C++: Having trouble using map with a static variable in a class, I get undefined symbol -

this error get

undefined symbols:   "config::fmap", referenced from:       register_function(unsigned int (*)(unsigned int))in config.o       print_config()     in shared.o ld: symbol(s) not found collect2: ld returned 1 exit status 

config.h

#include <map> #include <boost/preprocessor/stringize.hpp>  typedef unsigned int (*fptr_t)(unsigned int); typedef std::map<fptr_t, std::string> function_name_map_type;  void register_hash_names(); void register_function(fptr_t funct_pointer);  class config { public:      static function_name_map_type fmap; }; 

config.cpp

function_map_t fmap;   void register_hash_names() {     register_function(sha1);     ... }  void register_function(fptr_t funct_pointer) {     (config::fmap)[funct_pointer] = boost_pp_stringize(funct_pointer); } 

shared.cpp error originates from:

std::cout << "\t " << config::fmap[config::current_hash_function] << "\n"; 

the definition in .cpp should be:

function_map_t config::fmap;