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Obtains the size of the first character of a string.
std::size_t(*next_char)(span< const unsigned char >);
Given a range of bytes, r
,
this function must interpret r
as a string encoded using this character set, and return the number of
bytes that the first character in the string spans, or 0 in case of error.
r
is guaranteed to be non-empty
(r.size() > 0
).
In some character sets (like UTF-8), not all byte sequences represent valid characters. If this function finds an invalid byte sequence while trying to interpret the first character, it should return 0 to signal the error.
This function must not throw exceptions or have side effects.