std::basic_iostream<CharT,Traits>::operator=

basic_iostream& operator=( const basic_iostream& other ) = delete;

protected:
basic_iostream& operator=( basic_iostream&& other );

1) Copy assignment is not allowed.

2) Move assigns another stream object. Effectively calls swap(rhs). This move assignment operator is protected: it is called by the move assignment operators of the derived stream classes std::basic_stringstream and std::basic_fstream which know how to properly move-assign the associated stream buffers.