title: How to let inline functions from different classes call each other
body: <p>To make some core-codes run as fast as possible, I want to make three functions inline, those belong to two classes respectively.</p>
  
  <p>Ideally, I want to define them as following:</p>
  
  <pre><code>class ClassB;
  class ClassA {
      ClassB * pB;
  
      inline void func1() {
          // do something
      };
  
      inline void func2() {
          pB-&gt;func3();
      };
  };
  
  class ClassB {
      ClassA * pA;
  
      inline void func3() {
          pA-&gt;func1();
      };
  };
  </code></pre>
  
  <p>I know it's impossible if I simplly code as above.
  My question is : Is there a way in it, we can do similar things?
  At least, let them run as fast as possible.
  Thanks!</p>