Issue 3786: Flat maps' deduction guide needs to default Allocator to be useful

Title: Flat maps' deduction guide needs to default Allocator to be useful
Status: c++23
Section: [flat.map.defn][flat.multimap.defn]
Submitter: Johel Ernesto Guerrero Peña

Created on 2022-09-25.00:00:00 last changed 12 months ago

Messages

msg13385 (view)

Date: 2023-02-13.11:31:32

Proposed resolution:

This wording is relative to N4917.

Modify [container.adaptors.general] as indicated:

-8- The following exposition-only alias template may appear in deduction guides for container adaptors:

template<class Allocator, class T>
  using alloc-rebind =                                 // exposition only
    typename allocator_traits<Allocator>::template rebind_alloc<T>;

Modify [flat.map.defn], class template flat_map synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<byte>>
  flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_map(from_range_t, R&&, Allocator)
    -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;
[…]

Modify [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<byte>>
  flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_multimap(from_range_t, R&&, Allocator)
    -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>>;
[…]

Modify [flat.set.defn], class template flat_set synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
         class Allocator = allocator<ranges::range_value_t<R>>>
  flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_set<ranges::range_value_t<R>, Compare,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_set(from_range_t, R&&, Allocator)
    -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
[…]

Modify [flat.multiset.defn], class template flat_multiset synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
         class Allocator = allocator<ranges::range_value_t<R>>>
  flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multiset<ranges::range_value_t<R>, Compare,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_multiset(from_range_t, R&&, Allocator)
    -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
[…]

msg13303 (view)

Date: 2023-02-13.00:00:00

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate → WP. ]

msg13197 (view)

Date: 2023-02-09.02:30:45

[ Issaquah 2023-02-07; LWG ]

Edited proposed resolution to fix missing > in guides for maps. Move to Immediate for C++23

msg12871 (view)

Date: 2023-01-15.00:00:00

[ 2023-01-11; Jonathan Wakely provides improved wording ]

During LWG telecon Tim pointed out that because allocator<void> does not meet the Cpp17Allocator requirements, it might not "qualify as an allocator" and so would cause the deduction guides to not participate in overload resolution, as per [container.adaptors.general] p6 (6.4). Use allocator<byte> instead.

msg12841 (view)

Date: 2022-10-15.00:00:00

[ 2022-10-19; Jonathan provides improved wording ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

Modify [container.adaptors.general] as indicated:

-8- The following exposition-only alias templates may appear in deduction guides for container adaptors:

template<class Container>
  using cont-key-type =                                // exposition only
    remove_const_t<typename Container::value_type::first_type>;
template<class Container>
  using cont-mapped-type =                             // exposition only
    typename Container::value_type::second_type;
template<class Allocator, class T>
  using alloc-rebind =                             // exposition only
    typename allocator_traits<Allocator>::template rebind_alloc<T>;

Modify [flat.map.defn], class template flat_map synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<void>>
  flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;

template<ranges::input_range R, class Allocator>
  flat_map(from_range_t, R&&, Allocator)
    -> flat_map<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;
[…]

Modify [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<void>>
  flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;

template<ranges::input_range R, class Allocator>
  flat_multimap(from_range_t, R&&, Allocator)
    -> flat_multimap<range-key-type<R>, range-mapped-type<R>, less<range-key-type<R>>,
         vector<range-key-type<R>, alloc-rebind<Allocator, range-key-type<R>>,
         vector<range-mapped-type<R>, alloc-rebind<Allocator, range-mapped-type<R>>>;
[…]

Modify [flat.set.defn], class template flat_set synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
         class Allocator = allocator<ranges::range_value_t<R>>>
  flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_set<ranges::range_value_t<R>, Compare,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_set(from_range_t, R&&, Allocator)
    -> flat_set<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
[…]

Modify [flat.multiset.defn], class template flat_multiset synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<ranges::range_value_t<R>>,
         class Allocator = allocator<ranges::range_value_t<R>>>
  flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multiset<ranges::range_value_t<R>, Compare,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;

template<ranges::input_range R, class Allocator>
  flat_multiset(from_range_t, R&&, Allocator)
    -> flat_multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>,
         vector<ranges::range_value_t<R>, alloc-rebind<Allocator, ranges::range_value_t<R>>>>;
[…]

msg12814 (view)

Date: 2022-09-15.00:00:00

[ 2022-09-28; LWG telecon ]

We should not just ignore the allocator, it should be rebound and used for the two container types.

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

Modify [flat.map.defn], class template flat_map synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<void>>
  flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_map<range-key-type<R>, range-mapped-type<R>, Compare>;
[…]

Modify [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

[…]
template<ranges::input_range R, class Compare = less<range-key-type<R>>,
         class Allocator = allocator<void>>
  flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multimap<range-key-type<R>, range-mapped-type<R>, Compare>;
[…]

msg12813 (view)

Date: 2022-09-25.00:00:00

This originated from the editorial issue #5800.

P0429R9 added some deduction guides with a non-defaulted Allocator template parameter and a corresponding function parameter that is defaulted. Since the template parameter Allocator is not defaulted, these deduction guides are never used.

History
Date	User	Action	Args
2023-11-22 15:47:43	admin	set	status: wp -> c++23
2023-02-13 11:31:32	admin	set	messages: + msg13385
2023-02-13 11:31:32	admin	set	status: immediate -> wp
2023-02-09 02:30:45	admin	set	messages: + msg13303
2023-02-09 02:30:45	admin	set	status: open -> immediate
2023-01-11 18:22:42	admin	set	messages: + msg13197
2022-10-19 19:43:42	admin	set	messages: + msg12871
2022-10-06 17:09:10	admin	set	messages: + msg12841
2022-10-06 17:09:10	admin	set	status: new -> open
2022-09-25 11:23:04	admin	set	messages: + msg12814
2022-09-25 00:00:00	admin	create