c++ - How can I make the map::find operation case insensitive?

Question

Does the map::find method support case insensitive search? I have a map as follows:

map<string, vector<string> > directory;

and want the below search to ignore case:

directory.find(search_string);

Answer 1

It does not by default. You will have to provide a custom comparator as a third argument. Following snippet will help you...

  /************************************************************************/
  /* Comparator for case-insensitive comparison in STL assos. containers  */
  /************************************************************************/
  struct ci_less : std::binary_function<std::string, std::string, bool>
  {
    // case-independent (ci) compare_less binary function
    struct nocase_compare : public std::binary_function<unsigned char,unsigned char,bool> 
    {
      bool operator() (const unsigned char& c1, const unsigned char& c2) const {
          return tolower (c1) < tolower (c2); 
      }
    };
    bool operator() (const std::string & s1, const std::string & s2) const {
      return std::lexicographical_compare 
        (s1.begin (), s1.end (),   // source range
        s2.begin (), s2.end (),   // dest range
        nocase_compare ());  // comparison
    }
  };

Use it like std::map< std::string, std::vector<std::string>, ci_less > myMap;

NOTE: std::lexicographical_compare has some nitty-gritty details. String comparison isn't always straightforward if you consider locales. See this thread on c.l.c++ if interested.

UPDATE: With C++11 std::binary_function is deprecated and is unnecessary as the types are deduced automatically.

  struct ci_less
  {
    // case-independent (ci) compare_less binary function
    struct nocase_compare
    {
      bool operator() (const unsigned char& c1, const unsigned char& c2) const {
          return tolower (c1) < tolower (c2); 
      }
    };
    bool operator() (const std::string & s1, const std::string & s2) const {
      return std::lexicographical_compare 
        (s1.begin (), s1.end (),   // source range
        s2.begin (), s2.end (),   // dest range
        nocase_compare ());  // comparison
    }
  };