std::codecvt::max_length, std::codecvt::do_max_length

From cppreference.com
Defined in header <locale>
public:
int max_length() const
(1)
protected:
int do_max_length() const
(2)

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.

Contents

[edit] Return value

The maximum number of externT characters that could be consumed if converted by do_in() to produce one internT character.

The non-converting specialization std::codecvt<char, char, std::mbstate_t> returns 1

[edit] Notes

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

[edit] Exceptions

noexcept specification:  
noexcept
  (since C++11)

[edit] Example

#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

[edit] See also

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