On 2013-10-15 08:32, Daniel Davidson wrote:
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
in my use case i don't have multiple objects with the same state, so keying
them per obj_ptr (if this is what is happening) seems to be sufficient for me.
(the object classes are 3rd party so i cannot overwrite member functions
anyway.)
/det
