Re: [boost] Interval Library and comparison operators

From: George A. Heintzelman (georgeh_at_[hidden])
Date: 2002-09-06 09:01:10

Peter Dimov wrote:
> Once you decouple std::less from operator<, you now need to always define a
> less<> specialization in terms of less<> whenever you would define an
> operator< in terms of operator<.

Right. This settles it for me. std::less should be an alias for 
operator<. And, my belief is that if you want to put intervals in a 
std::set, you need std::less to be a total ordering. A strict weak 
ordering isn't good enough, unless you want to require the user to 
always use a multiset, and I don't think we want to do that. Then there 
are two choices that I see for the ordering of operator<. One is 
lexicographic. The other is to use the midpoint as the primary sort and 
the first element as the secondary sort (and the second point as the 
tertiary -- otherwise integer ranges might become degenerate...)

Both give the same ordering in the case of non-overlapping intervals, 
so the question is which to choose in the overlapping case. IMHO, 
lexicographic isn't really that bad -- it can be intuited as 'starts 
earlier', it's fast to compute, and makes no new requirements on the 
base type.  And as I argued before, if you're dealing with overlapping 
intervals you'd better be thinking about it carefully anyway, though 
the library should supply functions/comparators to make that job easier.

George Heintzelman
georgeh_at_[hidden]