void C::C::C::A::A::A::foo() - valid syntax monstrosity

Here’s an odd bit of C++ syntax for you. Say we have the following class hierarchy:

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

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

class C : public A, public B {
public:
    virtual void foo();
};

The following definitions are all well-formed:

void C::foo(){
  std::cout << "C";
}
void C::A::foo(){
  std::cout << "A";
}
void C::B::foo(){
  std::cout << "B";
}

The first one defines C::foo, the second defines A::foo and the third defines B::foo. This is valid because of an entity known as the injected-type-name:

A class-name is inserted into the scope in which it is declared immediately after the class-name is seen. The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name. For purposes of access checking, the injected-class-name is treated as if it were a public member name. […]

Since A is a base class of C and the name A is injected into the scope of A, A is also visible from C.

The injected-class-name exists to ensure that the class is found during name lookup instead of entities in an enclosing scope. It also makes referring to the class name in a template instantiation easier. But since we’re awful people, we can use this perfectly reasonable feature do horribly perverse things like this:

void C::C::C::A::A::A::foo(){
    std::cout << "A";
}

Yeah, don’t do that.