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std::map::emplace(3) C++ Standard Libary std::map::emplace(3) NAME std::map::emplace - std::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. 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. Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations. No iterators or references are invalidated. Parameters args - arguments to forward to the constructor of the element 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. Exceptions If an exception is thrown for any reason, this function has no ef- fect (strong exception safety guarantee). Complexity Logarithmic in the size of the container. Example // Run this code #include <iostream> #include <string> #include <utility> #include <map> int main() { std::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 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 (since C++17) (public member function) http://cppreference.com 2024.06.10 std::map::emplace(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Complexity | Example | Output: | See also
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