Memberspaces may access private members of parent class

c++, idioms

Solution

By language rules in C++11.

A nested class is a member and as such has the same access rights as any other member. Example:

class E {
   int x;
   class B { };
   class I {
     B b; // OK: E::I can access E::B
     void f(E* p, int i) 
     {
        p->x = i; // OK: E::I can access E::x
     }
   };
};

And in C++03 was

The members of a nested class have no special access to members of an enclosing class, nor to classes or functions that have granted friendship to an enclosing class; the usual access rules (clause 11) shall be obeyed.

So, example from C++11 should not work with c++03 compilers.

Problem

I've been reading this article, and was playing around with the memberspace idiom for a while when I noticed something that surprised me within this snippet (which compiles without problems: http://ideone.com/hRiV5B): ``` class HugeClass { public: struct memberspace { int f() const { return parent.f; } private: friend HugeClass; explicit memberspace(HugeClass & parent) : parent(parent) {} HugeClass & parent; } memberspace; HugeClass() : memberspace(*this), f(42) {} private: int f; }; ``` I would have expected a compiler error that the access of `HugeClass::f` is not allowed because `f` is private in that context. `HugeClass` is a `friend` of `memberspace`, so `HugeClass` may call the private constructor of `memberspace`, but why does it work the other way around without explicitly declaring `memberspace` a `friend` of `HugeClass`?

Original source