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In template definitions, typename provides a hint to the compiler that an unknown identifier is a type. In template parameter lists, it's used to specify a type parameter.
typenameidentifier;
The typename keyword must be used if a name in a template definition is a qualified name that is dependent on a template argument; it's optional if the qualified name isn't dependent. For more information, see Templates and Name Resolution.
typename can be used by any type anywhere in a template declaration or definition. It isn't 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()
{
}