C++ unordered_map problem

Question

This time I'm able to show a complete code:

#include <unordered_map>
#include <iostream>
#include <stdlib.h>

using namespace std;

bool mystrcmp(const char *s1, const char *s2) {
        int i = 0;
        do {
                if(s1[i] != s2[i])
                        return false;
        } while(s1[i++] != '\0');
        return true;
}

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return mystrcmp(s1, s2);
  }
};


int main(void) {
    char buffer[5] = {'h', 'e', 'd', 'e', '\0'};
    unordered_map<char *, int , hash<char *> , eqstr> int_from_symbols;
    int_from_symbols["hede"] = 1;
    int_from_symbols["hodo"] = 2;
    unordered_map<char *, int , hash<char *> , eqstr>::const_iterator it = int_from_symbols.find(buffer);
    eqstr myeq;
    if(myeq("hede",buffer))
        fprintf(stderr, "no problem here\n");
    if(it == int_from_symbols.end())
        fprintf(stderr, "dammit\n");
    else fprintf(stderr, "%d\n", int_from_symbols[buffer]);
    return 0;
}

This outputs:

no problem here
dammit

Any idea what's going on?

Thanks in advance,,
Onur

R Samuel Klatchko · Accepted Answer · 2009-12-25 07:17:13Z

2

The problem is that hash<char *> does not do what you want. It is not specialized to actually hash the 'string' but is simply returning the pointer as the hash.

Add this to your code and it will start working (although the hash function is not production quality and is for demonstration only):

namespace std
{
    template<>
    struct hash<char *> : public std::unary_function<char *, size_t>
    {
        size_t operator()(char* str) const
        { 
            size_t h = 0;
            for (; *str; ++str)
                h += *str;
            return h;
        }
    };
}

answered Dec 25, 2009 at 7:17

R Samuel Klatchko

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6 Comments

Pavel Minaev Over a year ago

I don't think that globally specializing hash<char*> for this is a good idea - among other things, this might make it very easy to break ODR. Why not just define a custom functor, and pass that as a template argument to unordered_map, as usual?

Hans Passant Over a year ago

Check your <functional> header for the hash<string> specialization.

Onur Cobanoglu Over a year ago

This is really annoying. I implemented other hash functions which encodes the structures as strings and used hash<char *> to hash those. Those hash functions worked correctly, if hash<char *> works incorrectly, why other ones worked correctly?

Onur Cobanoglu Over a year ago

OK, you are right. hash<char *> barely returns the pointer. Thank you for the answer, however, I need a good quality hash function for strings. Can you recommend me one?

Thomas Over a year ago

The namespace std is reserved. Never, ever drop anything of your own into it.

|

bliako · Accepted Answer · 2012-02-25 23:29:11Z

1

a review of a few hash function algoirthms for 'char *' here:

http://www.cse.yorku.ca/~oz/hash.html

the sum of ascii codes mentioned above is not the best as the author of the comment says.

answered Feb 25, 2012 at 23:29

bliako

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