std::multiset<Key,Compare,Allocator>::emplace
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std::multiset
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multiset::emplace
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Deduction guides (C++17)
template< class... Args >
iterator emplace( Args&&... args );
(since C++11) iterator emplace( Args&&... args );
(constexpr since C++26)
Inserts a new element into the container constructed in-place with the given args.
The constructor of the new element is called with exactly the same arguments as supplied to emplace, forwarded via std::forward <Args>(args)....
If value_type is not EmplaceConstructible into multiset from args, the behavior is undefined.
No iterators or references are invalidated.
[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
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
Run this code
#include <chrono> #include <cstddef> #include <functional> #include <iomanip> #include <iostream> #include <string> #include <set> class Dew { private: int a, b, c; public: Dew(int _a, int _b, int _c) : a(_a), b(_b), c(_c) {} bool operator<(const Dew& other) const { return (a < other.a) || (a == other.a && b < other.b) || (a == other.a && b == other.b && c < other.c); } }; constexpr int nof_operations{101}; std::size_t set_emplace() { std::multiset <Dew> set; for (int i = 0; i < nof_operations; ++i) for (int j = 0; j < nof_operations; ++j) for (int k = 0; k < nof_operations; ++k) set.emplace(i, j, k); return set.size(); } std::size_t set_insert() { std::multiset <Dew> set; for (int i = 0; i < nof_operations; ++i) for (int j = 0; j < nof_operations; ++j) for (int k = 0; k < nof_operations; ++k) set.insert(Dew(i, j, k)); return set.size(); } void time_it(std::function <int()> set_test, std::string what = "") { const auto start = std::chrono::system_clock::now (); const auto the_size = set_test(); const auto stop = std::chrono::system_clock::now (); const std::chrono::duration <double, std::milli > time = stop - start; if (!what.empty() && the_size) std::cout << std::fixed << std::setprecision (2) << time << " for " << what << '\n'; } int main() { time_it(set_insert, "cache warming..."); time_it(set_insert, "insert"); time_it(set_insert, "insert"); time_it(set_emplace, "emplace"); time_it(set_emplace, "emplace"); }
Possible output:
499.61ms for cache warming... 447.89ms for insert 436.77ms for insert 430.62ms for emplace 428.61ms for emplace