std::codecvt<InternT,ExternT,StateT>::out, do_out

From cppreference.com
< cpp‎ | locale‎ | codecvt
 
C++
 
Localization library
 
std::codecvt
 
Defined in header <locale>
public:

result out( StateT& state,
            const InternT* from,
            const InternT* from_end,
            const InternT*& from_next,
            ExternT* to,
            ExternT* to_end,

            ExternT*& to_next ) const;
 (1)
protected:

virtual result do_out( StateT& state,
                       const InternT* from,
                       const InternT* from_end,
                       const InternT*& from_next,
                       ExternT* to,
                       ExternT* to_end,

                       ExternT*& to_next ) const;
 (2)
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
LWG 76 C++98 it was unclear whether the conversion is required
to support taking one internal character at a time 		only required if used
by basic_filebuf

See also

[virtual]
 writes characters to the associated file from the put area
(virtual protected member function of std::basic_filebuf<CharT,Traits>)
converts a wide string into a byte string
(public member function of std::wstring_convert<Codecvt,Elem,Wide_alloc,Byte_alloc>)
converts a wide string to narrow multibyte character string, given state
(function)
[virtual]
 converts a string from ExternT to InternT, such as when reading from file
(virtual protected member function)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/locale/codecvt/out&oldid=160052"