Is the pImpl idiom really used in practice?

Question

I am reading the book \"Exceptional C++\" by Herb Sutter, and in that book I have learned about the pImpl idiom. Basically, the idea is to create a structure for the p

Answer 1

It is used in practice in a lot of projects. It's usefullness depends heavily on the kind of project. One of the more prominent projects using this is Qt, where the basic idea is to hide implementation or platformspecific code from the user (other developers using Qt).

This is a noble idea but there is a real drawback to this: debugging As long as the code hidden in private implemetations is of premium quality this is all well, but if there are bugs in there, then the user/developer has a problem, because it just a dumb pointer to a hidden implementation, even if he has the implementations source code.

So as in nearly all design decisions there a pros and cons.

Answer 2

As many other said, the Pimpl idiom allows to reach complete information hiding and compilation independency, unfortunately with the cost of performance loss (additional pointer indirection) and additional memory need (the member pointer itself). The additional cost can be critical in embedded software development, in particular in those scenarios where memory must be economized as much as possible. Using C++ abstract classes as interfaces would lead to the same benefits at the same cost. This shows actually a big deficiency of C++ where, without recurring to C-like interfaces (global methods with an opaque pointer as parameter), it is not possible to have true information hiding and compilation independency without additional resource drawbacks: this is mainly because the declaration of a class, which must be included by its users, exports not only the interface of the class (public methods) needed by the users, but also its internals (private members), not needed by the users.

Answer 3

I think this is one of the most fundamental tools for decoupling.

I was using pimpl (and many other idioms from Exceptional C++) on embedded project (SetTopBox).

The particular purpose of this idoim in our project was to hide the types XImpl class uses. Specifically we used it to hide details of implementations for different hardware, where different headers would be pulled in. We had different implementations of XImpl classes for one platform and different for the other. Layout of class X stayed the same regardless of the platfrom.

Answer 4

Here is an actual scenario I encountered, where this idiom helped a great deal. I recently decided to support DirectX 11, as well as my existing DirectX 9 support, in a game engine. The engine already wrapped most DX features, so none of the DX interfaces were used directly; they were just defined in the headers as private members. The engine utilizes DLLs as extensions, adding keyboard, mouse, joystick, and scripting support, as week as many other extensions. While most of those DLLs did not use DX directly, they required knowledge and linkage to DX simply because they pulled in headers that exposed DX. In adding DX 11, this complexity was to increase dramatically, however unnecessarily. Moving the DX members into a Pimpl defined only in the source eliminated this imposition. On top of this reduction of library dependencies, my exposed interfaces became cleaner as moved private member functions into the Pimpl, exposing only front facing interfaces.

Answer 5

Agree with all the others about the goods, but let me put in evidence a limit: doesn't work well with templates.

The reason is that template instantiation requires the full declaration available where the instantiation took place. (And that's the main reason you don't see template methods defined into CPP files)

You can still refer to templetised subclasses, but since you have to include them all, every advantage of "implementation decoupling" on compiling (avoiding to include all platoform specific code everywhere, shortening compilation) is lost.

Is a good paradigm for classic OOP (inheritance based) but not for generic programming (specialization based).