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std::codecvt<InternT,ExternT,StateT>::length, do_length

Defined in header <locale>

Defined in header `<locale>`
public: int length( StateT& state, const ExternT* from, const ExternT* from_end, std::size_t max ) const;	(1)
protected: virtual int do_length( StateT& state, const ExternT* from, const ExternT* from_end, std::size_t max ) const;	(2)

public:
int length( StateT& state, const ExternT* from, const ExternT* from_end,
            std::size_t max ) const;

(1)

protected:
virtual 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 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

#include <locale>
#include <string>
#include <iostream>
 
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
LWG 305	C++98	`std::codecvt<wchar_t, char, std::mbstate_t>::do_length` was required to return `std::min(max, from_end - from)`	not required

std::codecvt<InternT,ExternT,StateT>::length, do_length

Return value

Example

Defect reports

See also