Constructs a filebuf object. The object is not associated to any file (it is not open).
(2) copy constructor (deleted)
Deleted (no copy constructor).
(3) move constructor
Acquires the contents of x. x is a closed file stream buffer after the call, with an otherwise unspecified but valid state.
It is unspecified whether the internal sequence is the one in x before the call, or a copy of it. In any case, both objects use independent sequences after the call (if any).
A filebuf object of the same type (with the same class template parameters charT and traits), whose value is moved.
The move constructor (3) modifies x.
Strong guarantee: if an exception is thrown, there are no side effects.