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[range.utility.conv.general] Should reservable-container and container-insertable be concepts? #5328

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hewillk opened this issue Mar 2, 2022 · 11 comments
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[range.utility.conv.general] Should reservable-container and container-insertable be concepts? #5328

hewillk opened this issue Mar 2, 2022 · 11 comments

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@hewillk
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hewillk commented Mar 2, 2022 •
edited

In [range.utility.conv.general], reservable-container and container-insertable are defined as

template<class Container>
constexpr bool reservable-container =          // exposition only
  sized_­range<Container> &&
  requires(Container& c, range_size_t<Container> n) {
    c.reserve(n);
    { c.capacity() } -> same_­as<decltype(n)>;
    { c.max_size() } -> same_­as<decltype(n)>;
  };

template<class Container, class Ref>
constexpr bool container-insertable =          // exposition only
  requires(Container& c, Ref&& ref) {
    requires (requires { c.push_back(std::forward<Ref>(ref)); } ||
              requires { c.insert(c.end(), std::forward<Ref>(ref)); });
  };

I don't understand why they lack inline, and it seems to me that defining them as concept is more consistent with the name, since the inline constexpr bool variables usually start with is_.
@hewillk hewillk changed the title [range.utility.conv.general] Should reservable-container and container-insertable be concepts? [range.utility.conv.general] Should _reserve-container_ and _container-insertable_ be concepts? Mar 2, 2022
@hewillk hewillk changed the title [range.utility.conv.general] Should _reserve-container_ and _container-insertable_ be concepts? [range.utility.conv.general] Should reserve-container and container-insertable be concepts? Mar 2, 2022
@JohelEGP
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JohelEGP commented Mar 2, 2022

I agree that prepending is- results in a better name, unless they're made concepts. These don't need to be inline since their address is not leaked, and thus the difference is not observable.

@hewillk
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hewillk commented Mar 2, 2022

I know, but exposition only constexpr bool in <ranges> all are inline. Should we be consistent with them?

@JohelEGP
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JohelEGP commented Mar 2, 2022

That makes sense.

@hewillk
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hewillk commented Mar 2, 2022

In the original paper p1206r7, the description of reservable-container is

reservable-container concept
The exposition only reservable-container concepts checks for
• reserve
• max_size
• capacity
which is consistent with ranges-v3. This serves multiple purposes:
• Avoid calling reserve on unordered containers and others entities that have a reserve
method with different semantics
• Allow implementations to check that we are not trying to reserve more than max size

So I suspect it should be a concept in nature, and the opinion of the author of the paper may be required.

@jwakely
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jwakely commented Mar 2, 2022

No, they should not be inline. They don't actually exist, so we don't need to add noise to the declaration to avoid linker errors for things that aren't really there.

No, we don't want them to be concepts. They were concepts in an earlier revision of the paper and it was changed intentionally by LWG review. The reason it's a variable template not a concept is because we don't want the semantic requirements of the constraints to result in UB if violated. It's a purely syntactic check.

@jwakely
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jwakely commented Mar 2, 2022 •
edited

I know, but exposition only constexpr bool in <ranges> all are inline. Should we be consistent with them?

Maybe we should remove it from them too.

Looks like this affects is-derived-from-view-interface and is-initializer-list specifically.

@jensmaurer
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No, they should not be inline, because variable templates are implicitly inline.

@JohelEGP
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JohelEGP commented Mar 2, 2022

Indeed, see #4625.

@hewillk
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hewillk commented Mar 6, 2022 •
edited

Do we need to rename container-insertable to insertable-container to make it consistent with the naming of reservable-container? It seems a bit odd to me that two similar concepts have different name styles.
This also makes it consistent with output_iterator which has a similar requires-clause style, which is also a noun rather than an adjective.

@hewillk hewillk changed the title [range.utility.conv.general] Should reserve-container and container-insertable be concepts? [range.utility.conv.general] Should reservable-container and container-insertable be concepts? Mar 6, 2022
@jensmaurer
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No. reservable-container is a property of a container (regardless of element type etc), but container-insertable is about some Ref that wants to be inserted in some container, so it talks about something in relationship to a container. Different naming styles are fine.

@jensmaurer
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I'm not seeing any more open issues here. Closing.

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@jwakely @JohelEGP @jensmaurer @hewillk