c++ - Does std::unordered_map<std::string,std::function<void(std::string&) can hold only static functions? -

this continuation of question c++ function ptr in unorderer_map, compile time error

i trying use std::function instead function pointer, , can insert function if functions static. otherwise following error

main.cpp:15:11: error: no matching member function call 'insert'

  map.insert(std::make_pair("one",&example::processtring)); 

#include<string> #include <unordered_map> #include<functional>  namespace test {  namespace test {   class example   {   public:     example()     {        map.insert(std::make_pair("one",&example::processtring));     }     static void processtring(std::string & astring)     //void processtring(std::string & astring) -> compiler error      {      }     static  void processstringtwo(std::string & astring)     {      }      std::unordered_map<std::string,std::function<void(std::string&)>> map;   }; } }  int main() {   return 0; } 

in context, std::function type wrong. member function example::processstring(std::string&):

std::function<void(example*, std::string&)> 

however, can avoid , "eat up" this parameter binding:

using std::placeholders; map.insert(std::make_pair("one", std::bind(&example::processstring, this, _1)); 

now argument left unbound string reference, type can remain:

std::function<void(std::string&)>