In template definitions, provides a hint to the compiler that an unknown identifier is a type. In template parameter lists, is used to specify a type parameter.

  
typename identifier;  

This keyword must be used if a name in a template definition is a qualified name that is dependent on a template argument; it is optional if the qualified name is not dependent. For more information, see Templates and Name Resolution.

typename can be used by any type anywhere in a template declaration or definition. It is not allowed in the base class list, unless as a template argument to a template base class.

template <class T>  
class C1 : typename T::InnerType // Error - typename not allowed.  
{};  
template <class T>  
class C2 : A<typename T::InnerType>  // typename OK.  
{};  

The typename keyword can also be used in place of class in template parameter lists. For example, the following statements are semantically equivalent:

template<class T1, class T2>...  
template<typename T1, typename T2>...  

// typename.cpp  
template<class T> class X  
{  
   typename T::Y m_y;   // treat Y as a type  
};  
  
int main()  
{  
}  

Templates
Keywords