Is the pImpl idiom really used in practice?

问题:

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 private objects of a class and dynamically allocate them to decrease the compilation time (and also hide the private implementations in a better manner).

For example:

class X
{
private:
  C c;
  D d;  
} ;

could be changed to:

class X
{
private:
  struct XImpl;
  XImpl* pImpl;       
};

and, in the CPP, the definition:

struct X::XImpl
{
  C c;
  D d;
};

This seems pretty interesting, but I have never seen this kind of approach before, neither in the companies I have worked, nor in open source projects that I\'ve seen the source code. So, I am wondering it this technique is really used in practice?

Should I use it everywhere, or with caution? And is this technique recommended to be used in embedded systems (where the performance is very important)?

回答1:

So, I am wondering it this technique is really used in practice? Should I use it everywhere, or with caution?

Of course it is used, and in my project, in almost every class, for several reasons you mentioned :

• data hiding
• recompilation time is really decreased, since only the source file needs to be rebuilt, but not the header, and every file that includes it
• binary compatibility. Since the class declaration doesn\'t change, it is safe to just update the library (assuming you are creating a library)

is this technique recommended to be used in embedded systems (where the performance is very important)?

That depends on how powerful your target is. However the only answer to this question is : measure and evaluate what you gain and lose.

回答2:

It seems that a lot of libraries out there use it to stay stable in their API, at least for some versions.

But as for all things, you should never use anything everywhere without caution. Always think before using it. Evaluate what advantages it gives you, and if they are worth the price you pay.

The advantages it may give you are:

• helps in keeping binary compatibility of shared libraries
• hiding certain internal details
• decreasing recompilation cycles

Those may or may not be real advantages to you. Like for me, I don\'t care about a few minutes recompilation time. End users usually also don\'t, as they always compile it once and from the beginning.

Possible disadvantages are (also here, depending on the implementation and whether they are real disadvantages for you):

• Increase in memory usage due to more allocations than with the naïve variant
• increased maintenance effort (you have to write at least the forwarding functions)
• performance loss (the compiler may not be able to inline stuff as it is with a naïve implementation of your class)

So carefully give everything a value, and evaluate it for yourself. For me, it almost always turns out that using the pimpl idiom is not worth the effort. There is only one case where I personally use it (or at least something similar):

My C++ wrapper for the linux stat call. Here the struct from the C header may be different, depending on what #defines are set. And since my wrapper header can\'t control all of them, I only #include <sys/stat.h> in my .cxx file and avoid these problems.

回答3:

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).

回答4:

Other people have already provided the technical up/downsides, but I think the following is worth noting:

First and foremost, don\'t be dogmatic. If pImpl works for your situation, use it - don\'t use it just because \"it\'s better OO since it really hides implementation\" etc. Quoting the C++ FAQ:

encapsulation is for code, not people (source)

Just to give you an example of open source software where it is used and why: OpenThreads, the threading library used by the OpenSceneGraph. The main idea is to remove from the header (e.g. <Thread.h>) all platform-specific code, because internal state variables (e.g. thread handles) differ from platform to platform. This way one can compile code against your library without any knowledge of the other platforms\' idiosyncrasies, because everything is hidden.

回答5:

I would mainly consider PIMPL for classes exposed to be used as an API by other modules. This has many benefits, as it makes recompilation of the changes made in the PIMPL implementation does not affect the rest of the project. Also, for API classes they promote a binary compatibility (changes in a module implementation do not affect clients of those modules, they don\'t have to be recompiled as the new implementation has the same binary interface - the interface exposed by the PIMPL).

As for using PIMPL for every class, I would consider caution because all those benefits come at a cost: an extra level of indirection is required in order to access the implementation methods.

回答6:

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.

回答7:

I used to use this technique a lot in the past but then found myself moving away from it.

Of course it is a good idea to hide the implementation detail away from the users of your class. However you can also do that by getting users of the class to use an abstract interface and for the implementation detail to be the concrete class.

The advantages of pImpl are:

1. Assuming there is just one implementation of this interface, it is clearer by not using abstract class / concrete implementation

2. If you have a suite of classes (a module) such that several classes access the same \"impl\" but users of the module will only use the \"exposed\" classes.

3. No v-table if this is assumed to be a bad thing.

The disadvantages I found of pImpl (where abstract interface works better)

1. Whilst you may have only one \"production\" implementation, by using an abstract interface you can also create a \"mock\" inmplementation that works in unit testing.

2. (The biggest issue). Before the days of unique_ptr and moving you had restricted choices as to how to store the pImpl. A raw pointer and you had issues about your class being non-copyable. An old auto_ptr wouldn\'t work with forwardly declared class (not on all compilers anyway). So people started using shared_ptr which was nice in making your class copyable but of course both copies had the same underlying shared_ptr which you might not expect (modify one and both are modified). So the solution was often to use raw pointer for the inner one and make the class non-copyable and return a shared_ptr to that instead. So two calls to new. (Actually 3 given old shared_ptr gave you a second one).

3. Technically not really const-correct as the constness isn\'t propagated through to a member pointer.

In general I have therefore moved away in the years from pImpl and into abstract interface usage instead (and factory methods to create instances).

回答8:

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.

回答9:

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.

回答10:

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.

回答11:

One benefit I can see is that it allows the programmer to implement certain operations in a fairly fast manner:

X( X && move_semantics_are_cool ) : pImpl(NULL) {
    this->swap(move_semantics_are_cool);
}
X& swap( X& rhs ) {
    std::swap( pImpl, rhs.pImpl );
    return *this;
}
X& operator=( X && move_semantics_are_cool ) {
    return this->swap(move_semantics_are_cool);
}
X& operator=( const X& rhs ) {
    X temporary_copy(rhs);
    return this->swap(temporary_copy);
}

PS: I hope I\'m not misunderstanding move semantics.