std::collate::hash,std::collate::do_hash(3)

NAME
std::collate::hash,std::collate::do_hash - std::collate::hash,std::collate::do_hash

Synopsis
   Defined in header <locale>
   public: (1)
   long hash( const CharT* beg, const CharT* end ) const;
   protected: (2)
   virtual long do_hash( const CharT* beg, const CharT* end ) const;
   1) Public member function, calls the protected virtual member function do_hash of
   the most derived class.
   2) Converts the character sequence [beg, end) to an integer value that is equal to
   the hash obtained for all strings that collate equivalent in this locale (compare()
   returns 0). For two strings that do not collate equivalent, the probability that
   their hashes are equal should be very small, approaching 1.0 /
   std::numeric_limits<unsigned long>::max().

Parameters
   beg - pointer to the first character in the sequence to hash
   end - one past the end pointer for the sequence to hash

Return value
   The hash value that respects collation order.

Note
   The system-supplied locales normally do not collate two strings as equivalent
   (compare() does not return 0) if basic_string::operator== returns false, but a
   user-installed std::collate facet may provide different collation rules, for
   example, it may treat strings as equivalent if they have the same Unicode normalized
   form.

Example
   Demonstrates a locale-aware unordered container. // Run this code
 #include <iostream>
 #include <locale>
 #include <string>
 #include <unordered_set>
 struct CollateHash
 {
     template<typename CharT>
     long operator()(const std::basic_string<CharT>& s) const
     {
         return std::use_facet<std::collate<CharT>>(std::locale()).hash(
                    &s[0], &s[0] + s.size()
                );
     }
 };
 struct CollateEq
 {
     template<typename CharT>
     bool operator()(const std::basic_string<CharT>& s1,
                     const std::basic_string<CharT>& s2) const
     {
         return std::use_facet<std::collate<CharT>>(std::locale()).compare(
                      &s1[0], &s1[0] + s1.size(),
                      &s2[0], &s2[0] + s2.size()
                ) == 0;
     }
 };
 int main()
 {
     std::locale::global(std::locale("en_US.utf8"));
     std::wcout.imbue(std::locale());
     std::unordered_set<std::wstring, CollateHash, CollateEq> s2 = {L"Foo", L"Bar"};
     for (auto& str : s2)
         std::wcout << str << ' ';
     std::cout << '\n';
 }

Possible output:
 Bar Foo

See also
   std::hash<std::basic_string> hash support for strings
   (C++11) (class template specialization)