On Tuesday, 11 December 2012 at 07:19:07 UTC, monarch_dodra wrote:
Here is a example of using COW for a simple object that wraps
nothing more than an int:
[snip]
This means you only pay for the copy when you *actually* need
it.
Drawbaks from this approach include:
1. Needs actual code.
2. NOT actually cheap: The counter goes up, and down, on each
and
every copy/destruction/pass: Requires a destructor and a
postblit
for RAII.
3 Implements deep ownership (*)
*Imo, this is the worst part of COW for a language like D. In
theory, you could just allocate your payload, and let the GC and
default copy take care of everything else, and move on with your
life. COW is (IMO) a c++ relic. Not only is this approach easy,
but it is *also* efficient. If you *do* need actual object
duplication, then you can implement "dup".
Thank you for the example. I see the benefit if performance is an
issue ... but what happened to premature optimization? For
example, the difference between the simple code below and a
version with COW may be easy - but I don't think it is trivial.
There is a fair amount of boilerplate bookeeping. What if the
struct had two, three, or more maps: V1[K1], V2[K2], V3[K3].
Would you have three separate reference counter payloads, or just
one payload with three maps? If you choose the latter you are
still aggressively copying when unnecessary. It seems one could
quickly take this to extremes. I did not understand the part
about "In theory, you could just allocate your payload, and let
the GC and default copy take care of everything else".
---
struct AddressBook {
alias Address[string] EmailAddressMap;
this(this) { _emailAddressMap = _emailAddressMap.dup; }
void addAddress(string email, Address address) {
_emailAddressMap[email] = address;
}
private EmailAddressMap _emailAddressMap;
}
---
//---------------------------------------
BTW: There is a subtle bug in this code. Can you spot it ;) ?
For me it did not compile (No constructor for payload), maybe you
had a ctor for Payload? I think opAssign is not necessary - if I
remove it it still seems to work since default opAssign calls
postblit. I added the ctor, removed opAssign and put it here:
http://dpaste.dzfl.pl/6ecfe675
Other than that - is there still a subtle bug?
Thanks,
Dan
