How to override a function in another base class?

You cannot directly have a class override a method not of its base class. But you can sort-of of do it in a roundabout way. I'll present two such approaches - I prefer the second.

Approach 1

This is described by Daniel Paul in a post on thinkbottomup.com.au, entitled C++ Mixins - Reuse through inheritance is good... when done the right way.

In your case, this is what it would look like:

class Base {
public:
    virtual void test() = 0;
};

template <typename T>
class Mixin : public T {
public:
    virtual void test() override { /*... do stuff ... */ }
};

class UnmixedExample : public Base {
    /* definitions specific to the Example class _not_including_
       a definition of the test() method */
};

using Example = class Mixin<UnmixedExample>;

int main(int argc, char** argv) {
    Example{}.test();
    return 0;
}

Approach 2: CRTP!

CRTP is the "Curiously Recurring Template Pattern" - definitely follow that link if you haven't seen it before. With this approach, we'll be using the virtual inheritance specifier to avoid ambiguity, and unlike the previous approach - we will not be reversing the inheritance order of the Mixin and Example classes.

class Base {
public:
    virtual void test() = 0;
};

template <typename T>
class Mixin : virtual T {
public:
    virtual void test() override { /*... do stuff ... */ }
};

class Example : public virtual Base, public virtual Mixin<Base> {
    /* definitions specific to the Example class _not_including_
       a definition of the test() method */
};

int main(int argc, char** argv) {
    Example{}.test();
    return 0;
}

Note about both solutions:

• Ain't it curious how the CRTP keeps recurring all over the place? :-)
• The code I used is C++11 for pedagogical purposes, but the same would work in C++98.

You cannot have a class override an unrelated class's virtual function. There are different things you could do to work around this. You can make the mixin a template that derives (virtually) from the type argument and use it as class Example : public virtual Base, Mixin, or you can add code in the final class to dispatch to the mixing:

void Derived::test() { Mixin::test(); }