Containers, Allocators and Purity

[ZombineDev via Digitalmars-d-learn]

On Thursday, 24 March 2016 at 11:18:06 UTC, Nordlöw wrote:
> Could somebody briefly outline how the thread-locality 
> (non-GC-locked) of allocators  relates to the purity of the 
> containers using them?
>
> This because I want to move forward with optimizations in my 
> knowledge graph that requires GC-free array containers storing 
> value typed elements (integers) which preferrably has pure API.
>
> Specifically, I want to use something like
>
> https://github.com/economicmodeling/containers/blob/master/src/containers/dynamicarray.d
>
> that is `@safe pure` and uses lock-free allocations in a 
> heavily multi-threaded application.
>
> If I want purity aswell which `std.experimental.allocators` are 
> possible?

Currently almost nothing in `std.experimental.allocators` is 
explicitly marked as pure, except for some of the things in 
./common.d 
(https://github.com/D-Programming-Language/phobos/pull/3957 
*shameless plug*). Thankfully make, makeArray, dispose and some 
of the allocators are templates, so you can rely on attribute 
inference.

The most important thing is that you need to have pure `allocate` 
and `deallocate` methods. After this `make` and friends will work 
like magic (at least for types with pure `this` and `~this`). 
`pure` statefull allocator essentially means that it has to be 
thread-local, so there are no global side-effects. Essentially, 
to achieve this you can make a large initial allocation in each 
thread (even from a non-pure allocator) and then you can make 
`pure` sub-allocations out of it.

You can read more about my adventures with my `pure` smart 
pointer here:
http://forum.dlang.org/post/eegjluaiwvdxfnbxkxym@forum.dlang.org
http://forum.dlang.org/post/bvgyrfvuqrqcyvhkqkrt@forum.dlang.org