std::codecvt<InternT,ExternT,StateT>::length, do_length
From cppreference.com
| Defined in header <locale>
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| public: int length( StateT& state, const ExternT* from, const ExternT* from_end, |
(1) | |
| protected: virtual int do_length( StateT& state, const ExternT* from, const ExternT* from_end, |
(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
Run this code
#include <iostream> #include <locale> #include <string> int main() { using facet_type = std::codecvt<wchar_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::locale loc("en_US.UTF-8"); facet_type const& codecvt_facet = std::use_facet<facet_type>(loc); 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_lengthwas required to return std::min(max, from_end - from) |
not required |
See also
| [virtual] |
converts a string from ExternT to InternT, such as when reading from file (virtual protected member function) |