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std::codecvt<InternT,ExternT,StateT>::max_length, do_max_length - cppreference.com

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Defined in header <locale>

(1)

public: int max_length() const throw();

 (until C++11)

public: int max_length() const noexcept;

 (since C++11)
(2)

protected: virtual int do_max_length() const throw();

 (until C++11)

protected: virtual int do_max_length() const noexcept;

 (since C++11)

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

#include <codecvt>
#include <iostream>
#include <locale>

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

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