Visual C++ STL: How do I use hash maps with Visual C++?

Q: How to use the hash_map class in C++

A: If you have Visual Studio .NET 2003 you can try the 'hash_map' class instead of the map if you want to use hash tables instead of binary trees to store your pairs 'key -> value'. There follows a example of a hash_map.

I know that it's a lot easier to use the hash_map from the SGI's STL implementation, but if you must use Visual Studio .NET 2003 you'll need to know this.

Code:

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#include <string>
#include <hash_map>
#include <iostream>

// The following class defines a hash function for strings
class stringhasher : public stdext::hash_compare <std::string>
{
public:
  /**
  * Required by
  * Inspired by the java.lang.String.hashCode() algorithm
  * (it's easy to understand, and somewhat processor cache-friendly)
  * @param The string to be hashed
  * @return The hash value of s
  */
  size_t operator() (const std::string& s) const
  {
    size_t h = 0;
    std::string::const_iterator p, p_end;
    for(p = s.begin(), p_end = s.end(); p != p_end; ++p)
    {
      h = 31 * h + (*p);
    }
    return h;
  }

  /**
  *
  * @param s1 The first string
  * @param s2 The second string
  * @return true if the first string comes before the second in lexicographical order
  */
  bool operator() (const std::string& s1, const std::string& s2) const
  {
    return s1 < s2;
  }
};

typedef stdext::hash_map<std::string, std::string, stringhasher> HASH_S_S;

/**
 * Compile this program with "cl -GX hash_sample.cpp"
 * if using MSVC++ 7.1 (Visual Studio .NET 2003)
 */
int main (int argc, char *argv[])
{
  HASH_S_S hm;
  HASH_S_S::iterator it;
   
  //-- Inserting the names of months in a hash map
  //-- key = names of the months in Portuguese
  //-- value = names of the months in English
  hm[std::string("janeiro")] = std::string("January");
  hm[std::string("fevereiro")] = std::string("February");
  hm[std::string("março")] = std::string("March");
  hm[std::string("abril")] = std::string("April");
  hm[std::string("maio")] = std::string("May");
  hm[std::string("junho")] = std::string("June");
  hm[std::string("julho")] = std::string("July");
  hm[std::string("agosto")] = std::string("August");
  hm[std::string("setembro")] = std::string("September");
  hm[std::string("outubro")] = std::string("October");
  hm[std::string("novembro")] = std::string("November");
   
  //-- Searching for the translation of the months "março" and "dezembro" in the map
  //-- (dezembro was not put into the map, so you can not find the corresponding element "December")
  it = hm.find(std::string("março"));
  if(it != hm.end())
    std::cout << "The value corresponding to the key 'março' is " << it->second << std::endl;
  else
    std::cout << "The value corresponding to the key 'março' was not found" << std::endl;

  it = hm.find(std::string("dezembro"));
  if(it != hm.end())
    std::cout << "The value corresponding to the key 'dezembro' is " << it->second << std::endl;
  else
    std::cout << "The value corresponding to the key 'dezembro' was not found" << std::endl;
       
  //-- Listing the pairs key, value (they're not ordered.)
  for(it = hm.begin(); it != hm.end(); ++it)
  {
    std::cout << "Key = \'" << it->first << "\' -> value = \'" << it->second << "\'" << std::endl;
  }
}

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