It is used to find widen character and returns the transformation of c to its equivalent using the ctype::widen facet of the locale object currently imbued in the stream.
This function is designed for instantiations of basic_ios that use a different (wider) character type: see basic_ios::widen.
Following is the declaration for ios::widen function.
char widen (char c) const;
c − Character to be "widened".
The wide equivalent of c.
Strong guarantee − if an exception is thrown, there are no changes in the stream.
Accesses the stream object.
Concurrent access to the same stream object may cause data races.