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Subject: Re: [boost] Should Boost adopt pimpl/d-pointer as much as it can?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-03-20 22:01:17


Artyom wrote:
> There are recently lots of discussions about current development issues.
> I would like to point to one small but extremely important issue
> (from user point of view).
> Boost has too many logic in header part of library, by meaning too much
> I mean painfully too much.
> I think that Boost should adopt as policy:
> 1) All implementation related code **should** be put in sources.
> 2) Any non-template based class should have opaque/d/pimpl pointer in its
> body.

I am against such a policy. Such decisions should be left
to the library author and the review process.

> Why? I give you a simple example: one of the pearls of Boost: Boost.Asio
> is one of most painful libraries to use.
> I created a simple example of echo server written with Boost.Asio
> and written with a small pimpl-wrapped Boost.Asio (I had written myself
> as proof of concept - that uses boost.asio in implementation):
> These are results of compilation times with gcc-4.3 (seconds):
> -O0 -O2 -O3 -O2+link
> asio 2.56 4.65 4.96 4.75
> pimpl 0.59 0.85 0.89 0.94
> diff *4.3 *5.5 *5.6 *5.05
> And this is trivial echo server. When the code gets larger and bigger
> it becomes even more painful.
> For me, compilation of components based on Asio is total nightmare
> because of unacceptable compilation times.
> Another issue is executable size. I had took same echo sever
> placed in 10 object compiled under different namespaces
> once I did it with Boost.Asio and then I did it with pimpl-ed Asio.
> Result (liked executable size with gcc-4.3):
> Build Size (-Os stripped)
> Asio: 245K
> Pimpl: 190K
> ----------------------------
> Conclusions:
> ============
> There are too many good libraries (Asio as good example) that
> use headers too much or headers only, this terribly increases
> compilation times and makes executable sizes much bigger (this
> is especially relevant for embedded systems).
> Should Boost wellcome pimpled libraries more? I think yes.
> It would solve two problems:
> 1. It would give faster compilation times.
> 2. It would allow to provide changes easily without breaking
> compatibility (even would allow to provide backward ABI compatibility)
> Dear Boosters.
> What do you think about?

I think making a general policy about this would
be misguided. Looking over the list of libraries at
There are many more libraries for which pimpls
are either impossible or nonsensical than libraries
for which they might make sense. Not to mention
that there seems to be quite a bit of disagreement
around here about whether having separate source
is a good thing.

In Christ,
Steven Watanabe

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