Acquires the contents of rhs, by move-assigning its members and base classes.
Move assigning its internal basic_stringbuf object leaves unspecified whether the internal sequence of characters is the one in rhs before the call, or a copy of it. In any case, both objects use independent sequences after the call.
Another basic_ostringstream object with the same template parameters (charT, traits and Alloc).
Modifies both stream objects (*this and rhs).
No-throw guarantee: this member function never throws exceptions.