Visual Studio Compiler warning C4250 ('class1' : inherits 'class2::member' via dominance)

Question

The following code generates warning C4250. My question is, what's the best solution to it?

class A
{
  virtual void func1();
}

class B : public A
{
}

class C : public A
{
  virtual void func1();
}

class D : public B, public C
{
}

int main()
{
  D d;
  d.func1(); // Causes warning
}

According to what I've read it should be possible to do this:

class D : public B, public C
{
  using B::func1();
}

But, this doesn't actually do anything. The way I've currently solved it is:

class D : public B, public C
{
  virtual void func1() { B::func1(); }
}

What's everyone's view on this?

Answer 1

Did you try to inherit public virtual from class A? I think it should solve it.

    class B :public virtual A;
    class C :public virtual A;
    class D : public virtual B, public virtual C;

The virtual inheritance suppose to solve the ambiguity.

Answer 2

I think the solution you're using may be the way to go, sorry to say. The only thing I can think of that might help is if you're able to make A's func1 pure virtual. That might not be feasible in your real program, though.

Answer 3

I had the same warning for the following code:

class Interface
{
public:
    virtual void A() = 0;
};

class Implementation : public virtual Interface
{
public:
    virtual void A() {};
};

class ExtendedInterface : public virtual Interface
{
    virtual void B() = 0;
};

class ExtendedImplementation : public ExtendedInterface , public Implementation
{
public:
    virtual void B() {};
};

This bug report for Visual C++ 2005 in msdn suggests that this is a known bug that was considered not important enough to fix... They suggest to disable the warning in this case by using a pragma. I think it is safe also in your case, but you should use virtual inheritance as shown in the answer by Gal Goldman.