[Boost-users] Re: [Foreach]TroubleiteratingoverBoost.MultiIndexcontainers

"Peter Dimov" writes:

David Abrahams wrote:

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"Peter Dimov" writes:

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Ian McCulloch wrote:

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Peter Dimov wrote:

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Right. And if they did not collide, nothing would become a de-facto standard, because people could keep using their own versions. There would be no pressure to resolve these collisions.

So even on boost, such collisions occur - what about libraries that are developed completely independently? Which politburo is to decide how each name is supposed to be used? How is it to be enforced?

It is unenforceable, of course. But this doesn't mean that it can't happen. Library authors are, in general, reasonable people; they do cooperate and do respond to issues.

I don't know about that. The various placeholders in Boost still have usability problems when #included together. And the bind placeholders are still causing ODR violations.

The placeholders do have the same meaning; they are an example of a de-facto standard. The conflicts that exist are (a) resolvable, (b) caused by factors that do not apply to our discussion.

Not exactly. I'm thinking of MPL and bind actually. They don't have the same meaning, only a similar one, and they can't be unified. It's the very "standard-ness" that prompted MPL to use the same names in a very similar -- but not identical -- way.

And for the record, Lambda's placeholders cause exactly the same ODR violations, although I'm not sure whether "cause" is the proper term.

You're right, "lead to" would be more accurate. I hope you understand the following: 1. I'm arguing about this because I want to end up convinced of something, and soon -- I'll need to choose customization techniques imminently -- so thank you for engaging me on this. 2. I only brought up the ODR thing as a piece of evidence that problems don't neccessarily get resolved quickly

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It's all very well that library writers may adjust eventually, but users can't always upgrade, or they can't always upgrade quickly. There ought to be a mechanism for staying out of one another's way.

Such a mechanism does exist.

Yeah, but it's cumbersome, and has a few other problems some of which you've mentioned. With that sentence I was arguing that we ought to have a mechanism that works more smoothly, and suggesting that -- with the right language changes -- avoidance of the problems with the current mechanism does not _have_ to imply the use of ADL.

I maintain that not using this mechanism is better in the long term.

Why?

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This isn't really specific to ADL customization points. Consider, for example, container requirements, where a.begin() must return A::iterator or A::const_iterator. What happens if another library has its own requirements table that specifies different semantics for a.begin()? Can I make my container conform to these requirements as well? Of course not.

That's why generic library writers have developed a guideline to write concept requirements that don't depend on member functions.

Maybe they did. My point is that I never see anti-v.begin() campaigns;

I do. Why do you think Boost.Range is using free functions? Not putting member functions in concept requirements is a well-known generic programming guideline. For years even Scott Meyers has been saying that "interfaces should be extended with free functions," which isn't viewed through the generic customization-point lens, but means the same thing.

it's ADL that gets all the flack.

I think that's because ADL has too many problems besides the one we're discussing right here (intrusive name reservation). Also because once we understand that member function requirements are a bad idea, we're forced to face those other problems and (I at least) anguish over whether to live with them or take the leap into using specialization. And because GCC's liberal interpretation of ADL screws everyone eventually ;-)

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And wern't namespaces supposed to solve these issues anyway?

The central question is: can we, the C++ community, decide on a common vocabulary for the primitive operations a type supports? If we can, there is no problem.

I disagree most strongly.

One problem is that not all operations are going to be considered "primitive." More importantly, there should be a way to develop software with confidence that it has a future and scales well, without having to worry that all customizable names are "pending the community's verdict on common meaning."

See above; there is such a way

Sorry, I can't connect the dots. What way?

and the debate is whether not having to worry is a good thing. Customization points are very important and need to be treated with caution.

What kind of caution, if they are associated with a namespace, and why? We could use std::iterator_traits as an example.

Once they become de-facto standards, the library no longer owns them, even if they are in the library's own namespace.

Even if I understood what you meant by "the library no longer owns them, even if they are in the library's own namespace," what are the implications of that? Is std::iterator_traits an example of a de-facto standard customization point in a namespace? There are clearly some (just a very few, IMO) customization points like swap that are really intrinsic. They have to do with the semantics of what Stepanov and friends are calling "value types." It seems to me as though the rest are associated with a particular domain, and it's appropriate to name that domain.

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Just consider the widespread use of numeric types in all kinds of domain-specific applications. Is every library going to pick the same meaning for

zero(x)

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Yes, I hope so. I also hope that they would pick the same meaning for x+y.

Ok, quick (but serious) question: What are the semantics of zero(x) ?

I don't know; this is Dave's example. Whatever the community decides, as long as it's consistent. The user should learn what it means _once_.

It could take many years for a name like that to settle down, especially if two major libraries with strong user base and backward-compatibility considerations pick competing definitions.

OK; so what alternatives do we have?

1. lib1::numeric_traits<X>::zero( x ); // somewhat unwieldy 2. lib1::zero( x ); // syntactic sugar for the above 3. lib1_zero( x );

What, exactly, are the advantages of #2 over #3?

#3 invokes ADL.

That you can bring lib1::zero into scope with a using declaration and call it unqualified?

Of course not! Getting the ability to make unqualified calls is not the goal here. Rather, the ability to make _qualified_ calls is valuable: it's more predictable, and it says what it means, and it avoids potential name choice collision problems, and...

Current language assumed. In this discussion I do not consider your extension proposal; it will take even more years to settle down.

Fair enough; I need answers urgently for the current language myself. You can ignore my one allusion to extensions in this thread; the rest of what I've written here deals with the current language. Thanks again, -- Dave Abrahams Boost Consulting www.boost-consulting.com