Officially, what is typename for?

Question

On occasion I\'ve seen some really indecipherable error messages spit out by gcc when using templates... Specifically, I\'ve had problems where seemingly correc

Answer 1

I think all of the answers have mentioned that the typename keyword, is used in two different cases:

a) When declaring a template type parameter. e.g.

template class MyClass{};        // these two cases are
template class MyNewClass{};  // exactly the same.

Which there is no difference between them and they are EXACTLY the same.

b) Before using a nested dependent type name for a template.

template
void foo(const T & param)
{
   typename T::NestedType * value; // we should use typename here
}

Which not using typename leads to parsing/compilation errors.

What I want to add to the second case, as mentioned in Scot Meyers book Effective C++, is that there is an exception of using typename before a nested dependant type name. The exception is that if you use the nested dependant type name either as a base class or in a member initialization list, you should not use typename there:

template
class D : public B::NestedType               // No need for typename here
{
public:
   D(std::string str) : B::NestedType(str)   // No need for typename here
   {
      typename B::AnotherNestedType * x;     // typename is needed here
   }
}

Note: Using typename for the second case (i.e. before nested dependent type name) is not needed since C++20.