Question

Here is the code:

template <typename T>
struct Outer {
  struct InnerBase {
    int n;
    InnerBase() : n(42) { }
  };
  struct InnerChild : InnerBase {
    int getN() { return n; }
  };
};

And here is the problem:

In member function 'int Outer<T>::InnerChild::getN()':
error: 'n' was not declared in this scope

If the Outer is not a template, everything works fine. How so? How to fix it in the template?

Answer 1

InnerBase is not a free-standing type, but actually a dependent class, since it is really Outer<T>::InnerBase. Thus InnerChild doesn’t know from whom exactly it is inheriting (think of specializations*!), and thus it cannot know that n is actually a name.

To assert that n is indeed a name of a class member, say this->n or InnerBase::n, or add a declaration using InnerBase::n; at some point in InnerChild.

_{*) e.g. template <> struct Outer<int>::InnerBase { typedef int n; };}