模板类 的 typename 关键字

 

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.

例如：

// typename.cpp

template<class T> class X

{

typename T::Y m_y; // treat Y as a type

};

 

例如下面的程序，不加typename 关键字会编译错误。

 //-------------------------------------------------------------------

#include <QtCore/QCoreApplication>

#include <map>

template <class T>

class MySetting

{

    public:

    void function();

private:

    T   m_data;

};

//////////////////////////////////////////////////////////////////////////

template <class T>

void MySetting<T>::function()

{

   typename std::map<int, T>::iterator  myit;  //此处要用 typename 否则qt的mingw编译提示出错。vs2008不出错

   }

int main(int argc, char *argv[])

{

    QCoreApplication a(argc, argv);

    std::map<int, int>::iterator  myit;

    return a.exec();

}