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

Section: 23.6.8.2 [flat.map.defn], 23.6.9.2 [flat.multimap.defn] Status: C++23 Submitter: Johel Ernesto Guerrero Peña Opened: 2022-09-25 Last modified: 2023-11-22

Priority: 2

View all other issues in [flat.map.defn].

View all issues with C++23 status.

Discussion:

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.

[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Previous resolution [SUPERSEDED]: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.

  1. Modify 23.6.8.2 [flat.map.defn], class template flat_map synopsis, as indicated:

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

  2. Modify 23.6.9.2 [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

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

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

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

  1. Modify 23.6.1 [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;

  2. Modify 23.6.8.2 [flat.map.defn], class template flat_map synopsis, as indicated:

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

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

  3. Modify 23.6.9.2 [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

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

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

  4. Modify 23.6.11.2 [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
         >;

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>>
         >;
[…]

  5. Modify 23.6.12.2 [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
         >;

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>>
         >;
[…]

[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 23.6.1 [container.adaptors.general] p6 (6.4). Use allocator<byte> instead.

[Issaquah 2023-02-07; LWG]

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

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

Proposed resolution:

This wording is relative to N4917.

  1. Modify 23.6.1 [container.adaptors.general] as indicated:

  2. Modify 23.6.8.2 [flat.map.defn], class template flat_map synopsis, as indicated:

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

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

  3. Modify 23.6.9.2 [flat.multimap.defn], class template flat_multimap synopsis, as indicated:

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

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

  4. Modify 23.6.11.2 [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
         >;

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>>
         >;
[…]

  5. Modify 23.6.12.2 [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
         >;

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>>
         >;
[…]