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Section: 24.6.9 [flat.map], 24.6.10 [flat.multimap], 24.6.11 [flat.set], 24.6.12 [flat.multiset] Status: Tentatively Ready Submitter: Arthur O'Dwyer Opened: 2023-02-06 Last modified: 2023-03-23
Priority: Not Prioritized
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Discussion:
This issue is part of the "flat_foo" sequence, LWG 3786, 3802, 3803, 3804. flat_set, flat_multiset, flat_map, and flat_multimap all have implicitly defaulted copy and move constructors, but they lack allocator-extended versions of those constructors. This means that the following code is broken:
// https://godbolt.org/z/qezv5rTrW #include <cassert> #include <flat_set> #include <memory_resource> #include <utility> #include <vector> template<class T, class Comp = std::less<T>> using pmr_flat_set = std::flat_set<T, Comp, std::pmr::vector<T>>; int main() { std::pmr::vector<pmr_flat_set<int>> vs = { {1,2,3}, {4,5,6}, }; std::pmr::vector<int> v = std::move(vs[0]).extract(); assert(v.get_allocator().resource() == std::pmr::get_default_resource()); }
pmr_flat_set<int> advertises that it "uses_allocator" std::pmr::polymorphic_allocator<int>, but in fact it lacks the allocator-extended constructor overload set necessary to interoperate with std::pmr::vector.
[2023-03-22; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
Proposed resolution:
This wording is relative to N4928.
Modify 24.6.9.2 [flat.map.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_map { public: […] // 24.6.9.3 [flat.map.cons], construct/copy/destroy flat_map() : flat_map(key_compare()) { } template<class Allocator> flat_map(const flat_map&, const Allocator& a); template<class Allocator> flat_map(flat_map&&, const Allocator& a); […] }; […] }
Modify 24.6.9.3 [flat.map.cons] as indicated:
[Drafting note: As a drive-by fix, a missing closing > is inserted in p11.]
template<class Allocator> flat_map(const flat_map&, const Allocator& a); template<class Allocator> flat_map(flat_map&&, const Allocator& a); template<class Allocator> flat_map(const key_compare& comp, const Allocator& a); template<class Allocator> explicit flat_map(const Allocator& a); template<class InputIterator, class Allocator> flat_map(InputIterator first, InputIterator last, const key_compare& comp, const Allocator& a); template<class InputIterator, class Allocator> flat_map(InputIterator first, InputIterator last, const Allocator& a); […]-11- Constraints: uses_allocator_v<key_container_type, Allocator> is true and uses_allocator_v<mapped_container_type, Allocator> is true.
-12- Effects: Equivalent to the corresponding non-allocator constructors except that c.keys and c.values are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 24.6.10.2 [flat.multimap.defn] as indicated:
namespace std { template<class Key, class T, class Compare = less<Key>, class KeyContainer = vector<Key>, class MappedContainer = vector<T>> class flat_multimap { public: […] // 24.6.10.3 [flat.multimap.cons], construct/copy/destroy flat_multimap() : flat_multimap(key_compare()) { } template<class Allocator> flat_multimap(const flat_multimap&, const Allocator& a); template<class Allocator> flat_multimap(flat_multimap&&, const Allocator& a); […] }; […] }
Modify 24.6.10.3 [flat.multimap.cons] as indicated:
template<class Allocator> flat_multimap(const flat_multimap&, const Allocator& a); template<class Allocator> flat_multimap(flat_multimap&&, const Allocator& a); template<class Allocator> flat_multimap(const key_compare& comp, const Allocator& a); […]-11- Constraints: uses_allocator_v<key_container_type, Allocator> is true and uses_allocator_v<mapped_container_type, Allocator> is true.
-12- Effects: Equivalent to the corresponding non-allocator constructors except that c.keys and c.values are constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 24.6.11.2 [flat.set.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_set { public: […] // 24.6.11.3 [flat.set.cons], constructors flat_set() : flat_set(key_compare()) { } template<class Allocator> flat_set(const flat_set&, const Allocator& a); template<class Allocator> flat_set(flat_set&&, const Allocator& a); […] }; […] }
Modify 24.6.11.3 [flat.set.cons] as indicated:
template<class Allocator> flat_set(const flat_set&, const Allocator& a); template<class Allocator> flat_set(flat_set&&, const Allocator& a); template<class Allocator> flat_set(const key_compare& comp, const Allocator& a); […]-9- Constraints: uses_allocator_v<container_type, Allocator> is true.
-10- Effects: Equivalent to the corresponding non-allocator constructors except that c is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).
Modify 24.6.12.2 [flat.multiset.defn] as indicated:
namespace std { template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>> class flat_multiset { public: […] // 24.6.12.3 [flat.multiset.cons], constructors flat_multiset() : flat_multiset(key_compare()) { } template<class Allocator> flat_multiset(const flat_multiset&, const Allocator& a); template<class Allocator> flat_multiset(flat_multiset&&, const Allocator& a); […] }; […] }
Modify 24.6.12.3 [flat.multiset.cons] as indicated:
template<class Allocator> flat_multiset(const flat_multiset&, const Allocator& a); template<class Allocator> flat_multiset(flat_multiset&&, const Allocator& a); template<class Allocator> flat_multiset(const key_compare& comp, const Allocator& a); […]-9- Constraints: uses_allocator_v<container_type, Allocator> is true.
-10- Effects: Equivalent to the corresponding non-allocator constructors except that c is constructed with uses-allocator construction (20.2.8.2 [allocator.uses.construction]).