title: How to emit signals in another class in Qt?
body: <p>We can emit signals in the class defining the signals easily by <code>emit signal_a()</code> like</p>
  
  <pre><code>class A
  {
  signals:
      signal_a();
  
  public:
      void fun()
      {
          do_something();
          emit signal_a();
          do_something();
      }
  };
  </code></pre>
  
  <p>However, How to emit signals in another class in Qt? For example</p>
  
  <pre><code>class B
  {
  public:
      void fun()
      {
          do_something();
          (*a) emit signal_a(); // ???
          do_something();
      }
  
  A* a;
  };
  </code></pre>