Problems with std.experimental.allocator

[Igor via Digitalmars-d-learn]

On Saturday, 2 September 2017 at 11:23:00 UTC, Igor wrote:
> I realize these are not yet stable but I would like to know if 
> I am doing something wrong or is it a lib bug.
>
> My first attempt was to do this:
>
> 	theAllocator = allocatorObject(Region!MmapAllocator(1024*MB));
>
> If I got it right this doesn't work because it actually does 
> this:
>
> 1. Create Region struct and allocate 1024MB from MMapAllocator
> 2. Wrap the struct in IAllocator by copying it because it has 
> state
> 3. Destroy original struct which frees the memory
> 4. Now the struct copy points to released memory
>
> Am I right here?
>
> Next attempt was this:
>
> 	theAllocator = 
> allocatorObject(Region!()(cast(ubyte[])MmapAllocator.instance.allocate(1024*MB)));
>
> Since I give actual memory instead of the allocator to the 
> Region it can not dealocate that memory so even the copy will 
> still point to valid memory. After looking at what will the 
> allocatorObject do in this case my conclusion is that it will 
> take a "copyable" static if branch and create an instance of 
> CAllocatorImpl which will have a "Region!() impl" field within 
> itself but given Region!() struct is never copied into that 
> field.
>
> Am I right here?
>
> If I am right about both are then these considered as lib bugs?
>
> I finally got it working with:
>
> 	auto newAlloc = 
> Region!()(cast(ubyte[])MmapAllocator.instance.allocate(1024*MB));
> 	theAllocator = allocatorObject(&newAlloc);
>
> Next I tried setting processAllocator instead of theAllocator 
> by using:
>
>     auto newAlloc = 
> Region!()(cast(ubyte[])MmapAllocator.instance.allocate(1024*MB));
>     processAllocator = sharedAllocatorObject(&newAlloc);
>
> but that complained how it "cannot implicitly convert 
> expression `pa` of type `Region!()*` to `shared(Region!()*)`" 
> and since Region doesn't define its methods as shared does this 
> mean one can not use Region as processAllocator? If that is so, 
> what is the reason behind it?

After a lot of reading I learned that I need a separate 
implementation like SharedRegion and I tried implementing one. If 
anyone is interested you can take a look here 
https://github.com/igor84/dngin/blob/master/source/util/allocators.d. It doesn't have expand at the moment but I tried making it work with this:

     processAllocator = sharedAllocatorObject(shared 
SharedRegion!MmapAllocator(1024*MB));

I did it by reserving first two bytes of allocated memory for 
counting the references to that memory and then increasing that 
count on postblit and decreasing it in destructor and identity 
assignment operator. I then only release the memory if this count 
gets bellow 0. Problem is I couldn't get it to compile since I 
would get this error on above line:

     Error: shared method 
util.allocators.SharedRegion!(MmapAllocator, 8u, 
cast(Flag)false).SharedRegion.~this is not callable using a 
non-shared object

I couldn't figure out why is it looking for non-shared 
destructor. If I remove shared from destructor then I get that 
"non-shared method SharedRegion.~this is not callable using a 
shared object" in std/experimental/allocator/package.d(2067).

I currently use it with pointer construction: 
https://github.com/igor84/dngin/blob/master/source/winmain.d#L175