Re: Nested friend class in nested template problem

From:
"Victor Bazarov" <v.Abazarov@comAcast.net>
Newsgroups:
comp.lang.c++
Date:
Fri, 7 Dec 2007 10:02:06 -0500
Message-ID:
<fjbn9f$mg9$1@news.datemas.de>
want.to.be.professer wrote:

On Dec 7, 10:40 pm, tonvandenheu...@gmail.com wrote:

Hi all,

the following code does not compile in gcc 4.0.2:

  3 class Outer
  4 {
  5 class B;
  6
  7 template<typename T>
  8 class A
  9 {
 10 friend class Outer::B;
 11
 12 public:
 13 A(T t): a(t) { };
 14 A() { };
 15 protected:
 16 T a;
 17 };
 18
 19 class B : public A<int>
 20 {
 21 public:
 22 B(A<int> & a)
 23 {
 24 b = a.a;
 25 }
 26
 27 int b;
 28 };
 29 };

The error I get is:

friend.cpp: In constructor 'Outer::B::B(Outer::A<int>&)':
friend.cpp:16: error: 'int Outer::A<int>::a' is protected
friend.cpp:24: error: within this context

Note that when the Outer class is stripped, everything compiles just
fine. What is the problem here?


Note the error message:'int Outer::A<int>::a' is protected.

B is inherited form A as public inheritance,
So B can only call A's public and private members.


Actually your post made me think a bit more about this, and perhaps
I was too fast to judge GCC as buggy...

Usually the rule is, the derived class is not alloed to access
*another instance's protected members*, but only its own or of
another instace of _its own type_. IOW, you should get an error
in 'foo', but not in 'bar':

    class A {
    protected:
        int a;
    };

    class B : A {
        void foo(A& a) {
            a.a; /// error!
        }

        void bar() {
            this->a; // OK
        }
    };

However, in this particular case, since A<T> makes 'B' its
friend, all the rules about protected access are overridden by
the rules of friendship, which allows access to _all_ members
regardless of their declared access specifiers.

V
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