std::collate<CharT>::hash,std::collate<CharT>::do_hash (3) - Linux Manuals

std::collate<CharT>::hash,std::collate<CharT>::do_hash: std::collate<CharT>::hash,std::collate<CharT>::do_hash

NAME

std::collate<CharT>::hash,std::collate<CharT>::do_hash - std::collate<CharT>::hash,std::collate<CharT>::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 <string>
  #include <locale>
  #include <unordered_set>

  struct CollateHash {
      template<typename CharT>
      std::size_t 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