Difference between revisions of "cpp/container/unordered multimap/emplace"
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Latest revision as of 10:53, 4 December 2021
| template< class... Args > iterator emplace( Args&&... args ); |
(since C++11) | |
Inserts a new element into the container constructed in-place with the given args.
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)....
Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.
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.
Contents |
[edit] Parameters
| args | - | arguments to forward to the constructor of the element |
[edit] Return value
An iterator to the inserted element.
[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 <iostream> #include <string> #include <utility> #include <unordered_map> int main() { std::unordered_multimap<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 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')); for (const auto& p : m) std::cout << p.first << " => " << p.second << '\n'; }
Possible output:
a => a b => abcd c => cccccccccc d => ddd d => DDD
[edit] See also
| constructs elements in-place using a hint (public member function) | |
| inserts in-place if the key does not exist, does nothing if the key exists (public member function) | |
| inserts elements or nodes(since C++17) (public member function) |
