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# std::codecvt::length, do_length

From cppreference.com
< 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.

### [edit] 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)

### [edit] Example

Run this code

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

    int main()
    {
        //  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::mbstate_t mb = std::mbstate_t();
        std::cout << "Only the first " <<
                  std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                        std::locale("en_US.utf8")
                  ).length(mb, &s[0], &s[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

### [edit] See also

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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