c++ - Undefined reference to header file function -

i got error of c:\temp\hashtableproject\main.cpp|14|undefined reference hash::hash(std::string) know how solve problem?

hash.h

#include <iostream> #include <cstdlib> #include <string> using namespace std;  #ifndef hash_h #define hash_h  class hash{ public:      int hash(string key); };  #endif // hash_h 

hash.cpp

#include <iostream> #include <cstdlib> #include <string>  #include "hash.h"  using namespace std;   int hash::hash(string key){      //int hash = 0;     int index;     index = key.length();      return index; } 

main.cpp

#include <iostream> #include <cstdlib> #include <string>  #include "hash.h"  using namespace std;  int main() {     int index;     hash hashob;     string traget = "testing";     index = hashob.hash(traget);      cout << index << endl;      return 0; } 

im using codeblock 13.12 there main.o file in obj folder. dont know why hash.o isn't there.

hash inbuilt template in "std" namespace.

try removing using namespace std; lien , use namespace name , when required.