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std::unordered_map::insert(3) C++ Standard Libarystd::unordered_map::insert(3) NAME std::unordered_map::insert - std::unordered_map::insert 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) Inserts element(s) into the container, if the container doesn't al- ready contain an element with an equivalent key. 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::for- ward<P>(value)) and only participates in overload resolution if std::is_con- structible<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 in- serted (pending LWG2844). If [first, last) is not a valid range, or first and/or last are it- erators into *this, the behavior is undefined. 8) Inserts elements from initializer list ilist. If multiple ele- ments 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 unde- fined if nh is not empty and get_allocator() != nh.get_allocator(). 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 inval- idated. Otherwise (no rehashing), iterators are not invalidated. If the insertion is successful, pointers and references to the ele- ment 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. (since C++17) 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) 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, posi- tion points to the inserted element, and node is empty. * If the insertion failed, inserted is false, node has the previ- ous 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). 7,8) No exception safety guarantee. 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 in- sert. 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 boolean in order to be signature-compatible with positional insert on sequential contain- ers, such as std::vector::insert. This makes it possible to create generic in- serters such as std::inserter. One way to check success of a hinted insert is to compare size() before and after. 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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior overloads (3,6) would only only par- ticipates if LWG 2005 C++11 participate in overload value_type resolution if P is implicitly is con- structible from P&& convertible to value_type 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 inserts an element or assigns to the current ele- ment if the key (C++17) already exists (public member function) inserter creates a std::insert_iterator of type inferred from the argument (function template) http://cppreference.com 2024.06.10 std::unordered_map::insert(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Exceptions | Complexity | Notes | Example | Possible output: | See also
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