What is causing this template-related compile error?

When I try to compile this:

#include <map>
#include <string>

template <class T>
class ZUniquePool
{               
        typedef std::map< int, T* > ZObjectMap;
        ZObjectMap      m_objects;
public:
    T * Get( int id )
    {
        ZObjectMap::const_iterator it = m_objects.find( id );
        if( it == m_objects.end() )
        {
            T * p = new T;
            m_objects[ id ] = p;
            return p;
        }
        return m_objects[ id ];
    }
};

int main( int argc, char * args )
{
    ZUniquePool< std::string > pool;
    return 0;
}

I get this:

main.cpp: In member function ‘T* ZUniquePool<T>::Get(int)’:
main.cpp:12: error: expected `;' before ‘it’
main.cpp:13: error: ‘it’ was not declared in this scope

I'm using GCC 4.2.1 on Mac OS开发者_C百科 X. It works in VS2008.

I'm wondering whether it might be a variation of this problem: Why doesn't this C++ template code compile? But as my error output is only partially similar, and my code works in VS2008, I am not sure.

Can anyone shed some light on what I am doing wrong?

You need typename:

typename ZObjectMap::const_iterator it = m_objects.find( id )

Since the type of ZObjectMap is dependent on the template parameter, the compiler has no clue what ZObjectMap::const_iterator is (it could be a member variable). You need to use typename to inform the compiler to assume it will be some sort of type.

Also to mention, GCC 4.5.0 gives the following output which would help you to solve the problem:

main.cpp: In member function 'T* ZUniquePool<T>::Get(int)':
main.cpp:12:7: error: need 'typename' before 'ZUniquePool<T>::ZObjectMap:: const_iterator' because 'ZUniquePool<T>::ZObjectMap' is a dependent scope
main.cpp:12:34: error: expected ';' before 'it'
main.cpp:13:11: error: 'it' was not declared in this scope
main.cpp: At global scope:
main.cpp:23:5: warning: second argument of 'int main(int, char*)' should be 'char **'