std::codecvt::max_length,do_max_length(3)

NAME
std::codecvt::max_length,do_max_length - std::codecvt::max_length,do_max_length

Synopsis
   Defined in header <locale>
   public: (until C++11)
   int max_length() const throw();
   public: (since C++11)
   int max_length() const noexcept; (1)
   protected: (until C++11)
   virtual int do_max_length() const throw(); (2)
   protected: (since C++11)
   virtual int do_max_length() const noexcept;
   1) Public member function, calls the member function do_max_length of the most
   derived class.
   2) Returns the maximum value that do_length(state, from, from_end, 1) can return for
   any valid range [from, from_end) and any valid state.

Return value
   The maximum number of ExternT characters that could be consumed if converted by in()
   to produce one InternT character.
   The non-converting specialization std::codecvt<char, char, std::mbstate_t> returns 1

Notes
   If the encoding is state-dependent (encoding() == -1), then more than max_length()
   external characters may be consumed to produce one internal character

Example
// Run this code
 #include <locale>
 #include <iostream>
 #include <codecvt>
 int main()
 {
     std::cout << "In codecvt_utf8, the longest multibyte character is "
               << std::codecvt_utf8<wchar_t>().max_length() << " bytes long\n";
     std::cout << "In header-consuming codecvt_utf8, the longest multibyte character is "
               << std::codecvt_utf8<wchar_t,
                        0x10ffff,
                        std::consume_header>().max_length() << " bytes long\n";
     std::cout << "In this system's en_US.utf8, the longest multibyte character is "
               << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                     std::locale("en_US.utf8")
               ).max_length() << " bytes long\n";
     std::cout << "In this system's zh_CN.gb18030, the longest multibyte character is "
               << std::use_facet<std::codecvt<wchar_t, char, std::mbstate_t>>(
                     std::locale("zh_CN.gb18030")
               ).max_length() << " bytes long\n";
 }

Output:
 In codecvt_utf8, the longest multibyte character is 4 bytes long
 In header-consuming codecvt_utf8, the longest multibyte character is 7 bytes long
 In this system's en_US.utf8, the longest multibyte character is 6 bytes long
 In this system's zh_CN.gb18030, the longest multibyte character is 4 bytes long

See also
   MB_CUR_MAX maximum number of bytes in a multibyte character in the current C locale
               (macro variable)
   do_encoding returns the number of externT characters necessary to produce one
   [virtual] internT character, if constant
               (virtual protected member function)