std::codecvt::length, std::codecvt::do_length

From cppreference.com

Defined in header `<locale>`

public: int length( stateT& state, const externT* from, const externT* from_end, std::size_t max ) const	(1)

protected: 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 producer, 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)

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