title: c++ when assigning an object private
body: <p>all. I'm new to c++, and I want to ask a question about assign an object.</p>
  
  <p>for example
  I have 2 objects that come from same class , a1 and a2.
  and if there are some private member in the class</p>
  
  <p>why can we do the assignment like this
  a1 = a2;</p>
  
  <p>my question is that, I thought only member function has the authority 
  to change the content of the private member data, but through the steps
  i just go through, the private member data is easily changed.
  Why is that??</p>