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 protected virtual member function do_narrow
   of the most derived class.
   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).
   Narrowing, if successful, preserves all character classification categories known to
   is().
   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.

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 char to fill

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

Example
// Run this code
 #include <locale>
 #include <iostream>
 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');
 }

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

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)