What does C++ syntax “A::B:A {};” mean

Question

What does C++ syntax struct A::B:A {}; mean? Where is this name definition (or access) described in the C++ standard?

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Answer 1

First of all struct B; is a forward declaration of struct B in global namespace. It might be confusing because it is actually not relevant in this example. This global B can be accessed as ::B or as just B.

struct A {
    struct B;
};

Is a definition of struct A in global namespace with a forward declaration of nested struct B (not the same as previously declared B in global namespace). This nested B can be accessed as ::A::B or A::B.

struct A::B:A {
};

Is a definition of nested struct B of struct A that inherits from A (with access specifier omitted). It can be rewritten to:

struct A::B
:   public A
{
};

Note that writing definition of nested struct B inside of A definition like this won't work:

struct A {
    struct B: A { // error: A is incomplete at this point
    };
};

And finally A::B::A is referring to the base class of nested struct B, that is to A, so A::B::A::B is equivalent to just A::B.