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Subject: Re: [boost] [optional] operator<(optional<T>, T) -- is it wrong?
From: Olaf van der Spek (ml_at_[hidden])
Date: 2014-12-01 13:58:54

On Mon, Dec 1, 2014 at 5:29 PM, Matt Calabrese <rivorus_at_[hidden]> wrote:
> It is arbitrary until and unless people actually explain what they feel is
> the difference between cases that they consider "reasonable" and
> "unreasonable." This is particularly true when one container that uses a
> lexicographical compare is considered "reasonable" while another container
> that uses a lexicographical compare is considered "unreasonable." Why you
> want a default order is generally analogous in both situations and denying
> one while not the other without any objective rationale is a very weak
> decision. All types can be ordered and all types that have more than one
> value can be ordered in multiple ways. No particular ordering is is
> applicable in all situations for any type -- even integers. That does not
> mean that a default is unimportant since frequently you require ordering
> for a datastructure or algorithm to work, but the actual ordering may be
> unimportant to the user or to the implementation. This is the whole point,
> which I mentioned in every single reply.

IMO integers have a natural ordering, optionals don't.
In most cases the natural ordering for integers is fine, whether the
proposed ordering for optionals is fine isn't clear.

>> > much more controversial matter). The notion of default ordering is useful
>> > because many datastructures and algorithms can make use of it,
>> particularly
>> What std datastructures use it?
> The ones that we've been talking about for the entire thread in almost
> every reply. I'm not going to repeat again.

vector? Doesn't use std:less for operator< does it?
map/set? Sure, maybe, most of the time

> Again, I have done so multiple times in this thread, both with concrete
> examples and with reference to why it is important in more generic code.
> You can choose to put your fingers in your ears if you want, but that
> changes nothing. Frankly, it's sort of unfortunate that the usefulness of
> ordering for /any/ type needs to be re-explained in the context of C++,
> since it's such a fundamental concept that has been talked about many times
> before, not just by myself in this thread but by key C++ people, i.e.
> Stepanov. Especially when you are creating a generic composite type that
> you expect people to use, you should be conscious of ordering.

Lots of types survive just fine without (default) ordering.

>> The only thing you
>> > accomplish by not having a default ordering is that you arbitrarily make
>> > the type difficult to use with these datastructures and algorithms. The
>> Having to explicitly pass in a comparator is not difficult is it?
> As explained, it is both needless and can also be an arbitrary decision
> even in non-generic code, let alone in generic code where all you usually
> care about is that an ordering exists rather than what that particular
> ordering is. Ideally you always forward the comparator along when writing
> generic code that needs ordering, using a default of std::less just as the
> standard does for its generic containers, and yet the default is still
> useful. In fact, default comparators are so useful in the standard library
> that plenty of C++ programmers don't even know that the comparators are
> template arguments at all, yet they get along fine. I doubt that explicitly
> passing the comparator is "not difficult" to them. Regardless, stating that
> explicitly passing an argument where a default can be used is "not
> difficult" is a cop-out since you can always say the same thing regarding
> any default in any context. In this particular context, that hypothetical

I think I'm still thinking about operator<
Other then map, do you have a reference for all these algorithms etc
that would benefit from less<optional<>>?

> lack of a default comparator for the types in question has already led to
> the recommendation of preferring entirely different associative
> datastructures over passing in an explicit comparator in this very thread
> (ironically, the other containers that were recommended also depend on a
> subjective default, only it's the default hashing function instead of the
> default ordering function).

How is the hash function subjective w.r.t. semantics?

> You should not be afraid to have your types be ordered when they can be
> ordered. Most decisions in development have a subjective aspect to them and
> precisely which default ordering comparator you provide is just one of
> those many decisions. The fact that a default is subjective (and it always
> is, even for arithmetic types), does not mean that a default is not useful.
> Why do we use less as a default for arithmetic types with respect to a set?
> Wouldn't greater be just as sensible?

No, IMO, though for a lot of uses it probably wouldn't matter.

> What about the other orderings? We
> only accept that it is "reasonable" now to have less be the default
> ordering because that was the decision that was made decades ago and people
> are used to it. It. Is. Just. A. Default.
> --
> -Matt Calabrese


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