std::codecvt::out, do_out

 

C++

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Technical Specifications

 

Localizations library

Locales and facets
locale
use_facet	has_facet
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11) wbuffer_convert (C++11)
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
Facet categories
ctype ctype<char> codecvt collate messages time_get time_put	num_get num_put numpunct money_get money_put moneypunct
Locale-specific facets
ctype_byname codecvt_byname messages_byname collate_byname	time_get_byname time_put_byname numpunct_byname moneypunct_byname
Code conversion facets
codecvt_utf8 (C++11) codecvt_utf16 (C++11)	codecvt_utf8_utf16 (C++11) codecvt_mode (C++11)
C locale
setlocale localeconv lconv	LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME

 

std::codecvt

Member functions

codecvt::codecvt

codecvt::~codecvt

codecvt::outcodecvt::do_out

codecvt::incodecvt::do_in

codecvt::unshiftcodecvt::do_unshift

codecvt::encodingcodecvt::do_encoding

codecvt::always_noconvcodecvt::do_always_noconv

codecvt::lengthcodecvt::do_length

codecvt::max_lengthcodecvt::do_max_length

 

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.

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 <string>
#include <locale>
 
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\U0001d10b"; // L"zß水𝄋"
 
    // note that the following can be done with wstring_convert
    std::mbstate_t mb = std::mbstate_t(); // 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𝄋

See also

overflow [virtual]	writes characters to the associated file from the put area (virtual protected member function of std::basic_filebuf)
to_bytes	converts a wide string into a byte string (public member function of std::wstring_convert)
wcsrtombs	converts a wide string to narrow multibyte character string, given state (function)
do_in [virtual]	converts a string from externT to internT, such as when reading from file (virtual protected member function)