根据ISO 2003，内部类访问外部私有成员

Question

As described in ISO C++ 2003

§11.8 Nested classes [class.access.nest]

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. The members of an enclosing class have no special access to members of a nested class; the usual access rules (clause 11) shall be obeyed.

[Example:

 class E { int x; class B { }; class I { B b; // error: E::B is private ERROR 1 int y; void f(E* p, int i) { p->x = i; // error: E::x is private ERROR 2 } }; int g(I* p) { //return p->y; // error: I::y is private ERROR 3 } }; int main() {} 

—end example]

So I think that clang and g++ are wrong as they compile this code successfully.

Or do I understand something wrong?

Answer 1

Standard says about "have no special access", but not about "have no access at all". Nested class is a same member of outer class as any other member.

It is not clearly said in C++03 standard, but C++11 contains it explicitly:

11.7 Nested classes [class.access.nest]

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

Answer 2

This behaviour has changed since 2003. The relevant clause in Working Draft N4926 (C++17) now reads:

§11.7 Nested classes [class.access.nest]

A nested class is a member and as such has the same access rights as any other member. The members of an enclosing class have no special access to members of a nested class; the usual access rules (Clause 11) shall be obeyed.

So the access is one way: nested class members can access enclosing class members, but not vice versa.

For instance:

class Enclosing
{
  int n;
  class Nested
  {
    int n;
    int f (Enclosing& E)
    {
      return E.n; // OK
    }
  } ;
  int f (Nested& N)
  {
    return N.n; // Error: Nested::n is private
  }
} ;