Re: [boost] Review Request: Generalization of the Pimpl idiom

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2008-03-31 01:45:02

Vladimir.Batov_at_[hidden] wrote:
> The code is in the Boost Vault called Pimpl.zip
> It compiles with GCC 4.1.2 (I use it on Ubuntu 7.04) and MS Visual Studio 
> 2005.
> Documentation is at DDJ site: http://www.ddj.com/cpp/205918714
> Any questions/suggestions/critique - vladimir.batov_at_[hidden]

I have only quickly gone through your documentation on DDJ, so my 
comments may be off. Feel free to correct me if I misunderstood something.

Personally, I fail to see the advantage over a shared_ptr member.
The only differences I could notice are
  -  implementation class is not user-declared but rather is a 
specialization of a template.
  - the null pointer is called null (by the way, I think we shouldn't 
allow a null value, the private implementation should always be 
initialized somehow, if lazily)

There doesn't seem to be any useful help for allocating the private 
implementation.
Such potentially interesting allocation schemes, like stack allocation, 
seem also to be impossible.

Being able to replace the private implementation of a base with the 
private implementation of a derived class certainly is a necessity.
I have to admit, though, I do not understand your solution.
You're passing a null<Widget>() to the Widget constructor, but the 
constructor wasn't written to take any kind of special argument.

I'm not really sure of the point (and validity) of having pointer 
semantics. Do you really want your implementation details to be shared 
by some of your different instances? (maybe there is a point in sharing 
implementation details with all instances or all equivalent instances, 
but some?)
It seems more logical to me to have the same semantics for the object 
and its private implementation. If the object itself has pointer 
semantics, then you won't copy it. Using one or the other for the 
implementation makes no difference, then. You might as well use the 
value semantics, which would even be more lightweight in that case.