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std::codecvt::out,do_out(3) C++ Standard Libary std::codecvt::out,do_out(3)


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


Defined in header <locale>

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;

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
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.

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


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

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

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.


// Run this code

#include <iostream>
#include <string>
#include <locale>

int main()
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';


The string in narrow multibyte encoding: zß水🍌

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
wcsrtombs converts a wide string to narrow multibyte character string, given state
do_in converts a string from externT to internT, such as when reading from file
[virtual] (virtual protected member function)