clear() and UFCS

[Jonathan M Davis]

On Saturday, May 26, 2012 04:04:51 Mehrdad wrote:
> Would you mind explaining the difference between clear(), delete,
> and __dtor/etc? I think it's a little confusing for me.

clear destroys the object (by calling its finalizer or destructor) and then in 
the case of classes, it zeroes out its vtable so that if you accidentally the 
object after clearing it, it'll blow up rather than getting weird, non-
deterministic behavior. In the case of structs, I think that it puts the 
object in its init state, but I'm not sure.

clear does _not_ free the object's memory. It merely destroys the object so 
that whatever other resources it references can be released (or just 
unreferenced in the case of GC heap allocated objects that it references).

delete does what it does in C++. It destroys the object and then frees the 
memory. It was decided that having a language construct which freed memory on 
the GC heap was too dangerous and error-prone and shouldn't be encouraged, so 
it's being removed. It's still quite possible however, to create a function 
which destroys and object and then tells the GC to free that object's memory 
using the stuff in core.memory.

__dtor is simply the object's destructor. Calling it calls that function. 
However, I don't believe that it destroys what the object references (since 
that would require doing more than simply calling the destructor). As I 
recall, you have to do something else which is a bit involved to do that. 
There was a discussion on it recently, but I don't recall the details 
unfortunately.

In the long run, I expect that anyone who really wants to use something akin 
to delete will likely use custom allocators (e.g. using malloc and free) 
rather than using the GC heap, possibly with some sort of ref-counting. So, 
you'd get something like

//non-ref-counted
auto a = ca.allocate!MyClass(args);
ca.deallocate(a);

or

//ref-counted
{
    auto a = ca.allocate!MyClass(args);
}
//a has left scope, so its ref-count reach zero, and it was destroyed

Obviously we don't have the custom allocator stuff fully ironed out yet though. 
Andrei has been working on those, but he's been a bit swamped, so who knows 
when we'll see them.

- Jonathan M Davis