on Wed Sep 03 2008, "Giovanni Piero Deretta" <gpderetta-AT-gmail.com> wrote:
On Wed, Sep 3, 2008 at 7:21 PM, David Abrahams <dave@boostpro.com> wrote:
on Wed Sep 03 2008, "Giovanni Piero Deretta" <gpderetta-AT-gmail.com> wrote:
Do you think we are breaking the iterator abstraction? How? Or I've misunderstood the question?
Well, if we add "Factorable" or something like it, the iterator abstraction gets broken down into smaller bits for the purpose of storage, complicating some code [like a generic range for example ;-)] and it imposes an extra coding duty on writers of high-quality iterator adaptors. I'm asking if its worth it.
We are simply trying to come up with a simple and clean trick to keep iterator size under control.
Really? IIUC so far we're only discussing compressing the size of ranges, unless we're willing to pay for indirection inside of iterators (I'm not).
But iterators like filter_iterator that need to know iteration boundaries, can store a range instead of an iterator, so they can benefit from the compression.
Oh, I totally missed that; sorry! Let's see... strided_iterator<int*,5> stores range<int*> sizeof(range<int*>) == 2*sizeof(int*) filter_iterator<strided_iterator<int*,5> > stores range<strided_iterator<int*,5> > range<strided_iterator<int*,5> > is represented as unique_data<strided_iterator<int*,5> >::type start; unique_data<strided_iterator<int*,5> >::type finish; common_data<strided_iterator<int*,5> >::type common; unique_data<strided_iterator<int*,5> >::type == int* common_data<strided_iterator<int*,5> >::type == int* so its size is 3*sizeof(int*) ?? Did I get that right? If so, it seems like there's still redundant storage. finish and common will be representing essentially the same information. If I were to write a filter_strided_iterator adapter and apply it to int*, it would have the size of 2 int*s. a
I don't understand why you would implement any_iterator yourself when there are so many extant implementations, but maybe it's none of my business.
Mostly for experimenting. I'll might end up using something third party in real code.
Currently the iterators I use are often over 400 bytes, which is a bit to big :), thus the need to squeeze their size down as much as possible. A hundred bytes could be enough [1].
Again, have you measured?
If you mean the size, yes, I've measured it. If you mean my assertion that 100 byte SBO may be optimal, then I wont' let facts interfere with my opinions :)
I meant measured that a type-erased 100 byte iterator improves performance over a bald 400-byte iterator.
Now, honestly, I'm trying to rationalize the fear that I had for a while that complex iterator adaptor sequences really can grow large. For example, the current filter_iterator double its size at every stacking. If you couple it with a relatively heavy predicate, the stack usage might start to really matter. On heavy threaded applications, stack usage is a somewhat limited resource.
And I'm trying to say that the point of GP is to raise coding to the highest possible level of abstraction *without loss of efficiency*. If we can't achieve "relative efficiency" as described in http://www.stepanovpapers.com/BYTE_com.htm, I don't think we're doing the job right. So far, unless I'm mistaken, we're still missing the mark by a fairly wide margin. -- Dave Abrahams BoostPro Computing http://www.boostpro.com