NAME¶

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

Synopsis¶

Defined in header <locale>
public:

int length( StateT& state, const ExternT* from, const ExternT* from_end, (1)

std::size_t max ) const;
protected:

virtual int do_length( StateT& state, const ExternT* from, const ExternT* (2)
from_end,

std::size_t max ) const;

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 produced, or a conversion error occurred.

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

Example¶

// Run this code

#include <iostream>
#include <locale>
#include <string>

int main()
{
using facet_type = std::codecvt<char16_t, char, std::mbstate_t>;

// 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::setlocale(LC_ALL, "en_US.UTF-8");
facet_type const& codecvt_facet = std::use_facet<facet_type>(std::locale());
std::mbstate_t mb = std::mbstate_t();
std::cout << "Only the first "
<< codecvt_facet.length(mb, s.data(), s.data() + 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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 75 C++98 the effect on state was not specified specified
std::codecvt<wchar_t, char,
LWG 305 C++98 std::mbstate_t>::do_length not required
was required to return std::min(max, from_end -
from)

See also¶

do_in converts a string from ExternT to InternT, such as when reading from file
[virtual] (virtual protected member function)