std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::emplace_hint - cppreference.com (original) (raw)

| template< class... Args >iterator emplace_hint( const_iterator hint, Args&&... args ); | | (since C++23) (constexpr since C++26) | | ----------------------------------------------------------------------------------------- | | ------------------------------------- |

Inserts a new element into the container as close as possible to the position just before hint.

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)....

[edit] Parameters

hint	-	iterator to the position before which the new element will be inserted
args	-	arguments to forward to the constructor of the element

[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

[edit] Example

#include #include #include #include #include #include const int n_operations = 100'500'0; std::size_t map_emplace() { std::flat_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::flat_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::flat_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::flat_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::flat_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); }