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Subject: Re: [boost] Review Request: impl_ptr (pimpl)
From: Gary Furnish (gfurnish_at_[hidden])
Date: 2017-08-22 21:41:06

On Tue, Aug 22, 2017 at 3:18 PM, Vladimir Batov via Boost
<boost_at_[hidden]> wrote:
> On 08/23/2017 01:06 AM, Gary Furnish via Boost wrote:
>> While I like the idea in general (bikeshedding aside), it should not
>> be possible to end up with nulls when treating something as a value
>> type.
> Is it a general observation or you have a particular use-case in mind that
> involves the discussed impl_ptr? It that is the former, then IMO it is not
> an issue as impl_ptr is a base class and the user/interface/proxy class
> ultimately decides if it is "possible to end up with nulls" or not. If that
> is the latter, then I'd certainly like to look at it and to evaluate and to
> fix if necessary.
On the nulls: just looking it may be policy dependent if you can end
up with nulls in the sense of runtime, but the machinery for nulls
will still show up in auto completion,name resolution, etc unless it
is enable-ifed or specialized out. It is possible I mistook how this
works though. I don't see a problem with supporting shared situations
with policies, but it should be 100% disabled. No

static other_type null() { return boost_impl_ptr_detail<other_type>::null(); }
static user_type null() { return boost_impl_ptr_detail< user_type>::null(); }

>> All of that should be hidden away with some sort of enable if
>> machinery. Also your support for allocators doesn't work properly
>> because if you use something like an allocator that returns offset
>> pointers you assume that all allocators return normal pointers.
> Well, "your support for allocators doesn't work properly" seems like a
> sweeping statement. They "seem" to work for me but you must have a
> particular use-case in mind where they do not. Do you mind elaborating,
> sharing it? I am somewhat puzzled by the meaning of "offset pointers". I
> presume it is a pointer offset from the beginning of actually allocated
> block, right? If so, does std::allocator support that?.. impl_ptr uses the
> facilities provided by the allocator concept. Namely, if your particular
> allocator returns an "offset pointer" from allocate(), then that same
> pointer is given back to your allocator to deal with inside deallocate().
> So, where does "your support for allocators doesn't work properly" come in?

Offset pointers are pointers that you use with an allocator where the
pointer is actually a size_t that is added to the location of the
pointer. This is used in mmap situations where you do not know the
absolute address, only relative to the current pointer offset. See
 So you just define an allocator where Allocator::pointer =
boost::offset_ptr<T> and vector etc automatically work in shared
memory. The basic punchline is instead of hardcoding the allocator
return value as T* you want to capture a
std::allocator_traits<...>::pointer . You can then use
std::address_of(*raw memory) to get the raw memory address. (Note
address of because operator& could be overloaded).
(This is how to do it in c++17, earlier versions require a bit more
boiler plate). It is not true allocator support if you don't support
the rarer cases.

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