Re: [Boost-users] Pimpl framework comments/suggestions

Subject: Re: [Boost-users] Pimpl framework comments/suggestions
From: Rob Riggs (rob_at_[hidden])
Date: 2011-04-17 12:24:15

On 04/14/2011 11:32 PM, Clinton Mead wrote:
> Hi All
>
> I've been experimenting with pimpl recently. What I've seen written 
> about the pimpl idiom is that you have an interface class and an 
> implementation class, and in the interface class you put some kind of 
> pointer to the implementation.
>
> However, I thought, sometimes you might want that to be a shared 
> pointer. Or an intrusive pointer. Or unique pointer. Or maybe 
> sometimes you might decide not to use pimpl at all.
>
> I thought it was a bit silly that you had to make these decisions at 
> the time you create the interface class. I thought it would be nice if 
> you could write the interface independent of how it was going to be 
> connected to the implementation, and then hook them together however 
> you like.
>
> So this is what I've done. Basically I've broke things up into three 
> separate parts:
>
> (1) Interface
> (2) Implementation
> (3) What joins them together (i've created unique_pimpl and 
> shared_pimpl as examples)
>
> I've implemented a unique_ptr and shared_ptr pimpl implementations as 
> examples. The idea is that you can basically do this:
>
> unique_pimpl<interface, impl>::create(...) or
> shared_pimpl<interface, impl>::create(...)
> make_your_own<interface, impl>::create(...)
>
> for any interface and implementation you choose.
>
> I've included all the code in a question at 
> http://stackoverflow.com/questions/5666442 but it doesn't seem to be 
> getting much attention there, so I thought I'd bring it up on this list.
>
> If something like this isn't in Boost, and people like it, with a bit 
> of help I'd be happy to work on it to get it into Boost. Alternatively 
> I'm happy for someone else to Boostify this. I've used a bit of c++0x 
> in the code, gcc 4.4.5 will compile it though.
>
> However if I'm just (badly) reinventing the wheel, if someone could 
> direct me to where this has already been done that would be good to.
>
Hi Clinton,

The point of a Pimpl is that the user never knows or cares that it is a 
Pimpl -- it's just another object.  Pimpl is a very small implementation 
detail.  With this code, that detail is no longer very small -- the user 
needs to know too much about how the object was implemented.

While Pimpl initially stood for "pointer to implementation", a better 
way to think about it is "private implementation".  The implementation 
is completely hidden from the user (and the compiler).  With your 
proposal, a significant part of the implementation is no longer 
private.  In fact, it has leaked into the interface.

Take a look at this article for good insights into Pimpl and another 
approach: http://drdobbs.com/cpp/205918714

I still think this adds more complexity to Pimpl than is warranted.  
But, if I had to implement dozens of such classes for a library, I might 
change my mind...

Regards,

Rob