Re: [boost] [pimpl] Some comments

Subject: Re: [boost] [pimpl] Some comments
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-06-07 10:19:27

2014-06-07 16:08 GMT+02:00 Rob Stewart <robertstewart_at_[hidden]>:

> On June 7, 2014 8:05:39 AM EDT, Andrzej Krzemienski <akrzemi1_at_[hidden]>
> wrote:
> >2014-06-06 3:51 GMT+02:00 Vladimir Batov
> ><Vladimir.Batov_at_[hidden]>:
> >> On 06/06/2014 07:21 AM, Andrzej Krzemienski wrote:
> >
> >I still claim that pimpl::pointer_semantics breaches value semantics
> >expectations in an unprecedented way. Both non-const refs and
> >shared_ptr,
> >and raw pointer and normal value semantic classes have one property in
> >common: Their operator== reflects their *salient attributes*. Sorry for
> >a
> >fancy term. I borrowed it from John Lakos:
> >http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2479.pdf,
> >
> https://github.com/boostcon/cppnow_presentations_2014/blob/master/files/accu2015.140518.pdf?raw=true
>
> The point of the Pimpl Idiom is to move what would be private data members
> out of the header. Any semantic thing you would have done, had the members
> been declared in the header, should be possible after applying the Pimpl
> Idiom. If value_semantics precludes proper equality comparisons, that's a
> major problem. If it doesn't preclude them, but changes the default
> behavior, relative to non-Pimpl classes, it's unfortunate.
>

>From what I understand the, pimpl::value_semantics part behaves
intuitively. It is just the other part of the library:
pimpl::pointer_semantics that has the controversial behaviour, and I
wouldn't even call it "pointer semantics", because pointers expose value
semantics.

Regards,
&rzej