std::unordered_map::insert(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

🦋 std::unordered_map::insert(3) — C++ Standard Libary 🦋

okay bestie, sit down, hydrate, because this one is the classic: std::unordered_map::insert, straight out of the C++ Standard Libary. Ten overloads. Ten. But like… we're gonna get through it together, iykyk 💅

✨ SYNOPSIS ✨

   std::pair<iterator, bool> insert( const value_type& value );     (1)
(since C++11)
   std::pair<iterator, bool> insert( value_type&& value );          (2)
(since C++17)
   template< class P >                                              (3)
(since C++11)
   std::pair<iterator, bool> insert( P&& value );
   iterator insert( const_iterator hint, const value_type& value ); (4)
(since C++11)
   iterator insert( const_iterator hint, value_type&& value );      (5)
(since C++17)
   template< class P >                                              (6)
(since C++11)
   iterator insert( const_iterator hint, P&& value );
   template< class InputIt >                                        (7)
(since C++11)
   void insert( InputIt first, InputIt last );
   void insert( std::initializer_list<value_type> ilist );          (8)
(since C++11)
   insert_return_type insert( node_type&& nh );                     (9)
(since C++17)
   iterator insert( const_iterator hint, node_type&& nh );          (10)
(since C++17)

💖 DESCRIPTION 💖

So the whole vibe of this function: it inserts element(s) into the container, if the container doesn't already contain an element with an equivalent key. That's the main character rule. No duplicates allowed, periodt. 🔥

Now let's go one by one, because you deserve the full breakdown:

  • 1–3) Inserts value. Overload (3) is equivalent to emplace(std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.
  • 4–6) Inserts value, using hint as a non-binding suggestion to where the search should start. Overload (6) is equivalent to emplace_hint(hint, std::forward<P>(value)) and only participates in overload resolution if std::is_constructible<value_type, P&&>::value == true.
  • 7) Inserts elements from range [first, last). If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844). ⚠️ If [first, last) is not a valid range, or first and/or last are iterators into *this, the behavior is undefined. not me warning you twice, the standard says it 🫶
  • 8) Inserts elements from initializer list ilist. If multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
  • 9) If nh is an empty node handle, does nothing. Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key(). The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().
  • 10) If nh is an empty node handle, does nothing and returns the end iterator. Otherwise, inserts the element owned by nh into the container, if the container doesn't already contain an element with a key equivalent to nh.key(), and returns the iterator pointing to the element with key equivalent to nh.key() (regardless of whether the insert succeeded or failed). If the insertion succeeds, nh is moved from, otherwise it retains ownership of the element. hint is used as a non-binding suggestion to where the search should start. The behavior is undefined if nh is not empty and get_allocator() != nh.get_allocator().

🌸 the rehashing tea (since C++17) 🌸

  • If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.
  • If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.
  • If the insertion is successful, pointers and references to the element obtained while it is held in the node handle are invalidated, and pointers and references obtained to that element before it was extracted become valid.
lowkey this is the part where you screenshot the page and send it to the group chat bc it WILL be on the quiz 💀

🎀 PARAMETERS 🎀

  • hint — iterator, used as a suggestion as to where to insert the content
  • value — element value to insert
  • first, last — range of elements to insert
  • ilist — initializer list to insert the values from
  • nh — a compatible node handle

🌈 TYPE REQUIREMENTS 🌈

  • InputIt must meet the requirements of LegacyInputIterator.

👑 RETURN VALUE 👑

  • 1–3) A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.
  • 4–6) An iterator to the inserted element, or to the element that prevented the insertion.
  • 7,8) (none) — yeah, nothing, don't come for me 🙈
  • 9) An object of insert_return_type with the members initialized as follows:
    • If nh is empty, inserted is false, position is end(), and node is empty.
    • Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty.
    • If the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key().
  • 10) end iterator if nh was empty, iterator pointing to the inserted element if insertion took place, and iterator pointing to an element with a key equivalent to nh.key() if it failed.

💀 EXCEPTIONS 💀

  • 1–6) If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee). slay, honestly ✨
  • 7,8) No exception safety guarantee. ngl this one's giving chaos.
  • 9,10) If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

⭐ COMPLEXITY ⭐

  • 1–6) Average case: O(1), worst case O(size()).
  • 7,8) Average case: O(N), where N is the number of elements to insert. Worst case: O(N * size() + N).
  • 9,10) Average case: O(1), worst case O(size()).

🧚 NOTES 🧚

The hinted insert (4–6) does not return a bool in order to be signature-compatible with positional insert on sequential containers, such as std::vector::insert. This makes it possible to create generic inserters such as std::inserter. One way to check success of a hinted insert is to compare size() before and after.

iykyk — that's the trick. size() before, size() after, do the math bestie 🫶

🔥 EXAMPLE 🔥

// Run this code

#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
    std::unordered_map<int, std::string> dict = {{1, "one"}, {2, "two"}};
    dict.insert({3, "three"});
    dict.insert(std::make_pair(4, "four"));
    dict.insert({{4, "another four"}, {5, "five"}});

    const bool ok = dict.insert({1, "another one"}).second;
    std::cout << "inserting 1 => \"another one\" "
              << (ok ? "succeeded" : "failed") << '\n';

    std::cout << "contents:\n";
    for (auto& p : dict)
        std::cout << ' ' << p.first << " => " << p.second << '\n';
}

Possible output:

inserting 1 => "another one" failed
contents:
 5 => five
 1 => one
 2 => two
 3 => three
 4 => four
notice how 1 stays one? that's the "no equivalent key" rule doing its thing. iconic behavior honestly 💅

🌺 DEFECT REPORTS 🌺

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

  • LWG 2005 — applied to C++11
    • Behavior as published: overloads (3,6) would only participate in overload resolution if P is implicitly convertible to value_type
    • Correct behavior: only participates if value_type is constructible from P&&

🍓 SEE ALSO 🍓

  • emplace — constructs element in-place (public member function)
  • emplace_hint — constructs elements in-place using a hint (public member function)
  • insert_or_assign (C++17) — inserts an element or assigns to the current element if the key already exists (public member function)
  • inserter — creates a std::insert_iterator of type inferred from the argument (function template)

and that's the whole page bestie, you ate that 🫶✨ source: http://cppreference.com, version 2024.06.10 — go forth and insert responsibly, mwah 💖🎀🦋

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