std::codecvt<InternT,ExternT,State>::length,do_length (3) - Linux Manuals

std::codecvt<InternT,ExternT,State>::length,do_length: std::codecvt<InternT,ExternT,State>::length,do_length

NAME

std::codecvt<InternT,ExternT,State>::length,do_length - std::codecvt<InternT,ExternT,State>::length,do_length

Synopsis

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

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()
  {
      // 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