unordered_map with custom hashing/equal functions - functions don't get called [duplicate]

Question

This is weird.. the following code (which I managed to compile thanks to Cassio Neri) is compiling without any error.. by the way either hashing_func nor key_equal_func do get called (the couts aren't showing in the console window)

#include <iostream>
#include <string>
#include <unordered_map>
#include <algorithm>
#include <functional>

using namespace std;

unsigned long hashing_func(string key)
{
    cout << "Hashing called";
    unsigned long hash = 0;
    for(int i=0; i<key.size(); i++)
    {
        hash += (71*hash + key[i]) % 5;
    }
    return hash;
}

template<class T> bool key_equal_fn(T t1, T t2)
{
    return t1 == t2;
}

template <> bool key_equal_fn<string>(string t1, string t2)
{
    cout << "Equal called";
    return !(t1.compare(t2));
}

int main ()
{
    unordered_map<string, string>::size_type n = 5;
    unordered_map<string, string> mymap(n, (const std::hash<string> &)hashing_func, 
        (const std::equal_to<string> &)(function<bool(string,string)>(key_equal_fn<string>))) ;

    bool case_insensitive = mymap.key_eq()("test","TEST");

    mymap["paul"] = "jenna";
    mymap["frank"] = "ashley";

    if(mymap["paul"] == mymap["frank"])
        cout << "equal" << endl;


    return 0;
}

I'm using MSVC2012, any hint on what could be the problem?

Answer 1

You have to specify hash/compare functions with template arguments, not in the constructor. Here is an example:

class Hasher
{
public:
  size_t operator() (string const& key) const
  {
      cout << "Hashing called";
      size_t hash = 0;
      for(size_t i=0; i<key.size(); i++)
      {
          hash += (71*hash + key[i]) % 5;
      }
      return hash;
  }
};
class EqualFn
{
public:
  bool operator() (string const& t1, string const& t2) const
  {
    cout << "Equal called";
    return !(t1.compare(t2));
  }
};

unordered_map<string, string, Hasher, EqualFn> mymap(5);