GCAllocator goodAllocSize?

[Steven Schveighoffer via Digitalmars-d]

On 1/30/16 12:41 PM, Andrei Alexandrescu wrote:
> On 1/29/16 9:52 PM, Steven Schveighoffer wrote:
>> import std.experimental.allocator.gc_allocator;
>>
>> void main()
>> {
>>     //GCAllocator.instance.goodAllocSize(3000); // error
>>     writeln(theAllocator.goodAllocSize(3000)); // 3008, should be 4096
>> }
>>
>> This is known info, why is it not available? Is there a reason this
>> needs to be inaccurate? I can create a PR if desired.
>
> There's no need, I just put in reasonable conservative defaults. Please
> do follow up - thanks! -- Andrei
>

OK, I'm making a PR for this.

Incidentally, you can do aligned allocation with GC (beyond just 
sizeof(real)). Just make sure your allocation requested is at least that 
many bytes (GC puts data into bins, each bin having the amount of bytes 
requested).

So for example, if you want a 16-byte allocation aligned to 32-bytes, 
just allocate 32 bytes and slice the first 16.

Followup questions:

1. How does one detect that automatic memory cleanup will occur? In 
GCAllocator, deallocate is implemented, so I can't tell that "no you 
don't need to call deallocate"

2. How does one allocate with an allocator for typed info? In other 
words, the GC will call the dtor, but only if it knows what type you put 
in there. With std.experimental.allocator, there isn't a way to do that.

3. I'm working on a library that (hopefully) takes an allocator for 
allocating a ubyte array for buffer space. If I use GCAllocator, this 
will mark the array as having pointers conservatively. It will also 
initialize all the data to 0 (needlessly in this case). How to do this 
correctly?

Thanks

-Steve