3227. Ambiguity issue for extract in ordered and unordered associative containers

Section: 23.2.7 [associative.reqmts], 23.2.8 [unord.req] Status: New Submitter: Konstantin Boyarinov Opened: 2019-06-25 Last modified: 2022-04-24

Priority: 3

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Discussion:

Ordered and unordered associative containers in C++14 contained an issue, which caused an ambiguity while invoking std::map::erase when key_type of the map can be constructed from the iterator. In this case both overloads erase(const key_type&) and erase(const_iterator) could be chosen.

The issue LWG 2059 was reported and resolved in C++17 by adding an extra overload for erase in ordered and unordered associative containers which accepts iterator as an argument.

C++17 also introduced new functionality for splicing ordered and unordered maps and sets. One of the extensions allows to extract a node from the container by passing either key_type& or const_iterator to the extract() member function:

node_type extract(const key_type& x);
node_type extract(const_iterator position);

Providing these two extract overloads causes the same problem as for erase. Consider the following example:

#include <map>
#include <string>

struct Key
{
  template <typename T>
  Key(const T&) {}
};

bool operator<(const Key&, const Key&) { return false; }

int main()
{
  using map_type = std::map<Key, std::string>;

  map_type m({ {Key(1), "a"}, {Key(2), "b"} });
  map_type::iterator it = m.begin();
  auto nh = m.extract(it);
}

In this case, call to extract() is ambiguous, because the overloads which accept const_iterator and key_type are equally good matches for the argument it.

Consequently, this issue can be resolved in the same way as for std::map::erase by adding an overload for extract which accepts iterator as an argument.

[2019-07 Issue Prioritization]

Priority to 3 after discussion on the reflector.

Previous resolution [SUPERSEDED]:

This wording is relative to N4820.

1. Modify [tab:container.assoc.req], Table 69 — "Associative container requirements", as indicated:

   Table 69 — Associative container requirements (in addition to container) [tab:container.assoc.req]

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   a.extract(q)	node_type	Effects: Removes the element pointed to by q. Returns: A node_type owning that element.	amortized constant
   a.extract(r)	node_type	Effects: Removes the element pointed to by r. Returns: A node_type owning that element.	amortized constant
   …

2. Modify [tab:container.assoc.req], Table 70 — "Unordered associative container requirements", as indicated:

   Table 70 — Unordered associative container requirements (in addition to container) [tab:container.hash.req]

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   …
   a.extract(q)	node_type	Effects: Removes the element pointed to by q. Returns: A node_type owning that element.	Average case 𝒪(1), worst case 𝒪(a.size()).
   a.extract(r)	node_type	Effects: Removes the element pointed to by r. Returns: A node_type owning that element.	Average case 𝒪(1), worst case 𝒪(a.size()).
   …

3. Modify 23.4.3.1 [map.overview], class template map synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

4. Modify 23.4.4.1 [multimap.overview], class template multimap synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

5. Modify 23.4.6.1 [set.overview], class template set synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

6. Modify 23.4.7.1 [multiset.overview], class template multiset synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

7. Modify 23.5.3.1 [unord.map.overview], class template unordered_map synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

8. Modify 23.5.4.1 [unord.multimap.overview], class template unordered_multimap synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

9. Modify 23.5.6.1 [unord.set.overview], class template unordered_set synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

10. Modify 23.5.7.1 [unord.multiset.overview], class template unordered_multiset synopsis, as indicated:

    […]
    node_type extract(iterator position);
    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    […]
    

[2022-04-24; Daniel rebases wording on N4910]

Proposed resolution:

This wording is relative to N4910.

1. Modify 23.2.7.1 [associative.reqmts.general] as indicated:

   a.extract(q)
   

   -108- Result: node_type

   -109- Effects: Removes the element pointed to by q.

   -110- Returns: A node_type owning that element.

   -111- Complexity: Amortized constant.

   a.extract(r)
   

   -?- Result: node_type

   -?- Effects: Removes the element pointed to by r.

   -?- Returns: A node_type owning that element.

   -?- Complexity: Amortized constant.

2. Modify 23.2.8.1 [unord.req.general] as indicated:

   a.extract(q)
   

   -141- Result: node_type

   -142- Effects: Removes the element pointed to by q.

   -143- Returns: A node_type owning that element.

   -144- Complexity: Average case 𝒪(1), worst case 𝒪(a.size()).

   a.extract(r)
   

   -?- Result: node_type

   -?- Effects: Removes the element pointed to by r.

   -?- Returns: A node_type owning that element.

   -?- Complexity: Average case 𝒪(1), worst case 𝒪(a.size()).

3. Modify 23.4.3.1 [map.overview], class template map synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

4. Modify 23.4.4.1 [multimap.overview], class template multimap synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

5. Modify 23.4.6.1 [set.overview], class template set synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

6. Modify 23.4.7.1 [multiset.overview], class template multiset synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

7. Modify 23.5.3.1 [unord.map.overview], class template unordered_map synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

8. Modify 23.5.4.1 [unord.multimap.overview], class template unordered_multimap synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

9. Modify 23.5.6.1 [unord.set.overview], class template unordered_set synopsis, as indicated:

   […]
   node_type extract(iterator position);
   node_type extract(const_iterator position);
   node_type extract(const key_type& x);
   […]
   

10. Modify 23.5.7.1 [unord.multiset.overview], class template unordered_multiset synopsis, as indicated:

    […]
    node_type extract(iterator position);
    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    […]