On Tuesday, 15 October 2013 at 05:44:25 UTC, captaindet wrote:
hi,
i am a bit confused.
the official language ref ( http://dlang.org/hash-map.html )
states:
"
Classes can be used as the KeyType. For this to work, the class
definition must override the following member functions of
class Object:
hash_t toHash()
bool opEquals(Object)
int opCmp(Object)
...
"
but now i stumbled on
http://forum.dlang.org/post/mailman.2445.1354457588.5162.digitalmars-d-le...@puremagic.com
"
int[typeof(O)] rc;
rc[O] = 42;
auto O2 = O;
// [...]
if (auto r = O2 in rc)
return *r;
else
return rc[O2] = compute(O2);
IOW explicitly taking the address may not be necessary when
doing that kind of things.
"
and i did a quick test and indeed, it seems to work out of the
box - without overriding any member functions. in my use case,
i wouldn't be able to modify the class anyway.
so my questions:
why is it working, is it just syntactic sugar for using
cast(void*)Obj as key?
what is the danger of using objects as keys? when would it fail?
as it seems to be working against language specs, will this
'feature' eventually be removed?
(then maybe i should use cast(void*)Obj right away...)
thanks, det
Do you have an example where it is really working? The problem of
not overriding those functions is that maybe you are not getting
what you think. For example, the code below will print:
[aaa.S:42]
[aaa.S:42, aaa.S:43]
This is probably not what you want. If you don't override the
functions how is the implementation to know what are equivalent
keys? Below you would probably expect two S's that are default
constructed to hit the same key spot in the AA. But this does not
happen since the implementation does not know you want new S to
equal new S because there is no opEquals.
Thanks,
Dan
-------------------------
import std.stdio;
class S {
int[] o = [1,2,3];
}
void main() {
S O = new S;
int[typeof(O)] rc;
rc[O] = 42;
writeln(rc);
rc[new S] = 43;
writeln(rc);
}
