Hi,

I got compilation error with the following C++ program. This program runs perfectly in Microsoft Visual Studio 2008. I cannot understand why it produced compilation error ! As a result, I had to write it in C#.

#include <iostream>
#include <set>
#include <vector>
#include <hash_map>
#include <string>

using namespace std;

int main(){
    int n = 0;
    set<string> s;
    vector<string> v;
    hash_map<string, int> hm;

    cin>>n;
    for(int i = 0; i < n; i++){
        string temp = "";
        cin>>temp;
        s.insert(temp);
        v.push_back(temp);
    }

    set<string>::iterator it = s.begin();

    for(size_t i = 0; i < s.size(); i++){
        hm.insert(make_pair(*it, 0)); // ERROR IN THIS LINE FOR SOME REASON
        it++;
    }

    for(size_t i = 0; i < v.size(); i++){
        hash_map<string, int>::iterator it = hm.begin();
        it = hm.find(v[i]);
        (it->second)++;
    }

    hash_map<string, int>::iterator hit = hm.begin();

    for(size_t i = 0; i < hm.size(); i++){
        if(hit->second > 1){
            cout<<hit->first<<endl;
        }
        hit++;
    }
}

Here is the error message:-

7c683836-7a29-4ffc-849b-00c6921bbbe5
S:\checker\compile\\Vc7\include\xhash(38): error: no suitable conversion function from "const std::string" to "size_t={unsigned int}" exists
          return ((size_t)_Keyval);

                          ^
          detected during:
            instantiation of "size_t={unsigned int} std::hash_compare<_Kty, _Pr>::operator()(const _Kty &) const [with _Kty=std::string, _Pr=std::less<std::string>]" at line 217
            instantiation of "std::_Hash<_Tr>::_Pairib std::_Hash<_Tr>::insert(const std::_Hash<_Tr>::value_type &) [with _Tr=std::_Hmap_traits<std::string, int, std::hash_compare<std::string, std::less<std::string>>, std::allocator<std::pair<const std::string, int>>, false>]" at line 28 of "7c683836-7a29-4ffc-849b-00c6921bbbe5"

compilation aborted for 7c683836-7a29-4ffc-849b-00c6921bbbe5 (code 2)

Thanks

hash_map is not in standard C++ library yet. It is Microsoft extension. You use it on your own risk.

It seems that Intel C++ library can only hash values that can be converted into size_t using type cast operator, and std::string has not such cast.

It is _very_ simple problem, you do not need very fast data structure here. std::map can be used. It is the simplest solution.

If anyway it is required to use hash_map, then I'd suggest to write wrapper compatible with std::string and int and use the wrapper as key type of hash_map:

class HashableString {

   std::string _value;

public:

   HashableString(const std::string &value)
      : _value(value)
   {}

   operator std::string() const {
      return _value;
   }

   operator size_t() const {
      return _value.size(); // bad hash!
   }
};

std::ostream & operator << (std::ostream &out, const HashableString &value) {
   return out << (std::string)value;
}

Hi,

Thanks Fyodor. You are absolutely right. The hashmap over here does not support any other keys except for size_t. But the next C++ standard will have hash_map as part of C++.

But thanks for your detailed explanation and time (may be that's why you are No. 40 :-))

Varun