 Answers1