Manual memory management in D2

[Andrei Alexandrescu]

On 07/13/2010 03:16 PM, Andrei Alexandrescu wrote:
> On 07/13/2010 01:59 PM, Vladimir Panteleev wrote:
>> On Tue, 13 Jul 2010 21:36:02 +0300, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> Not quite. New is different because it is a primitive - it can't be
>>> implemented as a function (well it could if it user placement new, but
>>> we're back to square one). In contrast, delete already knows the type
>>> of the object it's destroying and can call the destructor manually so
>>> it is easy to implement as a regular function.
>>
>> Sorry, not following you here. Why can't new be implemented as a
>> templated function which takes the type of the object to create as a
>> template parameter? Isn't it just allocation, copying over T.init, then
>> calling the constructor with whatever arguments the user passed?
>
> In D it's possible to call the constructor by issuing __ctor(). In C++
> there is no way, hence the presence of a language primitive.
>
>>> But manual deletion has no business in the garbage collected heap.
>>
>> Isn't that making the assumption that all D programs are
>> garbage-collected?
>
> No. It is not making that assumption.
>
>>> That currently druntime provides it is an accident caused by
>>> thecurrent implementation; most GC's cannot provide efficient manual
>>> reclamation. And they shouldn't anyway.
>>
>> Why not? And what if I don't use the GC (just the
>> allocation/deallocation aspects of it)?
>
> You should use malloc() and free() for manual memory management and the
> GC for automatic memory management. Each of these two is ill-suited for
> carrying the other's job.
>
>>> (There's a longer discussion underneath this concerning what memory
>>> really is and what GCs really are and do.)
>>
>> I understand your points regarding leaving deallocation to happen on a
>> GC run being more efficient than manually deallocating objects
>> individually,
>
> I didn't make that point.

Apologies. I now understand how you were referring to that point.

To further clarify: on certain GC implementation, the cost of one 
individual object deletion is comparable to the cost of a full collection.


Andrei