std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace_hint
iterator emplace_hint( const_iterator hint, Args&&... args );
(constexpr since C++26)
Inserts a new element into the container, using hint as a suggestion where the element should go.
The constructor of value_type (i.e., std::pair <const Key, T>) is called with exactly the same arguments as supplied to the function, forwarded with std::forward <Args>(args)....
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, or to the element that prevented the insertion.
[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] Example
#include <chrono> #include <cstddef> #include <functional> #include <iomanip> #include <iostream> #include <unordered_map> const int n_operations = 100'500'0; std::size_t map_emplace() { std::unordered_map <int, char> map; for (int i = 0; i < n_operations; ++i) map.emplace(i, 'a'); return map.size(); } std::size_t map_emplace_hint() { std::unordered_map <int, char> map; auto it = map.begin(); for (int i = 0; i < n_operations; ++i) { map.emplace_hint(it, i, 'b'); it = map.end(); } return map.size(); } std::size_t map_emplace_hint_wrong() { std::unordered_map <int, char> map; auto it = map.begin(); for (int i = n_operations; i > 0; --i) { map.emplace_hint(it, i, 'c'); it = map.end(); } return map.size(); } std::size_t map_emplace_hint_corrected() { std::unordered_map <int, char> map; auto it = map.begin(); for (int i = n_operations; i > 0; --i) { map.emplace_hint(it, i, 'd'); it = map.begin(); } return map.size(); } std::size_t map_emplace_hint_closest() { std::unordered_map <int, char> map; auto it = map.begin(); for (int i = 0; i < n_operations; ++i) it = map.emplace_hint(it, i, 'e'); return map.size(); } double time_it(std::function <std::size_t ()> map_test, std::string what = "", double ratio = 0.0) { const auto start = std::chrono::system_clock::now (); const std::size_t map_size = map_test(); const auto stop = std::chrono::system_clock::now (); std::chrono::duration <double, std::milli > time = stop - start; if (what.size() && map_size) std::cout << std::setw (8) << time << " for " << what << " (ratio: " << (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n"; return time.count(); } int main() { std::cout << std::fixed << std::setprecision (2); time_it(map_emplace); // cache warmup const auto x = time_it(map_emplace, "plain emplace"); time_it(map_emplace_hint, "emplace with correct hint", x); time_it(map_emplace_hint_wrong, "emplace with wrong hint", x); time_it(map_emplace_hint_corrected, "corrected emplace", x); time_it(map_emplace_hint_closest, "emplace using returned iterator", x); }
Possible output:
143.48ms for plain emplace (ratio: 1.00) 164.78ms for emplace with correct hint (ratio: 0.87) 171.22ms for emplace with wrong hint (ratio: 0.84) 166.55ms for corrected emplace (ratio: 0.86) 167.41ms for emplace using returned iterator (ratio: 0.86)