# std::codecvt<InternT,ExternT,State>::max_length, do_max_length

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< cpp‎ | locale‎ | codecvt

Defined in header `<locale>` |  |
public: int max_length() const; | (1) |
protected: virtual 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`.

### 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

### Exceptions

(none) | (until C++11)
`noexcept` specification: noexcept | (since C++11)

### 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 [virtual] | returns the number of externT characters necessary to produce one internT character, if constant (virtual protected member function)

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