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std::unorde...map::emplace(3) C++ Standard Libarystd::unorde...map::emplace(3) NAME std::unordered_map::emplace - std::unordered_map::emplace Synopsis template< class... Args > (since C++11) std::pair<iterator,bool> emplace( Args&&... args ); 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 ele- ment (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 invali- dated. Otherwise iterators are not affected. References are not invalidated. Rehash- ing occurs only if the new number of elements is greater than max_load_fac- tor()*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 denot- ing whether the insertion took place (true if insertion happened, false if it did not). Exceptions If an exception is thrown by any operation, this function has no ef- fect (strong exception guarantee). Complexity Amortized constant on average, worst case linear in the size of the container. Example // Run this code #include <iostream> #include <utility> #include <string> #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 emplace_hint constructs elements in-place using a hint (C++11) (public member function) try_emplace inserts in-place if the key does not exist, does noth- ing if the key (C++17) exists (public member function) inserts elements insert or nodes (C++11) (since C++17) (public member function) http://cppreference.com 2022.07.31 std::unorde...map::emplace(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Complexity | Example | Possible output: | See also
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