std.allocator issues

[Steven Schveighoffer via Digitalmars-d]

On 2/20/16 12:22 AM, Andrei Alexandrescu wrote:
> On 02/19/2016 10:12 PM, Steven Schveighoffer wrote:
>> On 2/19/16 9:21 PM, Steven Schveighoffer wrote:
>>> I'm trying to use the std.experimental.allocator API more in my new io
>>> library, and I'm having a few stumbling points:
>>
>> Another thought:
>>
>> Allocators return void[] when requesting allocations, reallocations,
>> etc. For C malloc, the assumption must be that what you requested is
>> what you got, because the actual block size isn't given. However, for GC
>> (and I assume other allocators), we have mechanisms to know upon
>> allocation the amount of data received.
>>
>> I would assume since we are returning both pointer and length, it would
>> be possible for an allocator to return more data than requested (why
>> waste it?). But it appears that the GC allocator just returns the amount
>> of data requested, even though it could return the extra data that was
>> received.
>>
>> Should the API assumptions allow returning more data than requested? It
>> means code has to be wary of this, but creating a wrapper allocator that
>> truncates the data would be trivial, no?
>>
>> I want to write some PRs to fix this, but I'm unclear what is expected.
>
> Allocators are guaranteed to return the size requested. This is by
> design. -- Andrei
>

Given that there is "goodAllocSize", this seems reasonable. But for 
ease-of-use (and code efficiency), it may be more straightforward to 
allow returning more data than requested (perhaps through another 
interface function? allocateAtLeast?) and then wrap that with the other 
allocation functions that simply slice the result down to size.

In the GC, expanding is done a page at a time. If I request an expansion 
of 1 byte, that's 4095 wasted bytes. I don't expect goodAllocSize to 
help me here.

BTW, just pushed an update to my i/o library that uses allocators 
exclusively for buffers. But I make the default a custom GC allocator 
that has the properties I need. I'm hoping there will be a way to get 
the desired behavior from the phobos version at some point.

-Steve