std::mbrlen (3) - Linux Man Pages
Defined in header <cwchar>
std::size_t mbrlen( const char* s, std::size_t n, std::mbstate_t* ps);
Determines the size, in bytes, of the remainder of the multibyte character whose first byte is pointed to by s, given the current conversion state ps.
This function is equivalent to the call std::mbrtowc(nullptr, s, n, ps?ps:&internal) for some hidden object internal of type std::mbstate_t, except that the expression ps is evaluated only once.
s - pointer to an element of a multibyte character string
n - limit on the number of bytes in s that can be examined
ps - pointer to the variable holding the conversion state
0 if the next n or fewer bytes complete the null character.
The number of bytes (between 1 and n) that complete a valid multibyte character
(size_t)-1 if encoding error occurs
(size_t)-2 if the next n bytes are part of a possibly valid multibyte character, which is still incomplete after examining all n bytes
// Run this code
do_length calculates the length of the externT string that would be consumed by conversion into given internT buffer