Why is diamond of death and not triangle of death [duplicate]

Question

I've seen a lot examples that illustrate the dangerous of multiple inheritance.

The example is usually like Class B and C extends Class A , Class D extends both B and C.

B and C both override a method from A, say for example, equals();

then when call D.equals(); it doesn't know which one from its parent should be called

provided that equals() is not overridden in D.

From what I can see, isn't Class A in this example redundant? If we remove A from this hierarchy and just look at B and C, if B and C both have method called equals(), then when D extends B and C, it will still have the same problem, so isn't it really a triangle of death?

I am not sure if what I assumed will cause compile time error in some other language.

Hope someone can clarify it for me.

Answer 1

If D extends B and C and not overrides the method equals(), which is implemented in B and C, there is no ambiguity - D can use B.equals() or C.equals().

With diamond structure on the other hand, if D calls A.equals() and both B and C override it, you don't know which method should be invoked B.equals() or C.equals().

Answer 2

You're actually right, there is no need for A to exist at all to show the problems of multiple inheritance.

The following code (the "V of death" as Craig eloquently puts it in a comment) is enough:

#include <iostream>

class xyzzy {
    public: virtual int get (void) { return 7; }
};

class plugh {
    public: virtual int get (void) { return 42; }
};

class twisty: public xyzzy, public plugh {
};

int main() {
    twisty passages;
    std::cout << passages.get() << '\n';
    return 0;
}

If you try to compile this, you get:

testprog.cpp: In function ‘int main()’:
testprog.cpp:16:24: error: request for member ‘get’ is ambiguous
testprog.cpp:8:14: error: candidates are: virtual int plugh::get()
testprog.cpp:4:14: error:                 virtual int xyzzy::get()

However, keep in mind you can explicitly choose which one you want with something like:

    std::cout << passages.plugh::get() << '\n';

There's more information on the diamond problem here, including why it's actually a different problem.