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4587. Missing complexity for unordered_map::at

Section: 23.5.3.3 [unord.map.elem] Status: Ready Submitter: Pablo Halpern Opened: 2026-06-09 Last modified: 2026-06-15

Priority: Not Prioritized

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Discussion:

During review of P3091R5 Pablo pointed out that there is no Complexity: element for unordered_map::at.

[Brno 2026-06-12; Status changed: New → Ready.]

Proposed resolution:

This wording is relative to N5046.

1. Modify 23.5.3.3 [unord.map.elem] as indicated:

   constexpr mapped_type& at(const key_type& k);
   constexpr const mapped_type& at(const key_type& k) const;
   

   -5- Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k`.

   -6- Throws: An exception object of type out_of_range if no such element is present.

   -?- Complexity: Average case constant, worst case linear in size().

   template<class K> constexpr mapped_type&       at(const K& k);
   template<class K> constexpr const mapped_type& at(const K& k) const;
   

   -7- Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

   -8- Preconditions: The expression find(k) is well-formed and has well-defined behavior.

   -9- Returns: A reference to find(k)->second.

   -10- Throws: An exception object of type out_of_range if find(k) == end() is true.

   -?- Complexity: Average case constant, worst case linear in size().