0001 template <typename T> T & Vector<T>::operator[] ( Rank r ) //重载下标操作符
0002 { return _elem[r]; } // assert: 0 <= r < _size
0003 
0004 template <typename T> const T & Vector<T>::operator[] ( Rank r ) const //仅限于做右值
0005 { return _elem[r]; } // assert: 0 <= r < _size