std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace_hint - cppreference.com (original) (raw)

| template< class... Args >iterator emplace_hint( const_iterator hint, Args&&... args ); | | (since C++11) (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 #include #include #include #include #include   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)

[edit] See also

| | constructs element in-place (public member function) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | inserts elements or nodes(since C++17) (public member function) [edit] |