title: Composition of classes
body: <p>How come, when setting up class composition, contained classes can be called with a default constructor but not with one that takes parameters?</p>
  
  <p>That's kind of confusing; let me offer an example.</p>
  
  <pre><code>#include "A.h"
  class B
  {
  private:
      A legal; // this kind of composition is allowed
      A illegal(2,2); // this kind is not.
  };
  </code></pre>
  
  <p>Assuming both the default constructor and one that takes 2 integers exists, only one of these is allowed.  Why is this?</p>