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# std::codecvt::out, do_out

From cppreference.com
< cpp‎ | locale‎ | 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.

### [edit] 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

### [edit] 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.

### [edit] 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𝄋

### [edit] See also

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

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