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:
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?
Does XImpl require a pointer back to the X object?
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