std::collate<CharT>::transform,do_transform (3) - Linux Man Pages

std::collate<CharT>::transform,do_transform: std::collate<CharT>::transform,do_transform


std::collate<CharT>::transform,do_transform - std::collate<CharT>::transform,do_transform


Defined in header <locale>
public: (1)
string_type transform( const CharT* low, const CharT* high ) const;
protected: (2)
virtual string_type do_transform( const CharT* low, const CharT* high ) const;

1) Public member function, calls the protected virtual member function do_transform of the most derived class.
2) Converts the character sequence [low, high) to a string that, compared lexicographically (e.g. with operator< for strings) with the result of calling transform() on another string, produces the same result as calling do_compare() on the same two strings.


low - pointer to the first character in the sequence to transform
high - one past the end pointer for the sequence to transform

Return value

The string transformed so that lexicographic comparison of the transformed strings may be used instead of collating of the originals. In the "C" locale, the returned string is the exact copy of [low, high). In other locales, the contents of the returned string are implementation-defined, and the size may be considerably longer.


In addition to the use in collation, the implementation-specific format of the transformed string is known to std::regex_traits<>::transform_primary, which is able to extract the equivalence class information.


// Run this code

  #include <iostream>
  #include <iomanip>
  #include <locale>

  int main()
      auto& f = std::use_facet<std::collate<wchar_t>>(std::locale());

      std::wstring in1 = L"\u00e4ngel";
      std::wstring in2 = L"\u00e5r";

      std::wstring out1 = f.transform(&in1[0], &in1[0] + in1.size());
      std::wstring out2 = f.transform(&in2[0], &in2[0] + in2.size());

      std::wcout << "In the Swedish locale: ";
      if(out1 < out2)
           std::wcout << in1 << " before " << in2 << '\n';
           std::wcout << in2 << " before " << in1 << '\n';

      std::wcout << "In lexicographic comparison: ";
      if(in1 < in2)
           std::wcout << in1 << " before " << in2 << '\n';
           std::wcout << in2 << " before " << in1 << '\n';



  In the Swedish locale: år before ängel
  In lexicographic comparison: ängel before år

See also

        transform a string so that strcmp would produce the same result as strcoll
strxfrm (function)
        transform a wide string so that wcscmp would produce the same result as wcscoll
wcsxfrm (function)