std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::emplace
std::pair <iterator, bool> emplace( Args&&... args );
(constexpr since C++26)
Inserts a new element into the container constructed in-place with the given args, if there is no element with the key in the container.
Initializes an object t of type std::pair <key_type, mapped_type> with std::forward <Args>(args)...; if the map already contains an element whose key is equivalent to t.first, *this is unchanged. Otherwise, equivalent to:
auto key_it = ranges::upper_bound (c.keys, t.first, compare); auto value_it = c.values.begin() + std::distance (c.keys.begin(), key_it); c.keys.insert(key_it, std::move(t.first)); c.values.insert(value_it, std::move(t.second));
This overload participates in overload resolution only if std::is_constructible_v <std::pair <key_type, mapped_type>, Args...> is true.
If value_type is not EmplaceConstructible into flat_map from args, the behavior is undefined.
[edit] Parameters
[edit] Return value
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.
[edit] Exceptions
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
[edit] Complexity
If insertion occurs, linear in the size of the container, otherwise logarithmic 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 <flat_map> int main() { std::flat_map <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 has no effect 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')); // an alternative is: m.try_emplace("c", 10, 'c'); for (const auto& p : m) std::cout << p.first << " => " << p.second << '\n'; }
Output:
a => a b => abcd c => cccccccccc d => ddd
[edit] See also
(public member function) [edit]