Why can I use auto on a private type?

Question

I was somehow surprised that the following code compiles and runs (vc2012 & gcc4.7.2)

class Foo {
    struct Bar { int i; };
public:
    Bar Baz() { retu         


        

Answer 1

To add to the other (good) answers, here's an example from C++98 that illustrates that the issue really doesn't have to do with auto at all

class Foo {
  struct Bar { int i; };
public:
  Bar Baz() { return Bar(); }
  void Qaz(Bar) {}
};

int main() {
  Foo f;
  f.Qaz(f.Baz()); // Ok
  // Foo::Bar x = f.Baz();
  // f.Qaz(x);
  // Error: error: ‘struct Foo::Bar’ is private
}

Using the private type isn't prohibited, it was only naming the type. Creating an unnamed temporary of that type is okay, for instance, in all versions of C++.