The typename keyword (C++ only)

Use the keyword typename if you have a qualified name that refers to a type and depends on a template parameter. Only use the keyword typename in template declarations and definitions. Consider the following example:

template<class T> class A
{
  T::x(y);
  typedef char C;
  A::C d;
}

The statement T::x(y) is ambiguous. It could be a call to function x() with a nonlocal argument y, or it could be a declaration of variable y with type T::x. C++ compiler interprets this statement as a function call. In order for the compiler to interpret this statement as a declaration, you must add the keyword typename to the beginning of T:x(y). The statement A::C d; is ill-formed. The class A also refers to A<T> and thus depends on a template parameter. You must add the keyword typename to the beginning of this declaration:

  typename A::C d;