Re: [boost] [STLInterfaces] only three operations?

On Wed, Nov 27, 2019 at 3:49 PM Andrzej Krzemienski wrote:

śr., 27 lis 2019 o 15:54 Zach Laine napisał(a):

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On Wed, Nov 27, 2019 at 8:38 AM Andrzej Krzemienski via Boost < boost@lists.boost.org> wrote:

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Hi, While the formal review of STLInterfaces has not yet started, some early feedback may be of help.

The idea to implement relational operators from binary operator- for random_access_iterator looks too clever for me.

I don't find that to be so. All those operations (the relops and op-) go together semantically. Could you elaborate on why you don't like the approach?

I am sorry. I was writing that mail in a hurry an I failed to communicate my thoughts clearly. I do like your approach. In fact, I find it really illuminating. It is just that I find it so novel, that it could easily be taken for a mistake. I was staring at the docs for a long while, where it said the only operations I need for random access iterator was *, -, and +=, and I concluded that there must be some some error in the script that produces the documentation and the guy surely wanted to type operator== as well. Only after looking at the implementation did I realize that it was not a mistake.

Hence my suggestion: make it very clear in the documentation that relops are generated from operator- for random access iterators.

Got it, thanks. Based on your original email, I've made this clear in the docs (though that change is not live on the website yet). [snip] It's true that the deque case will be suboptimal. Here are my

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considerations though:

- The point of the library is to reduce the amount of code you must write to the bare minimum, and basing all the relops on op- and op== does not serve that goal. - Any generated operation (not just the relops) can be replaced by the user at will, just be having the user define that operation. In the deque case, that means defining op-, op==, and op!=. Deque is not the common case -- most random-access containers are also contiguous. I want to maintain the brevity of the common case. That means the solution is clear documentation, IMO.

I agree with aiming at the minimum boilerplate from the user, and I agree that non-contiguous random access iterators are so rare that they could be sacrificed. However, I believe that what I propose does not compromise your goals. As Hans pointed demonstrated, if you implement operator== using operator-, and then implement operator!= using operator== (rather than using operator-), then all your use cases work as before, but the additional gain is that if I additionally volunteer to provide my own operator== (because it is faster) you can give me operator!= for free.

Does this make sense?

Yes, it does. I'll try this, but I vaguely remember that I tried things this way and got an ambiguity. Hopefully I'm misremembering. Zach