std::set<Key,Compare,Allocator>::emplace

 

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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 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.

No iterators or references are invalidated.

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

Logarithmic in the size of the container.

Example

Run this code

#include <chrono>
#include <functional>
#include <iomanip>
#include <iostream>
#include <set>
#include <string>
 
class Dew
{
  private:
    int a;
    int b;
    int c;
 
  public:
    Dew(int _a, int _b, int _c)
      : a(_a), b(_b), c(_c)
    {}
 
    bool operator<(const Dew& other) const
    {
        if (a < other.a)
            return true;
        if (a == other.a && b < other.b)
            return true;
        return (a == other.a && b == other.b && c < other.c);
    }
};
 
const int nof_operations = 120;
 
int set_emplace()
{
    std::set<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();
}
 
int set_insert()
{
    std::set<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 timeit(std::function<int()> set_test, std::string what = "")
{
    auto start = std::chrono::system_clock::now();
    int setsize = set_test();
    auto stop = std::chrono::system_clock::now();
    std::chrono::duration<double, std::milli> time = stop - start;
    if (what.size() > 0 && setsize > 0)
        std::cout << std::fixed << std::setprecision(2)
                  << time.count() << "  ms for " << what << '\n';
}
 
int main()
{
    set_insert();
    timeit(set_insert, "insert");
    timeit(set_emplace, "emplace");
    timeit(set_insert, "insert");
    timeit(set_emplace, "emplace");
}

Possible output:

638.45  ms for insert
619.44  ms for emplace
609.43  ms for insert
652.55  ms for emplace

See also

emplace_hint (C++11)	constructs elements in-place using a hint (public member function)
insert	inserts elements or nodes (since C++17) (public member function)