std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace
| template< class... Args > std::pair<iterator, bool> emplace( Args&&... args ); |
(since C++11) | |
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.
Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.
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)....
The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately.
If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor() * bucket_count().
Parameters
| args | - | arguments to forward to the constructor of the element |
Return value
Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place (true if insertion happened, false if it did not).
Exceptions
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
Complexity
Amortized constant on average, worst case linear in the size of the container.
Example
#include <iostream> #include <string> #include <utility> #include <unordered_map> int main() { std::unordered_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"); // uses pair's piecewise constructor m.emplace(std::piecewise_construct, std::forward_as_tuple("c"), std::forward_as_tuple(10, 'c')); // as of C++17, m.try_emplace("c", 10, 'c'); can be used for (const auto& p : m) std::cout << p.first << " => " << p.second << '\n'; }
Possible output:
a => a b => abcd c => cccccccccc d => ddd
See also
| (C++11) |
constructs elements in-place using a hint (public member function) |
| (C++17) |
inserts in-place if the key does not exist, does nothing if the key exists (public member function) |
| (C++11) |
inserts elements or nodes (since C++17) (public member function) |