Virtual inheritance polymorphic call resolution

kshegunov

Hello,
This happens so rarely that I seem to have forgotten this and that. Suppose I have a diamond inheritance (virtual inheritance for base classes is provided), such as:

class A
{
public:
   virtual void method();
};

class B : public virtual A
{
public:
    void method() override;
};

class C : public virtual A
{
};

class D : public B, public C
{
};

I was wondering what happens if one calls a virtual function from the last derived class with the base pointer? As far as I can tell the call should be ambiguous, so am I required to provide an override in the last class of the hierarchy to resolve it? From the example above something along the lines:

class D : public B, public C
{
public:
    void method() override
    {
        B::method();
    }
};

mrjj

@kshegunov
Hi
It seems spot on.
My memory was even more vague about the diamond case so i consulted
http://www.cprogramming.com/tutorial/virtual_inheritance.html
and yes using virtual should be the solution.
But let's see if there are other opinions or solutions.
Being in C++ Gurus, ill just keep low profile ;)

Chris Kawa

Ah yes, the famous problem deemed unsolvable by Java creators, and yet solved and working in c++ decades ago ;)

@kshegunov Draw it on a piece of paper. There's no diamond here. Just a triangle with a dangling A outside of it ;) Having that, you've got your virtuals wrong. The classes that inherit the same base (B) are C and D, so that's where you need the virtual keyword:

class A
{
public:
   virtual void method();
};

class B : public A
{
public:
    void method() override;
};

class C : public virtual B
{
};

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

In the example you posted B::method() would be indeed ambiguous and as such - is invalid and won't compile.

kshegunov

There's a typo, Chris, C should derive from A and it is a diamond.
Read as:

class C : public virtual A

PS. Corrected in the original post.

Chris Kawa

Oh, ok, then your original solution was indeed correct.

kshegunov

Okay. Thanks!