Re: [boost] [optional] operator<(optional<T>, T) -- is it wrong?

On Mon, Dec 1, 2014 at 8:44 PM, Matt Calabrese wrote:

Not vector. I was referring to set and map, whose use frequently doesn't

Just two? That's not what I'd call many datastructures.

depend on what the ordering actually is, but often only that there simply is an ordering. While on the topic though, the standard containers such as std::vector /do/ use std::less for their operator< (even though the requirements of the operation are [inconsistently] specified on the value type's operator< rather than their default ordering function). This was

I'm not sure what you're saying. Requirements specify operator< but they're using std::less? I'm confused.

I think I'm still thinking about operator<

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Other then map, do you have a reference for all these algorithms etc that would benefit from less>?

Any algorithm that takes a comparator and has a default (there are plenty in the standard library). This is no different from the set and map cases where you often don't care about the specific order, but rather, only that an order exists. Depending on your specific use, this can be true with respect to algorithms such as std::sort and std::binary_search, for example.

True

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lack of a default comparator for the types in question has already led to

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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?

Not all hash functions are created equal and there are reasons to explicitly specify one as opposed to using the default, just as there are reasons to explicitly specify an ordering function.

True, but not an answer to the question.

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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.

Why no? Why is "less" as a default objectively better than "greater" as a default?

Because in the majority of cases people order stuff in ascending order.

It's simply not, we just use it by convention. The same is true for why we frequently define other operators in terms of operator<. We could just as easily have decided that operator> was the "base" operation to be used as a default and around which we built other operators. < is nothing special and not intrinsically better. I understand that you personally intuitively like < best, but you're only kidding yourself if you think that there is something deeper.

True, operator> could've been used. But then default order would probably still have been ascending.. -- Olaf