Error - boost::unordered_map is not derived from type A

When we use STL or boost template classes like boost::unordered_map together with our own template class, as below:

#include "boost/unordered_map.hpp"#include <string> 
template < typename T >
class A
{
typedef boost::unordered_map< std:;string, T* > FtObjsTypeMap;} 

When we compile above class definition with gcc, following compilation error will be reported: 

A.h:7: error: type âboost::unordered_map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, T*, boost::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::equal_to<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, T*> > >â is not derived from type âA<T>â

The reason is that This because boost::unordered_map< std:;string, T* > is dependent on the template parameter T of the class template A To enable correct parsing of the template without having to make any assumptions about possible specializations of any other templates, the language rules require you to indicate which dependent names denote types by using the typename keyword.

Following class A definition will pass the compilation: 

#include "boost/unordered_map.hpp"#include <string> 
template < typename T >
class A
{
typedef typename boost::unordered_map< std:;string, T* > FtObjsTypeMap;} 

The reason of the error is found at http://stackoverflow.com/questions/2841757/c-template-is-not-derived-from-type for my own recording and its mix usage with typedef, I posted my version of code here for future reference.