Why is typename required in such a case?

Asked Aug 06 '19 at 12:07

Active Aug 06 '19 at 12:13

Viewed 47 times

#include <string>

template<typename>
void g()
{}

template<typename T>
void f()
{
    g<typename T::value_type>(); // ok
    g<T::value_type>(); // ok
    typename T::value_type n1; // ok
    T::value_type n2; // failure        
}

int main()
{
    f<std::string>();
}

Compiled with vc++.

In the code above, whether T::value_type is a type or a variable can be determined at compile-time. There is no ambiguity here, or g<T::value_type>() will also be required written as g<typename T::value_type>().