# std::codecvt<InternT,ExternT,StateT>::length, do_length

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< cpp‎ | locale‎ | codecvt

Defined in header `<locale>` |  |
public: int length( StateT& state, const ExternT* from, const ExternT* from_end, std::size_t max ) const; | (1) |
protected: virtual int do_length( StateT& state, const ExternT* from, const ExternT* from_end, std::size_t max ) const; | (2) |

1) Public member function, calls the member function `do_length` of the most derived class.
2) Attempts to convert the `ExternT` characters from the character array defined by `[from, from_end)`, given initial conversion state state, to at most max `InternT` characters, and returns the number of `ExternT` characters that such conversion would consume. Modifies state as if by executing do_in(state, from, from_end, from, to, to + max, to) for some imaginary `[to, to + max)` output buffer.

### Return value

The number of `ExternT` characters that would be consumed if converted by do_in() until either all from_end - from characters were consumed or max `InternT` characters were produced, or a conversion error occurred.

The non-converting specialization std::codecvt<char, char, std::mbstate_t> returns std::min(max, from_end - from).

### Example

Run this code

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

    int main()
    {
        using facet_type = std::codecvt<char16_t, char, std::mbstate_t>;

        // narrow multibyte encoding
        std::string s = "z\u00df\u6c34\U0001d10b"; // or u8"zß水𝄋"
                          // or "\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b";

        std::setlocale(LC_ALL, "en_US.UTF-8");
        facet_type const& codecvt_facet = std::use_facet<facet_type>(std::locale());
        std::mbstate_t mb = std::mbstate_t();
        std::cout << "Only the first "
                  << codecvt_facet.length(mb, s.data(), s.data() + s.size(), 2)
                  << " bytes out of " << s.size() << " would be consumed"
                     " to produce the first 2 characters\n";
    }

Output:

    Only the first 3 bytes out of 10 would be consumed to produce the first 2 characters

### 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 75 | C++98 | the effect on state was not specified | specified
LWG 305 | C++98 | `std::codecvt<wchar_t, char, std::mbstate_t>::do_length` was required to return std::min(max, from_end - from) | not required

### See also

do_in [virtual] | converts a string from `ExternT` to `InternT`, such as when reading from file (virtual protected member function)

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