Question

Suppose I have class like this (simplified):

class Foo_p;
class Foo
{
private:
  Foo_p *p;
public:
  Foo();
  /* methods, etc... */
};

This class is a part of an API.
The Foo_p is all the private parts of the class, which are not
declared in the class Foo itself as usual, but rather in a separate forward-declared class that is only used by the underlying implementation not visible on the outside.

I’ve seen this pattern used in a couple of projects, is there a name for it?

Also, how do I use it properly (e.g. exception safety, etc.)? Where should the actual implementation go? In class Foo, as usual, only using Foo_p for storage of data, or in the Foo_p class with Foo being just a wrapper?

Answer 1

That is the pimpl idiom

See

• Handle/Body Idiom – close cousin and the seminal idea for pimpl
  EDIT Found an online copy of this original James Coplien talk
• GotW #100: Compilation Firewalls (Difficulty: 6/10)
• GotW #101: Compilation Firewalls, Part 2 (Difficulty: 8/10) for the most recent on this