std::hash specialization remains unused by std::unordered_map

Question

I am trying to extend std::hash<T> by supplying a specialization for const char , so that I can use const char* as key type in std::unordered_map .

This is what I tried:

#include <unordered_map>
#include <functional>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

namespace std
{
    template<>
    struct hash<const char*>
    {
        size_t operator()(const char* const& s) const
        {
           size_t h = 0;
           const char* tmp = s;

           while (*tmp)
              h = (h << 5) + h + (unsigned char)toupper(*tmp++);

           printf("hash of '%s' is (%ld)\n", s, h);
           return h;
        }
    };
}

int main(int argc, char* argv[])
{
   const char* name1= "Mark Nelson";
   const char* name2= strdup(name1);

   std::unordered_map<const char*, int> map;

   printf("Insert (%s)\n", name1);
   map[name1]= 100;

   printf("Lookup (%s)\n", name1);
   printf("map[%s](name1) = %d\n", name1, map.find(name1) != map.end() ? map.find(name1)->second : -1);
   printf("Lookup (%s)\n", name2);
   printf("map[%s](name2) = %d\n", name2, map.find(name2) != map.end() ? map.find(name2)->second : -1);

   return 0;
}

What the output is:

Insert (Mark Nelson)
hash of 'Mark Nelson' is (121066894705597050)
Lookup (Mark Nelson)
hash of 'Mark Nelson' is (121066894705597050)
hash of 'Mark Nelson' is (121066894705597050)
map[Mark Nelson](name1) = 100
Lookup (Mark Nelson)
hash of 'Mark Nelson' is (121066894705597050)
map[Mark Nelson](name2) = -1

So to me it seems, std::unordered_map is using my provided hash implementation for both the insertion as well as the lookup. But for some reason, it does not find the value via name2 , which seems like it is still using pointer comparison instead.

What is wrong here? I am using GCC 4.8.2, and the file was compiled with g++ -std=c++11 main.cc

Answer 1

You need two things for an unordered map ( ref ):

1. A hash function
2. A comparison function for equality

So you have (1) which appears to work but then the map has to check the const char * for equality which devolves into a pointer comparison. To do this you can specify a custom comparator ( ref ):

struct cmp_str
{
   bool operator()(char const *a, char const *b)
   {
      return std::strcmp(a, b) == 0;
   }
};

std::unordered_map<const char*, int, std::hash<const char *>, cmp_str> BlahBlah;