title: why can get address of member function by class type
body: <p>I met this code:</p>
  
  <pre><code> auto f3 = std::bind(&amp;Foo::print_sum, &amp;foo, 95, _1);
  </code></pre>
  
  <p>in the previous code, we can apply the address operator to a member function while there is no instance 
  object been created, how can that be possible?</p>