Question

This code compiles on msvc/g++:

class A{
protected:
    int i;
    class B{
    public:
        A* a;
        B(A* a_)
        :a(a_){
        }
        void doSomething(){
            if (a)
                a->i = 0;//<---- this part
        }       
    };
public:
    A()
    :i(0){
    }
};

As you can see, B gets access to “protected” section of enclosing class, although it isn’t declared as friend.

Is this a standard(standard-conforming) behavior?

I use this feature sometimes, but I don’t remember a rule saying that nested protected class should automatically get access to all protected data of enclosing class.

Answer 1

In the C++03 standard, 11.8p1 says:

The members of a nested class have no special access to members of an enclosing class.

However, the resolution for defect report 45 (to the standard) states the opposite, and hence defines the behavior you see:

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

In C++0x the text of 11.8 has been changed to reflect this fact, so this is valid behavior for both C++03 and C++0x conforming compilers.

See also this thread from the cprogramming.com forums.