16.2.8 The typename Keyword

The typename keyword was added to C++ after the initial specification and is not recognized by all compilers. It is a hint to the compiler that a name following the keyword is the name of a type. In the usual case, the compiler has sufficient context to know that a symbol is a defined type, as it must have been encountered earlier in the compilation:

class Foo { public: typedef int map_t; }; void func () { Foo::map_t m; }

Here, map_t is a type defined in class Foo. However, if func happened to be a function template, the class which contains the map_t type may be a template parameter. In this case, the compiler simply needs to be guided by qualifying T::map_t as a type name:

class Foo { public: typedef int map_t; }; template <typename T> void func () { typename T::map_t t; }