templatetypedef templatetypedef - 1 year ago 87
C++ Question

Are inner classes in C++ automatically friends?

If I define an inner class in C++, is it automatically a friend of the class that contains it? For example, is this legal:

class Outer {
class Inner {
void mutateOuter(Outer& o);

int value;

void Outer::Inner::mutateOuter(Outer& o) {
o.value ++; // Legal? Or not?

I ask because on some compilers I've tried (VS2003) this code won't work, but I've heard at least anecdotally that it does work on some compilers. I can't find a relevant section in the C++ spec about this, and if anyone can cite something specific that would say that it is or is not legal that would be great.

Answer Source

After having asked more or less the same question here myself, I wanted to share the (apparently) updated answer for C++11:

Quoted from http://stackoverflow.com/a/14759027/1984137:

standard $11.7.1

"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 shall be obeyed"

and the usual access rules specify that:

"A member of a class can also access all the names to which the class has access..."

specific examples has been given in the standard:

class E {
    int x;
    class B { };

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