Can't make a hash_map with std::string for key

[Jonathan Gilligan]

How can I make a hash map with an std::string for a key? VC++ .NET barfs, complaining that it cannot convert from type 'std::string' to 'size_t'. Why couldn't Science, in the long run, serve As well as one's uncleared lunch-table or Mme X en Culottes de Matador?     James Merrill

[John M Drescher]

I don't know how to do this but you need to write a hash function. John

[John M Drescher]

I found this on a web search:

#include <iostream>
#include <string>
#include <hash_map>
using namespace std;

/** BEGIN FIX **/
namespace std
{
template<> struct hash< std::string >
{
size_t operator()( const std::string& x ) const
{
return hash< const char* >()( x.c_str() );
}
};
}
/** END FIX **/

hash_map <string, int> months;
int main(void) {
months["january"] = 31;
months["february"] = 28;
return 0;
}

John

[ZoogieZork]

Here's the hash function I've been using:

namespace std {
template<>
size_t hash_compare<string>::operator()(const string &s) const
{
unsigned long retv = 0;
for (string::const_iterator iter = s.begin();
iter != s.end(); ++iter)
{
retv = 5 * retv + *iter;
}
return size_t(retv);
}
}

There's probably a better way to do it, though. :) - Mike

[Stuart Dootson]

The other ways, of course, to get working hash_maps would be to either

1. use STLPort (where hash_map and hash_set have a hash comparison function defined for std::strings - and also you get nice features like debug mode, which tells you when you've made boo-boos like pushing an iterator outside a container, or used mismatched iterators)
2. Upgrade to VS.NET 2003. It cost me GBP19 (USD30 roughly) and was worth every penny - especially for the way more compliant compiler and faster IDE (well, I think it is - subjectively).

Of course, the solution in the other posts is rather more immediate :-D Stuart Dootson 'Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p'

[Alexandru Savescu]

Here's how Java get a hash code for the string. The algorithm should be fine: /** * Returns a hash code for this string. The hash code for a * String object is computed as *

 \* s\[0\]\*31^(n-1) + s\[1\]\*31^(n-2) + ... + s\[n-1\]
 \* 

* using int arithmetic, where s[i] is the * _i_th character of the string, n is the length of * the string, and ^ indicates exponentiation. * (The hash value of the empty string is zero.) * * @return a hash code value for this object. */ Best regards, Alexandru Savescu P.S. Interested in art? Visit this!

[Jonathan Gilligan]

Thanks to everyone who helped explain this. Thanks especially for the code fragments. Why couldn't Science, in the long run, serve As well as one's uncleared lunch-table or Mme X en Culottes de Matador?     James Merrill