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

std::codecvt::out,do_out(3) 💅✨

hey bestie, welcome to the C++ Standard Libary manual page for std::codecvt::out,do_out — yes, both of them: the public wrapper and the protected virtual girlie. we're about to make multibyte conversion make sense, no cap. 🫶

📛 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;

okay so here's the tea, bestie. 🎀

1. This is the public member function, and it calls the member function do_out of the most derived class.
2. If this codecvt facet defines a conversion, it translates the internal characters from the source range [from, from_end) to external characters, placing the results in the subsequent locations starting at to. It converts no more than from_end - from internal characters and writes no more than to_end - to external characters. It 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. periodt. 💅

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 is 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 LWG 76 — applied to C++98. Behavior as published: it was unclear whether the conversion is required to support taking one internal character at a time. Correct behavior: only required if used by basic_filebuf.

👀 See also 👀

• overflow — writes characters to the associated file from the put area; [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
• to_bytes — converts a wide string into a byte string; (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)

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💖 source: http://cppreference.com · version/date: 2024.06.10 · now go be the main character of your locale, bestie 💅✨