On 07/12/2012 06:15 AM, Andrei Alexandrescu wrote:
Required reading prior to this: http://goo.gl/eXpuX
You destroyed, we listened.
I think Christophe makes a great point.
+1.
We've been all thinking inside
the box but we should question the very existence of the box. Once the
necessity of opCmp, opEquals, toHash, toString is being debated, we get
to some interesting points:
1. Polymorphic comparisons for objects has problems even without
considering interaction with qualifiers. I wrote quite a few pages about
that in TDPL, which add to a lore grown within the Java community.
2. C++ has very, very successfully avoided the necessity of planting
polymorphic comparisons in base classes by use of templates. The issue
is template code bloat. My impression from being in touch with the C++
community for a long time is that virtually nobody even talks about code
bloat anymore. For whatever combination of industry and market forces,
it's just not an issue anymore.
3. opCmp, opEquals, and toHash are all needed primarily for one thing:
built-in hashes. (There's also use of them in the moribund .sort
method.) The thing is, the design of built-in hashes predates the
existence of templates. There are reasons to move to generic-based
hashes instead of today's runtime hashes (such as the phenomenal success
of templated containers in C++), so it can be argued that opCmp,
opEquals, and toHash exist for reasons that are going extinct.
4. Adding support for the likes of logical constness is possible, but
gravitates between too lax and onerously complicated. Walter and I don't
think the aggravation is justified.
There are of course more angles and considerations. Walter and I
discussed such for a while and concluded we should take the following
route:
1. For the time being, rollback the changes. Kenji, could you please do
the honors? There's no need to undo everything, only the key parts in
object.d. Apologies for having to undo your work!
2. Investigate a robust migration path from the current use of opCmp,
opEquals, toHash (we need to also investigate toString) to a world in
which these methods don't exist in Object. In that world, associative
arrays would probably be entirely generic. Ideally we should allow
existing code to still work, while at the same time fostering a better
style for new code.
What say you?
Andrei
Thank you for taking the time.
Removing the default methods completely is actually a lot better than
making inheriting from Object optional or tweaking const beyond
recognition and/or usefulness.
I was afraid to suggest this because it breaks all code that assumes
that the methods are present in object (most code?), but I think it is
a great way to go forward.
Regarding toString, getting rid of it would imply that the default way
of creating a textual representation of an object would no longer be
part of Object, paving the way for the proposal that uses buffers and
scope delegates - this will be purely a library thing.
Regarding backwards-compatibility, an issue that is trivial to fix is
the invalidation of 'override' declarations in the child classes.
They can be allowed with the -d switch for those methods. And if they
use 'super', the compiler could magically provide the current default
implementations.
