How to initialize a static field of which type is a private nested class?

Question

Outer.hpp :

class Outer {
  class Inner {
    Inner() {}
  };
  static Inner inner;
}

Outer.cpp (at top-level, eg not within a function body):

Outer::Inner Outer::inner;

I get the following error:

error C2248: 'Outer::Inner::inner' : cannot access private member declared in class 'Outer::Inner'

I'm not using a compiler that is fully compliant with C++11 (Visual Studio 2010), so it is not possible to define the field at declaration.

Answer 1

The trick is to make Outer a friend of Inner :

Outer.hpp :

class Outer {
  class Inner {
    Inner() {}
    friend Outer;
  }
  static Inner inner;
}

Now, Outer can see Inner 's type as if it weren't private even in the implementation file, so the initialization in Outer.cpp succeeds.

Answer 2

No, you don't need to Friend it. Bad idea.

class Inner is a private member of class Outer . There's nothing wrong with that.

The problem depends on where you've placed your definition.

Outer::Inner Outer::inner; // is fine in the global space.
int main()
{
    Outer::Inner Outer::inner; // Fails because it's used as a local variable to function main.
}