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New issue from Arthur: "Inconsistent constraints on flat_foo::emplace"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4180" status="New"> | ||
| <title>Inconsistent constraints on <tt>flat_<i>foo</i>::emplace</tt></title> | ||
| <section><sref ref="[flat.multiset.modifiers]"/><sref ref="[flat.map.modifiers]"/></section> | ||
| <submitter>Arthur O'Dwyer</submitter> | ||
| <date>09 Dec 2024</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| The usual pattern in <sref ref="[containers]"/> is that `x.emplace(args...)` has a precondition | ||
| (<sref ref="[sequence.reqmts]"/> p20, <sref ref="[associative.reqmts.general]"/> p48) but no | ||
| <i>Constraints</i> element. That is, `emplace` is not SFINAE-friendly. And it has only the one overload, | ||
| so it doesn't need a constraint for purposes of overload resolution. | ||
| <p/> | ||
| No Constraints on `emplace`: `deque`, `list`, `vector`, `containers`, `associative containers`, `unordered containers`, | ||
| `priority_queue`, `optional`. | ||
| <p/> | ||
| Constraints on `emplace`: `flat_map`, `flat_multiset`, `any`, `expected`, `variant`. | ||
| <p/> | ||
| I believe a <i>Constraints</i> element was accidentally copy-pasted from the spec of <tt>flat_map::insert(P&&)</tt> | ||
| — which does need the constraint because it's part of `insert`'s large overload set — to | ||
| `flat_map::emplace`, and then from there to `flat_multiset::emplace`. The constraint is already (correctly) absent | ||
| `from flat_set::emplace`. | ||
| <p/> | ||
| While we're touching this paragraph, also resolve the vague word "initializes" to "direct-non-list-initializes." | ||
| Editorially, <tt>pair<…></tt> is a verbose way to spell the `value_type` of a `flat_map`; we should | ||
| be consistent and just say `value_type`. | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| This wording is relative to <paper num="N4993"/>. | ||
| </p> | ||
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| <blockquote class="note"> | ||
| <p> | ||
| [<i>Drafting note</i>: <sref ref="[flat.set.modifiers]"/> is already OK as far as this issue is concerned: | ||
| it has no wording for `emplace`. | ||
| <p/> | ||
| [flat.multimap.modifiers] is already OK ditto: it does not exist. ] | ||
| </p> | ||
| </blockquote> | ||
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| <ol> | ||
| <li><p>Modify <sref ref="[flat.multiset.modifiers]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<class... Args> iterator emplace(Args&&... args); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i><ins>Mandates</ins><del>Constraints</del></i>: <tt>is_constructible_v<value_type, Args...></tt> is <tt>true</tt>. | ||
| <p/> | ||
| -2- <i>Effects</i>: First, <ins>direct-non-list-</ins>initializes an object `t` of type `value_type` with | ||
| <tt>std::forward<Args>(args)...</tt>, then inserts `t` as if by: | ||
| </p> | ||
| <blockquote><pre> | ||
| auto it = ranges::upper_bound(c, t, compare); | ||
| c.insert(it, std::move(t)); | ||
| </pre></blockquote> | ||
| <p> | ||
| -3- <i>Returns</i>: An iterator that points to the inserted element. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
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| <li><p>Modify <sref ref="[flat.map.modifiers]"/> as indicated:</p> | ||
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| <blockquote> | ||
| <pre> | ||
| template<class... Args> pair<iterator, bool> emplace(Args&&... args); | ||
| </pre> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i><ins>Mandates</ins><del>Constraints</del></i>: <tt>is_constructible_v<<ins>value_type</ins><del>pair<key_type, mapped_type></del>, Args...></tt> | ||
| is <tt>true</tt>. | ||
| <p/> | ||
| -2- <i>Effects</i>: <ins>First, direct-non-list-i</ins><del>I</del>nitializes an object `t` of type | ||
| <tt><ins>value_type</ins><del>pair<key_type, mapped_type></del></tt> with <tt>std::forward<Args>(args)...</tt>; | ||
| if the map already contains an element whose key is equivalent to `t.first`, `*this` is unchanged. Otherwise, equivalent to: | ||
| </p> | ||
| <blockquote><pre> | ||
| auto key_it = ranges::upper_bound(c.keys, t.first, compare); | ||
| auto value_it = c.values.begin() + distance(c.keys.begin(), key_it); | ||
| c.keys.insert(key_it, std::move(t.first)); | ||
| c.values.insert(value_it, std::move(t.second)); | ||
| </pre></blockquote> | ||
| <p> | ||
| -3- <i>Returns</i>: The `bool` component of the returned pair is `true` if and only if the insertion took place, and | ||
| the iterator component of the pair points to the element with key equivalent to `t.first`. | ||
| </p> | ||
| </blockquote> | ||
| </blockquote> | ||
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| </li> | ||
| </ol> | ||
| </resolution> | ||
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| </issue> |