C ++中的模板中的typename

Question

What is the difference between following two usages of templates?

Usage 1:

template<class T> class U : A<T>
{
  T::B x;
  void f(A<T>& y) { *y++; }
};

Usage 2:

template<class T> class U : A<T>
{
  typename T::B x;
  void f(A<T>& y) { *y++; }
};

Is there any ambiguity in 1?

Answer 1

The difference is that "Usage 1" should not compile. When a dependent name like T::B is not preceded by typename , it is assumed to refer to an object or function member. In which case, T::B x; is invalid syntax.

Answer 2

According to the standard, $14.6/2 Name resolution [temp.res]

A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword typename.

That means, if you don't use the keyword typename for T::B x; , T::B will be considered as a non-type name, such as a member B of T , whick makes T::B x; ill-formed.