std::codecvt::length, std::codecvt::do_length

 

C++

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Localizations library

Locales and facets
locale
use_facet	has_facet
Character classification
isspace isblank (C++11) iscntrl isupper islower isalpha	isdigit ispunct isxdigit isalnum isprint isgraph
Conversions
toupper tolower	wstring_convert (C++11) wbuffer_convert (C++11)
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
Facet categories
ctype ctype<char> codecvt collate messages time_get time_put	num_get num_put numpunct money_get money_put moneypunct
Locale-specific facets
ctype_byname ctype_byname<char> codecvt_byname messages_byname collate_byname	time_get_byname time_put_byname numpunct_byname moneypunct_byname
Code conversion facets
codecvt_utf8 (C++11) codecvt_utf16 (C++11)	codecvt_utf8_utf16 (C++11) codecvt_mode (C++11)
C locale
setlocale localeconv lconv	LC_ALL LC_COLLATE LC_CTYPE LC_MONETARY LC_NUMERIC LC_TIME

 

std::codecvt

Member functions

codecvt::codecvt

codecvt::~codecvt

codecvt::out codecvt::do_out

codecvt::in codecvt::do_in

codecvt::unshift codecvt::do_unshift

codecvt::encoding codecvt::do_encoding

codecvt::always_noconv codecvt::do_always_noconv

codecvt::length codecvt::do_length

codecvt::max_length codecvt::do_max_length

 

Defined in header <locale>
public: int length( stateT& state,             const externT* from,             const externT* from_end,             std::size_t max ) const	(1)
protected: int do_length( stateT& state,                const externT* from,                const externT* from_end,                std::size_t max ) const	(2)

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 producer, or a conversion error occurred.

The non-converting specialization std::codecvt<char, char, std::mbstate_t> returns std::min(max, from_end-from)

Example

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