std::codecvt::out,do_out(3)

NAME
std::codecvt::out,do_out - std::codecvt::out,do_out

Synopsis
   Defined in header <locale>
   public:
   result out( StateT& state,
               const InternT* from,
               const InternT* from_end, (1)
               const InternT*& from_next,
               ExternT* to,
               ExternT* to_end,
               ExternT*& to_next ) const;
   protected:
   virtual result do_out( StateT& state,
                        const InternT* from,
                        const InternT* from_end, (2)
                        const InternT*& from_next,
                        ExternT* to,
                        ExternT* to_end,
                        ExternT*& to_next ) const;
   1) Public member function, calls the member function do_out of the most derived
   class.
   2) If this codecvt facet defines a conversion, translates the internal characters
   from the source range [from, from_end) to external characters, placing the results
   in the subsequent locations starting at to. Converts no more than from_end - from
   internal characters and writes no more than to_end - to external characters. Leaves
   from_next and to_next pointing one beyond the last element successfully converted.
   If this codecvt facet does not define a conversion, no characters are converted.
   to_next is set to be equal to to, state is unchanged, and std::codecvt_base::noconv
   is returned.
   do_out(state, from, from + 1, from_next, to, to_end, to_next) must return ok if
     * this codecvt facet is used by basic_filebuf, and
     * do_out(state, from, from_end, from_next, to, to_end, to_next) would return ok
       where from != from_end.

Return value
   A value of type std::codecvt_base::result, indicating the success status as follows:
   ok conversion completed
   partial not enough space in the output buffer or unexpected end of source buffer
   error encountered a character that could not be converted
   noconv this facet is non-converting, no output written
   The non-converting specialization std::codecvt<char, char, std::mbstate_t> always
   returns std::codecvt_base::noconv.

Notes
   Requires that from <= from_end && to <= to_end and that state either representing
   the initial shift state or obtained by converting the preceding characters in the
   sequence.
   While codecvt supports N:M conversions (e.g. UTF-16 to UTF-8, where two internal
   characters may be necessary to decide what external characters to output),
   std::basic_filebuf can only use codecvt facets that define a 1:N conversion, that is
   it must be able to process one internal character at a time when writing to a file.
   When performing N:M conversions, this function may return std::codecvt_base::partial
   after consuming all source characters (from_next == from_end). This means that
   another internal character is needed to complete the conversion (e.g. when
   converting UTF-16 to UTF-8, if the last character in the source buffer is a high
   surrogate).
   The effect on state is deliberately unspecified. In standard facets, it is used to
   maintain shift state like when calling std::wcsrtombs, and is therefore updated to
   reflect the shift state after the last successfully converted character, but a
   user-defined facet is free to use it to maintain any other state, e.g. count the
   number of special characters encountered.

Example
// Run this code
 #include <iostream>
 #include <locale>
 #include <string>
 int main()
 {
     std::locale::global(std::locale("en_US.utf8"));
     auto& f = std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(std::locale());
     std::wstring internal = L"z\u00df\u6c34\U0001f34c"; // L"zß水🍌"
     // note that the following can be done with wstring_convert
     std::mbstate_t mb{}; // initial shift state
     std::string external(internal.size() * f.max_length(), '\0');
     const wchar_t* from_next;
     char* to_next;
     f.out(mb, &internal[0], &internal[internal.size()], from_next,
               &external[0], &external[external.size()], to_next);
     // error checking skipped for brevity
     external.resize(to_next - &external[0]);
     std::cout << "The string in narrow multibyte encoding: " << external << '\n';
 }

Output:
 The string in narrow multibyte encoding: zß水🍌
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
                     it was unclear whether the conversion is
   LWG 76 C++98 required only required if used
                     to support taking one internal character at by basic_filebuf
                     a time

See also
   overflow writes characters to the associated file from the put area
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
             converts a wide string into a byte string
   to_bytes (public member function of
             std::wstring_convert<Codecvt,Elem,Wide_alloc,Byte_alloc>)
   wcsrtombs converts a wide string to narrow multibyte character string, given state
             (function)
   do_in converts a string from ExternT to InternT, such as when reading from file
   [virtual] (virtual protected member function)