View Compilation Error

0_0_21973366_14742.cpp: In member function 'Matrix<T>& Matrix<T>::operator+=(Matrix<T>)':
0_0_21973366_14742.cpp:70:6: error: 'null' was not declared in this scope
    p=null;
      ^
0_0_21973366_14742.cpp: In member function 'Matrix<T>& Matrix<T>::operator-=(Matrix<T>)':
0_0_21973366_14742.cpp:81:6: error: 'null' was not declared in this scope
    p=null;
      ^