Question

Please educate me. Why does this compile:

struct compiles
{
    struct A;
    struct B
    {
        B(const A &a) : member(a.member) { }
        int member;
    };
    struct A
    {
        A(const B &b) : member(b.member) { }
        int member;
    };
};

while this does not:

namespace doesnt
{
    struct A;
    struct B
    {
        B(const A &a) : member(a.member) { }
        int member;
    };
    struct A
    {
        A(const B &b) : member(b.member) { }
        int member;
    };
}

(in MSVC 9.0)

Answer 1

In C++, class scope is special. Any declaration that extends to or past then end of the class definition is automatically extended to the regions defined by its member definitions (3.3.6 [basic.scope.class]).

This means that in the first case both the first declaration of struct A and the full definition of struct A are visible in the body of B and its constructor.

This doesn’t apply to namespace scope so in the second case a.member in the constructor of B is an error because a definition of struct A isn’t yet visible.