If you are using gcc version 3.3.6 you'll need to wrap operator() in a class or struct:

class HashString 
{ 
        public: 
                unsigned operator()(std::string const &str) const 
                { 
                        return __gnu_cxx::hash<char const*>()(str.c_str()); 
                } 
}; 

then declare the hash_map like so:

hash_map<string,string,HashString> _blahBlahMemberVariable ;

If you are using hashmap as an extension:

#include <ext/hash_map> 

#define __gnu_cxx std 
#include <ext/hash_map> 
using namespace std; 

link