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                       Standard C++ Library
             Copyright 1998, Rogue Wave Software, Inc.


NAME
     collate, collate_byname

      - A string collation, comparison, and hashing facet.



SYNOPSIS
     #include <locale>
     template <class charT> class collate;
     template <class charT> class collate_byname;



DESCRIPTION
     The collate and collate_byname facets allow for string  col-
     lation,  comparison,  and hashing. Use the collate facet for
     the  "C"  locale,  and  use  the  collate_byname  for  named
     locales.



INTERFACE
     template <class charT>
     class collate : public locale::facet {
     public:
      typedef charT               char_type;
      typedef basic_string<charT> string_type;
      explicit collate(size_t refs = 0);
      int compare(const charT*, const charT*,
                  const charT*, const charT*) const;
      string_type transform(const charT*, const charT*) const;
      long hash(const charT*, const charT*) const;
      static locale::id id;
     protected:
       ~collate();  // virtual
      virtual int do_compare(const charT*, const charT*,
                             const charT*, const charT*) const;
      virtual string_type do_transform(const charT*,
                                  const charT*) const;
      virtual long do_hash (const charT*, const charT*) const;
     };

     template <class charT>
     class collate_byname : public collate<charT> {
     public:
      typedef basic_string<charT> string_type;
      explicit collate_byname(const char*, size_t = 0);
     protected:
       ~collate_byname();  // virtual
      virtual int do_compare(const charT*, const charT*,
                             const charT*, const charT*) const;
      virtual string_type do_transform(const charT*,
                                  const charT*) const;
      virtual long do_hash(const charT*, const charT*) const;
     };



TYPES
     char_type


        Type of character the facet is instantiated on.



     string_type


        Type of character string returned by member functions.



CONSTRUCTORS
     explicit collate(size_t refs = 0)


        Construct a collate facet. If the  refs  argument  is  0,
        destruction  of the object is delegated to the locale, or
        locales, containing it. This allows the  user  to  ignore
        lifetime management issues. On the other hand, if refs is
        1, the object must be explicitly deleted: the locale does
        not do so.



     explicit collate_byname(const char* name, size_t refs = 0);


        Construct a collate_byname facet. Use  the  named  locale
        specified  by the name argument. The refs argument serves
        the same purpose as it does for the collate constructor.

DESTRUCTORS
     ~collate();  // virtual and protected
     ~collate_byname();  // virtual and protected


        Destroy the facet.



FACET ID
     static locale::id id;


        Unique identifier for this type of facet.



PUBLIC MEMBER FUNCTIONS
     The public members of the collate facet include an interface
     to   protected   members.  Each  public  member  xxx  has  a
     corresponding virtual protected member do_xxx. All  work  is
     delegated  to  these  protected members.   For instance, the
     long version of the public compare function simply calls its
     protected cousin do_compare.


     int
     compare(const charT* low1, const charT* high1,
            const charT* low2, const charT* high2) const;
     long
     hash(const charT* low, const charT* high) const;
     string_type
     transform(const charT* low, const charT* high) const;


        Each of these public member functions xxx simply call the
        corresponding protected do_xxx function.



PROTECTED MEMBER FUNCTIONS
     virtual int
     do_compare(const charT* low1, const charT* high1,
               const charT* low2, const charT* high2) const;


        Returns 1 if the  character  string  represented  by  the
        range  [low1,high1)  is greater than the character string
        represented by the range [low2,high2), -1 if first string
        is  less than the second, or 0 if the two are equal.  The
        default      instantiations,      collate<char>       and
        collate<wchar_t>, perform a lexicographical comparison.



     virtual long
     do_hash( const charT* low, const charT* high)


        Generates a hash value from a string defined by the range
        of  characters [low,high). Given two strings that compare
        equal (in other words,  do_compare  returns  0),  do_hash
        returns  an  integer  value  that  is  the  same for both
        strings. For differing strings the probability  that  the
        return value is equal is approximately:

        1.0/numeric_limits<unsigned long>::max()



     virtual string_type
     do_transform(const charT* low, const charT* high) const;


        Returns a string that yields the same result in a lexico-
        graphical  comparison  with  another string returned from
        transform as does the do_compare function applied to  the
        original  strings. In other words, the result of applying
        a lexicographical comparison to two strings returned from
        transform  is the same as applying do_compare to the ori-
        ginal strings passed to transform.



EXAMPLE
     //
     // collate.cpp
     //
     #include <iostream>

     int main ()
     {
      using namespace std;
      locale loc;
      string s1("blue");
      string s2("blues");
       // Get a reference to the collate<char> facet
      const collate<char>& co =
     #ifndef _RWSTD_NO_TEMPLATE_ON_RETURN_TYPE
          use_facet<collate<char> >(loc);
     #else
          use_facet(loc,(collate<char>*)0);
     #endif
       // Compare two strings
      cout << co.compare(s1.begin(),s1.end(),
                         s2.begin(),s2.end()-1) << endl;
      cout << co.compare(s1.begin(),s1.end(),
                         s2.begin(),s2.end()) << endl;
       // Retrieve hash values for two strings
      cout << co.hash(s1.begin(),s1.end()) << endl;
      cout << co.hash(s2.begin(),s2.end()) << endl;
      return 0;
     }



SEE ALSO
     locale, facets, ctype

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