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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../header/../locale/codecvt/length.html below:

std::codecvt<InternT,ExternT,StateT>::length, do_length - cppreference.com

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 
#include <iostream>
#include <locale>
#include <string>
 
int main()
{
    using facet_type = std::codecvt<wchar_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::locale loc("en_US.UTF-8");
    facet_type const& codecvt_facet = std::use_facet<facet_type>(loc);
    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
[edit] 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 [edit] See also converts a string from ExternT to InternT, such as when reading from file
(virtual protected member function) [edit]

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