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

c++, friend, initialization, private, static

Solution

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.

Problem

Outer.hpp: ``` class Outer { class Inner { Inner() {} }; static Inner inner; } ``` Outer.cpp (at top-level, e.g. 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.

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