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typename

Tells the compiler that an unknown identifier is a type.

typename identifier;

Use this keyword only in template definitions.

This keyword must be used if the name is a qualified name 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 identical:

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()
{
}

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