automem v0.0.7 - C++ style smart pointers using std.experimental.allocator

[Martin Nowak via Digitalmars-d-announce]

On Sunday, 9 April 2017 at 13:59:14 UTC, Andrei Alexandrescu 
wrote:
>> . The allocator has to be specified as part of the type: this 
>> means the user can choose how to store it in the smart 
>> pointer, which for singletons (e.g. Mallocator) or stateless 
>> allocators means they can take up zero space. If a singleton 
>> (or the default theAllocator), the allocator doesn't need to 
>> be passed in to the constructor, otherwise it does. 
>> Specifying, e.g. Mallocator also means the relevant code can 
>> be marked @nogc.
>
> After extensively studying how C++ allocator framework works, I 
> got to the notion that making the allocator part of the type is 
> an antipattern.

Just repeating an older argument of mine which didn't make it 
during the std.allocator discussions 
(http://forum.dlang.org/post/ubithltzbtdypaegnhvi@forum.dlang.org).

Yes, we clearly want type erasure for the Allocator, because 
having the Allocator as part of the type tends to push Allocator 
choices/forwarding everywhere into the APIs. It also prevents 
conversion/copying values with different allocators.
The obvious solution is to use an Allocator interface and/or a 
custom deleter function (deleter was needed for attribute correct 
destruction with polymorphic RC!Klass, see [¹]).

Now as Chandler Carruth mentions,

https://www.youtube.com/watch?v=fHNmRkzxHWs&t=3950
https://www.youtube.com/watch?v=fHNmRkzxHWs&t=4037

, an interface with dynamic dispatch would prevent optimizing 
away redundant allocations.
Allocations are complex enough that I'm not too worried about the 
virtual call overhead itself.

I think we might be able to solve this problem in D by making 
IAllocator.allocate pure, which tells the compiler that this 
function returns a fresh piece of memory without any side-effect, 
i.e. enough information to optimize away allocations.

Pure might be too restrictive for some allocators, but maybe this 
can be solved with a little type system hack (or at worse a 
compiler exemption).

[¹]: 
https://github.com/MartinNowak/phobos/commit/8cf0ec29ad65ac2a13bd6917b4ff3da0fdea5ab0#diff-4e008aedb3026d4a84f58323e53bf017R4896