Question

Is the following code valid with C++ standard? It’s strange that C::B would work because struct B is in A’s namespace. But it does compile fine with gcc.

struct A { struct B {}; };
struct C : public A::B {};
struct D : public C::B {};

If this is standard conforming C++, what are reasonable applications for this construct?
Thanks.

Answer 1

Yes, it’s valid C++. A class in its own scope (so both B and B::B refer to the same class B), and a class’s parent class is in its own scope. So since B is in C‘s scope and B is in its own scope, C::B refers to B which is A::B.

(Side note: do not confuse a namespace with a scope.)

C++03 §9 paragraph 2 says:

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.