std::codecvt::length,do_length(3)

NAME
std::codecvt::length,do_length - std::codecvt::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

See also
   do_in converts a string from externT to internT, such as when reading from file
   [virtual] (virtual protected member function)