Re: [boost] Review Request: impl_ptr (pimpl)

Subject: Re: [boost] Review Request: impl_ptr (pimpl)
From: Vladimir Batov (Vladimir.Batov_at_[hidden])
Date: 2017-08-18 21:44:13

On 2017-08-18 23:05, degski via Boost wrote:
> On 18 August 2017 at 13:47, Vladimir Batov via Boost 
> <boost_at_[hidden]>
> wrote:
> 
>> I can be dead wrong but it does not seem (unfortunately) applicable to
>> impl_ptr. It appears to be the standard allocator-usage pattern, i.e. 
>> I
>> explicitly create an allocator (and memory arena) and then take from 
>> that
>> arena. It's not applicable in the impl_ptr::onstack situation.
> 
> Aren't you doing something wrong (in your design) then? You called it:
> "standard allocator-usage pattern.". Standard is the key word here 
> isn't
> it, that's what should be used IMHO.

It's not about "doing" but rather "not doing". :-) It is an important 
design decision. Namely, where the allocator deployment is to be 
declared. One choice is to follow pimpl's philosophy and to deploy it in 
the implementation, i.e. totally hide it from the user, i.e. the 
developer decides how his pimpl implementation uses memory. The other 
choice is the standard (external) allocator deployment, i.e. applying it 
to the proxy declaration, i.e. giving it to the user.

The advantage of the first choice is the same as the main pimpl's 
advantage -- the developer decides to optimize or not memory usage, when 
and how. Seems attractive.

The advantage of the second choice is that it is the standard allocator 
deployment with all the perks. Attractive no less.

That appears to be a serious design decision with long-term impact and 
the potential of alienating some users (who are staunchly for one design 
or the other). Consequently, I was not implementing that hard bit as I 
personally do not use it. My idea was that, if impl_ptr gets over the 
"endorsement request" hurdle and gets to the review stage, then I'd 
bring that design problem to the list and we'd decide one way or the 
other. If impl_ptr does not get anywhere and is only used in my 
backyard, then I'll leave it as-is.