std::ctype::narrow,do_narrow(3)

NAME
std::ctype::narrow,do_narrow - std::ctype::narrow,do_narrow

Synopsis
   Defined in header <locale>
   public: (1)
   char narrow( CharT c, char dflt ) const;
   public:
   const CharT* narrow( const CharT* beg, const CharT* end, (2)
                        char dflt, char* dst ) const;
   protected: (3)
   virtual char do_narrow( CharT c, char dflt ) const;
   protected:
   virtual const CharT* do_narrow( const CharT* beg, const CharT* end, (4)
                        char dflt, char* dst ) const;
   1,2) Public member function, calls the corresponding protected virtual member
   function do_narrow overload of the most derived class. Overload (1) calls
   do_narrow(c, dflt), overload (2) calls do_narrow(beg, end, dflt, dst).
   3) Converts the (possibly wide) character c to multibyte representation if the
   character can be represented with a single byte (for example, ASCII characters in
   UTF-8 encoding are single bytes). Returns dflt if such conversion does not exist.
   4) For every character in the character array [beg, end), writes narrowed characters
   (or dflt whenever narrowing fails) to the successive locations in the character
   array pointed to by dst.
   Narrowing is always successful and is always reversible (by calling widen()) for all
   characters from the
   basic source character set
   (until C++23)
   basic character set
   (since C++23).
     * i.e. do_widen(do_narrow(c, 0)) == c always holds for any character c in the
       basic source character set
       (until C++23)
       basic character set
       (since C++23).
   Narrowing, if successful, preserves all character classification categories known to
   is().
     * i.e. is(m, c) || !ctc.is(m, do_narrow(c, dflt)) is always true for any named
       ctype category with a ctype<char> facet ctc and ctype_base::mask value m (unless
       do_narrow returns dflt).
   Narrowing of any digit character guarantees that if the result is subtracted from
   the character literal '0', the difference equals the digit value of the original
   character.
     * i.e. for any digit character c, the expression (do_narrow(c, dflt) - '0')
       evaluates to the digit value of the character.

Parameters
   c - character to convert
   dflt - default value to produce if the conversion fails
   beg - pointer to the first character in an array of characters to convert
   end - one past the end pointer for the array of characters to convert
   dst - pointer to the first element of the array of characters to fill

Return value
   1,3) Narrowed character or dflt if narrowing fails.
   2,4) end

Example
// Run this code
 #include <iostream>
 #include <locale>
 void try_narrow(const std::ctype<wchar_t>& f, wchar_t c)
 {
     char n = f.narrow(c, 0);
     if (n)
         std::wcout << '\'' << c << "' narrowed to " << +(unsigned char)n << '\n';
     else
         std::wcout << '\'' << c << "' could not be narrowed\n";
 }
 int main()
 {
     std::locale::global(std::locale("en_US.utf8"));
     std::wcout.imbue(std::locale());
     std::wcout << std::hex << std::showbase << "In US English UTF-8 locale:\n";
     auto& f = std::use_facet<std::ctype<wchar_t>>(std::locale());
     try_narrow(f, L'A');
     try_narrow(f, L'Ａ');
     try_narrow(f, L'ě');
     std::locale::global(std::locale("cs_CZ.iso88592"));
     auto& f2 = std::use_facet<std::ctype<wchar_t>>(std::locale());
     std::wcout << "In Czech ISO-8859-2 locale:\n";
     try_narrow(f2, L'A');
     try_narrow(f2, L'Ａ');
     try_narrow(f2, L'ě');
 }

Possible output:
 In US English UTF-8 locale:
 'A' narrowed to 0x41
 'Ａ' could not be narrowed
 'ě' could not be narrowed
 In Czech ISO-8859-2 locale:
 'A' narrowed to 0x41
 'Ａ' could not be narrowed
 'ě' narrowed to 0xec
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                      1. the code representing reversibility
                      was
                      do_widen(do_narrow(c), 0) == c
   LWG 126 C++98 2. the code representing category corrected both
                      preservation was
                      is(m, c) || !ctc.is(m, do_narrow(c),
                      dflt)
   LWG 153 C++98 narrow always called overload (4) calls the corresponding
                        overload

See also
   widen invokes do_widen
          (public member function)
   narrow narrows characters
          (public member function of std::basic_ios<CharT,Traits>)
   wctob narrows a wide character to a single-byte narrow character, if possible
          (function)