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




what is type casting in c++ (2)

This definition

struct A {
    struct B;
};

Defines a struct A with a declaration of a nested struct B 1 . The fully qualified name of B is A::B , you could say B is inside the "namespace" of A . Then this:

struct A::B : A { // Note I added spaces
};

Is the definition of A::B , and the single : specifies that it is derived from A .

Now, the interesting part is A::B::A::B . Let's dissect it:

  1. A::B names the nested structure.
  2. A::B::A accesses the injected class name A inside B . The injection is due to the inheritance.
  3. A::B::A::B names the nested structure B in A again.

And you can continue ad-infinitum, or at least until your compiler meets its translation limit 2 .

A fun intellectual exercise, but avoid like the plague in actual code.

[class.qual]/1 explains how the lookup works

If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class ([class.member.lookup]), except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (Clause [class.derived]).

And the text above allows us to name the base class because [class]/2

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.

The above clearly says that starting a fully qualified name with A:: allows you to specify a member or a base class. Since A has no bases, you can only specify A::B (a "member type"). But A::B also nominates a class. So we may specify a base or member of that as well with A::B:: , which allows us to name A::B::A . Now rinse and repeat.

1 - Note it's a completely other B . Not at all related to the global struct B .
2 - A recommended minimum of 256 according to [implimits]/2.36

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

#include <iostream>

struct B;

struct A {
    struct B;
};

struct A::B:A {
};

int main() {
    A::B::A::B b;
    std::cout<<"Sizeof A::B::A::B is " << sizeof(A::B::A::B)<<std::endl;
    return 0;
}

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 .





scope-resolution