The C++ standard requires types with qualified names that depend upon template arguments to be explicitly noted as type names with the typename keyword. This is true even if the compiler can “know” that it should be a type. The comments in the following example show the types with qualified names that require the typename keyword.
struct simple { typedef int a_type; static int a_datum; }; int simple::a_datum = 0; // not a type template <class T> struct parametric { typedef T a_type; static T a_datum; }; template <class T> T parametric<T>::a_datum = 0; // not a type template <class T> struct example { static typename T::a_type variable1; // dependent static typename parametric<T>::a_type variable2; // dependent static simple::a_type variable3; // not dependent }; template <class T> typename T::a_type // dependent example<T>::variable1 = 0; // not a type template <class T> typename parametric<T>::a_type // dependent example<T>::variable2 = 0; // not a type template <class T> simple::a_type // not dependent example<T>::variable3 = 0; // not a type |