std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::emplace
std::unordered_multimap iterator emplace( Args&&... args );
(constexpr since C++26)
Inserts a new element into the container constructed in-place with the given args.
The constructor of the new element (i.e. std::pair <const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via std::forward <Args>(args)....
If value_type is not EmplaceConstructible into unordered_multimap from args, the behavior is undefined.
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.
[edit] Parameters
[edit] Return value
An iterator to the inserted element.
[edit] Exceptions
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
[edit] Complexity
Amortized constant on average, worst case linear in the size of the container.
[edit] Notes
Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.
[edit] Example
#include <iostream> #include <string> #include <utility> #include <unordered_map> int main() { std::unordered_multimap <std::string, std::string > m; // uses pair's move constructor m.emplace(std::make_pair (std::string ("a"), std::string ("a"))); // uses pair's converting move constructor m.emplace(std::make_pair ("b", "abcd")); // uses pair's template constructor m.emplace("d", "ddd"); // emplace with duplicate key m.emplace("d", "DDD"); // uses pair's piecewise constructor m.emplace(std::piecewise_construct, std::forward_as_tuple ("c"), std::forward_as_tuple (10, 'c')); for (const auto& p : m) std::cout << p.first << " => " << p.second << '\n'; }
Possible output:
a => a b => abcd c => cccccccccc d => ddd d => DDD