The class template describes an object that controls conversions between wide
string objects of class
basic_string<Elem, char_traits<Elem>,
WideAlloc> and byte string objects of class
basic_string<char,
char_traits<char>, ByteAlloc>. Conversion between a sequence of
Elem values (stored in a
wide_string object) and multibyte sequences (stored in a
byte_string object) is performed by an object of class
Codecvt, which meets the
requirements of the standard code-conversion facet
codecvt<Elem,
char, mbstate_t>.