View source for cpp/string/multibyte/mbrlen

===Return value===
* {{c|0}} if the next {{c|n}} or fewer bytes complete the null character.
* The number of bytes (between {{c|1}} and {{c|n}}) that complete a valid multibyte character.
* {{c|std::size_t(-1)}} if encoding error occurs.
* {{c|std::size_t(-2)}} if the next {{c|n}} bytes are part of a possibly valid multibyte character, which is still incomplete after examining all {{c|n}} bytes.