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Subject: Re: [boost] Pimpl Again?
From: Vladimir Batov (Vladimir.Batov_at_[hidden])
Date: 2016-05-29 20:29:00

On 05/30/2016 10:08 AM, Gavin Lambert wrote:
> On 28/05/2016 17:41, Vladimir Batov wrote:
>> I feel that pimpl and its deployment pattern need to be codified,
>> described, "standardized" and in our toolboxes to stop/avoid everyone
>> re-inventing the pimpl and making the same mistakes. IMO Boost is best
>> positioned for that.
>> Do you think we might review what I've got and/or maybe collectively
>> come up with something better... It's no MPL or Hana but as
>> std::unique_ptr it's one of the first "little things" I personally reach
>> out for in my everyday work. IMO having pimpl in Boost would save quite
>> a few hours of frustration for many.
> I like the idea, certainly. My main concerns about Boost-ification of
> this are:
> 1. The way the docs are structured suggest that the "natural"
> implementation is the shared one and the unique implementation is an
> extension. Standard C++ language and performance guidelines suggest
> the reverse should be preferred (or as Chris suggested, one that
> avoids heap allocation entirely). (This is mostly just a doc issue;
> the actual implementation seems neutral.)

As you point out the described is "just a doc issue". In that light I do
not quite understand how it can be a "main concern". I'll certainly be
re-working docs when/if the time comes. At this point IMO we should
decide if we want pimpl in Boost and if the suggested design could be
used as the starting point.

> 2. This introduces a symbol (pimpl) into the global namespace, which
> is probably against Boost guidelines.

Again, I feel you are rushing things. Adapting doccs and putting it into
Boost namespace are minor issues to be addressed in due course.

> But putting it into the boost namespace doesn't seem like a good
> solution either as usage requires explicit template specialisation,
> which is more clunky if the template to be specialised is in a
> different namespace.

Indeed. Visual Studio did not have that problem for quite some time. GCC
had that "clunkiness" a few years ago. Now I try with gcc-4.8:

namespace boost
     template<class user_type> struct pimpl {...};

struct Shared : public boost::pimpl<Shared>::shared { ... };

Works fine.

> (Though even being in the global namespace doesn't avoid this
> clunkiness, if the user classes are themselves in a non-global
> namespace.)

That's something I do not understand. I have plenty of classes like:

template<> struct pimpl<chart::panel::leg>::implementation.

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