Re: [boost] Interval Library and comparison operators

From: Guillaume Melquiond (gmelquio_at_[hidden])
Date: 2002-09-06 07:18:52

On 6 Sep 2002, Gabriel Dos Reis wrote:

> Guillaume Melquiond <gmelquio_at_[hidden]> writes:
> 
> | I can imagine a lot of situations where somebody would like to use
> | intervals as keys. However I don't really see a case where it is needed
> | to have another order than the lexicographic one.
> 
> For intervals in general, me neither.
>
> | In particular, if the
> | intervals don't overlap, it can be used as a positional relation.
> | 
> | Could you explain what kind of order you need in your example about the
> | roots of a polynomial?
> 
> I didn't say I needed a *particular* order; what is important is the
> _ability_ to use it as a key.  Yes, one can define one each time one
> needs it (I used the lexicographical compare).
>
> Similar arguments would apply for pointers (in general) also, but it
> is much more convenient to say std::map<T*, U>, than 
> std::map<T*, U, MyCompare> when the exact ordering MyCompare doesn't
> really matter.

My opinion on this two points are the same as yours: the lexicographic is
the only useful order for interval keys in the genral case; and it should
be better if we didn't have to specify it whenever we create an ordered
data structure.

So maybe it's time to put back 'less< interval<...> >' and to define it
with the lexicographic order. But as someone (sorry to say "someone", I
can't find the mail anymore) pointed it, 'less' should be equivalent to
'<'.

However, one of the predefined comparison policy class is the
lexicographic order. So, if the interval type has been defined by using
this policy, there is no more problem. The operator '<' will then be the
lexicographic order and 'std::map' will be happy.

Guillaume