# std::codecvt<InternT,ExternT,StateT>::in, std::codecvt<InternT,ExternT,StateT>::do_in

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< cpp‎ | locale‎ | codecvt

Defined in header `<locale>` |  |
public: result in( StateT& state, const ExternT* from, const ExternT* from_end, const ExternT*& from_next, InternT* to, InternT* to_end, InternT*& to_next ) const; | (1) |
protected: virtual result do_in( StateT& state, const ExternT* from, const ExternT* from_end, const ExternT*& from_next, InternT* to, InternT* to_end, InternT*& to_next ) const; | (2) |

1) Public member function, calls the member function `do_in` of the most derived class.
2) If this `codecvt` facet defines a conversion, translates the external characters from the source range `[from, from_end)` to internal characters, placing the results in the subsequent locations starting at to. Converts no more than from_end - from external characters and writes no more than to_end - to internal 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_in(state, from, from_end, from_next, to, to + 1, to_next) must return `ok` if

- this `codecvt` facet is used by `basic_filebuf`, and
- do_in(state, from, from_end, from_next, to, to_end, to_next) would return `ok` where to != to_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.

The effect on state is deliberately unspecified. In standard facets, it is used to maintain shift state like when calling std::mbsrtowcs, and is therefore updated to reflect the conversion state after the last processed external 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 const& f = std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>
            (std::locale());
        std::string external = "z\u00df\u6c34\U0001d10b"; // or u8"zß水𝄋"
                              // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";

        // note that the following can be done with wstring_convert
        std::mbstate_t mb = std::mbstate_t(); // initial shift state
        std::wstring internal(external.size(), '\0');
        const char* from_next;
        wchar_t* to_next;
        f.in(mb, &external[0], &external[external.size()], from_next,
                 &internal[0], &internal[internal.size()], to_next);
        // error checking skipped for brevity
        internal.resize(to_next - &internal[0]);

        std::wcout << L"The string in wide encoding: " << internal << '\n';
    }

Output:

    The string in wide 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 producing one internal character at a time | only required if used by `basic_filebuf`

### See also

underflow [virtual] | reads from the associated file (virtual protected member function of `std::basic_filebuf<CharT,Traits>`)
from_bytes | converts a byte string into a wide string (public member function of `std::wstring_convert<Codecvt,Elem,Wide_alloc,Byte_alloc>`)
mbsrtowcs | converts a narrow multibyte character string to wide string, given state (function)
do_out [virtual] | converts a string from `InternT` to `ExternT`, such as when writing to file (virtual protected member function)

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