Friendship and inheritance

Question

I'm working on a small project and i'm sort of stuck because I don't really understand how friendship and inheritance interact with each other. I'll show you some sample code.

namespace a
{
    class Foo
    {
    public:
        Foo(int x) : m_x(x) {}
    protected:
        friend class b::Derived;
        friend class a::Base;
        int m_x;
    };

    class Base
    {
    public:
        Base(Foo foo) : m_foo(foo) {}
    protected:
        Foo m_foo;
    };
}
namespace b
{
    class Derived : public a::Base
    {
    public:
        Derived(a::Foo foo)
            : Base(foo)
        {
            m_foo.m_x;
        }
    };
}

e0265: at line 29: member a::Foo::m_x (declared at line 10) is inaccessible

Apparently Derived can't access protected members of Foo, seemingly because Derived::m_foo is a derived member, so constructing Derived will fail. Can anyone explain this in detail to me?

Answer 1

I found the problem. The namespace b and therefore the class derived were not visible to the friend declaration in Foo. When I Forward declared b and derived everything worked as intended and derived could access private/protected members.