On 10/15/2010 08:34 PM, Walter Bright wrote:
Max Samukha wrote:
I don't see why. They're useful enough.
I might have exaggerated. But, for example, this use case:
struct S
{
int x, y, z;
}
ref S foo();
void bar(ref S s);
void baz()
{
auto s = &foo();
s.x = 1;
s.y = 2;
s.z = 3;
bar(*s);
}
will not be easy. One will have to use tricks like that unsafe Ref
struct or to move the code accessing the referenced object to another
function. Pretty awkward.
What I like about Go's solution is that it is consistent with
closures. When a group of locals escape with a closure (that is when
the address of the local function using that group is taken) they are
copied to heap. When a local escape by ref (that is when the address
of the local is taken), it is also copied to heap.
I understand how it works.
Of course, you do!
There is a downside to it, though. In D2,
closures get copied to the GC heap if there is a possibility of an
escaping reference. A lot of people complain about this being unexpected
hidden overhead.
Yeah, I don't like these hidden allocations either.
The trouble with "copy any ref'd local to the heap" automatically
happening is the biggest advantage of passing by ref (efficiency) is
automatically lost. Even if it does not escape, it is copied to the heap
anyway, as you point out below.
Indeed. Why create a stack-allocated local if it is going to be copied
to the heap anyway?
What I don't like about Go's closures/addresses-to-locals and D's
delegates is that stuff is heap-allocated implicitly and by default.
Go has even gone (sorry) as far as allocating copies *every* time the
address of a local is taken.
Exactly. (Though D won't do the copy if it can prove that the delegate
does not escape.)
That reminds me of the argument about "new" being necessary for
classes because it makes the heap allocation explicit. It is difficult
to say good-bye to "new" but at the same time we are somehow happy
with implicitly allocated closures.
I think that implicitly allocated closures are a lot less common than
passing a local by reference.
I have no idea how often closures are used. I use them rarely but some
people do crazy things with them.
