On 2010-10-06 19:03:45 -0400, Andrei Alexandrescu
<[email protected]> said:
On 10/6/10 16:38 CDT, Michel Fortin wrote:
On 2010-10-06 16:14:58 -0400, Andrei Alexandrescu
<[email protected]> said:
On 10/6/10 13:59 CDT, Michel Fortin wrote:
Could algorithms use "move" when possible (when "front" returns a ref)
and do a copy when move isn't available? That'd be a good compromise I
think.
T tmp = moveOrCopy(r1.front);
r1.front = moveOrCopy(r2.front);
r2.front = moveOrCopy(tmp);
Then problem is, people sort ranges of their own objects and are
amazed at the unbearable performance.
I'll repeat myself in clearer terms:
I agree with you that copy-constructor should be of linear complexity.
But I'm asking for constant complexity. We can only simplify if copy
construction has no impact on overall complexity.
Sorry, I was just confused and said linear when I meant constant.
I agree that you should use a copy when you can't do a move (when front
doesn't return a ref).
But that makes it impossible for an algorithm to guarantee good complexities.
Given my "by linear I meant constant" correction above, that should be
the same as you're proposing?
I disagree that you should use a copy even when you can do a move (when
front *does* return a ref).
Alright.
Therefore, I suggest that you use "move" when you can (when front
returns a ref) and fallback to a copy otherwise (when front doesn't
return a ref). Hence "moveOrCopy", which moves when it can move and
copies when it cannot move.
How can this result in poorer performances than to always copy? It can
only increase performance because you're copying less. (I understand
that it was probably just a misunderstanding of my suggestion.)
It doesn't lead to poorer performance than copying. The only issue is
that now you need to carry cost of copy everywhere as a possible
term/factor. Also, many algorithms would approach things differently if
the copy were O(n). This complicates, not simplifies, things.
Does this still apply given my correction above?
--
Michel Fortin
[email protected]
http://michelf.com/
