Item 29. Compilation Firewalls

Difficulty: 6

Using the Pimpl Idiom can dramatically reduce code interdependencies and build times. But what should go into a pimpl_ object, and what is the safest way to use it?

In C++, when anything in a class definition changes (even private members), all users of that class must be recompiled. To reduce these dependencies, a common technique is to use an opaque pointer to hide some of the implementation details.

class X 
{
public:
  /* ... public members ... */
protected:
  /* ... protected members? ... */
private:
  /* ... private members? ... */
  struct XImpl;
  XImpl* pimpl_;         // opaque pointer to
                         // forward-declared class
};

The questions for you to answer are:

1. What should go into XImpl? There are four common disciplines.

   • Put all private data (but not functions) into XImpl.

   • Put all private members into XImpl.

   • Put all private and protected members into XImpl.

   • Make XImpl entirely the class that X would have been, and write X as only the public interface made up entirely of simple forwarding functions (a handle/body variant).

     What are the advantages/drawbacks of each? How would you choose among them?

2. Does XImpl require a pointer back to the X object?