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

On 08/23/2017 07:41 AM, Gary Furnish via Boost wrote:

On Tue, Aug 22, 2017 at 3:18 PM, Vladimir Batov via Boost wrote:

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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

On 08/23/2017 01:06 AM, Gary Furnish via Boost wrote: 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::null(); } static user_type null() { return boost_impl_ptr_detail< user_type>::null(); }

Sure. If taking these convenience interfaces out makes people happy, I have nothing against it. No drama. I guess, I'll go take it out right away. :-)

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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 http://www.boost.org/doc/libs/1_65_0/doc/html/interprocess/offset_ptr.html 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.

Uh, right. Thank you for the link and explanations. Appreciated. And you are certainly correct, I should be using std::allocator_traits<...>::pointer. That said, I consider it an oversight, a nit to be addressed in due course... Agree?