multiset::emplace() in C++ STL

Multisets are a type of associative containers similar to set, with an exception that multiple elements can have same values.

multiset::emplace()

This function is used to insert a new element into the multiset container.

Syntax :

multisetname.emplace(value)
Parameters :
The element to be inserted into the multiset
is passed as the parameter.
Result :
The parameter is added to the multiset.

Examples:

Input  : mymultiset{1, 2, 3, 4, 5};
         mymultiset.emplace(6);
Output : mymultiset = 1, 2, 3, 4, 5, 6

Input  : mymultiset{};
         mymultiset.emplace("This");
         mymultiset.emplace("is");
         mymultiset.emplace("Geeksforgeeks");
Output : mymultiset = Geeksforgeeks, This, is 

Errors and Exceptions
1. It has a strong exception guarantee, therefore, no changes are made if an exception is thrown
2. Parameter should be of same type as that of the container, otherwise an error is thrown

// INTEGER EXAMPLE
// CPP program to illustrate
// Implementation of emplace() function
#include <iostream>
#include <set>
using namespace std;

int main()
{
    multiset<int> mymultiset{};
    mymultiset.emplace(1);
    mymultiset.emplace(56);
    mymultiset.emplace(4);
    mymultiset.emplace(9);
    mymultiset.emplace(0);
    // multi set becomes 0, 1, 4, 9, 56

    // adding another element
    mymultiset.emplace(87);

    // printing the multiset
    for (auto it = mymultiset.begin();
         it != mymultiset.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

0 1 4 9 56 87

// STRING EXAMPLE
// CPP program to illustrate
// Implementation of emplace() function
#include <iostream>
#include <set>
#include <string>
using namespace std;

int main()
{
    multiset<string> mymultiset{};
    mymultiset.emplace("This");
    mymultiset.emplace("is");
    mymultiset.emplace("a");
    mymultiset.emplace("computer science");
    mymultiset.emplace("portal");
    // multi set becomes This, a,
    // computer science, is, portal

    // adding element
    mymultiset.emplace("GeeksForGeeks");

    // printing the multiset
    for (auto it = mymultiset.begin();
         it != mymultiset.end(); ++it)
        cout << ' ' << *it;
    return 0;
}

Output:

GeeksForGeeks This a computer science is portal

Application
Input an empty multi set with the following numbers and order using emplace() function and find sum of elements. The advantage of emplace() over insert is, it avoids unnecessary copy of object.

Input :  7, 9, 4, 6, 2, 5, 3
Output : 36

// CPP program to illustrate
// Application of emplace() function
#include <iostream>
#include <set>
using namespace std;

int main()
{
    // sum variable declaration
    int sum = 0;

    // multiset declaration
    multiset<int> mymultiset{};
    mymultiset.emplace(7);
    mymultiset.emplace(9);
    mymultiset.emplace(4);
    mymultiset.emplace(6);
    mymultiset.emplace(2);
    mymultiset.emplace(5);
    mymultiset.emplace(3);

    // iterator declaration
    set<int>::iterator it;

    // finding sum of elements
    while (!mymultiset.empty()) {
        it = mymultiset.begin();
        sum = sum + *it;
        mymultiset.erase(it);
    }

    // printing the sum
    cout << sum;
    return 0;
}

Output :

36

Time Complexity : O(logn)

emplace() vs insert()
When we use insert, we create an object and then insert it into the multiset. With emplace(), the object is constructed in-place.

// C++ code to demonstrate difference between
// emplace and insert
#include<bits/stdc++.h>
using namespace std;

int main()
{
    // declaring multiset of pairs
    multiset<pair<char, int>> ms;

    // using emplace() to insert pair in-place
    ms.emplace('a', 24);

    // Below line would not compile
    // ms.insert('b', 25);

    // using insert() to insert pair in-place
    ms.insert(make_pair('b', 25));

    // printing the multiset
    for (auto it = ms.begin(); it != ms.end(); ++it)
        cout << " " << (*it).first << " "
             << (*it).second << endl;

    return 0;
}

Output :

 a 24
 b 25

Please refer Inserting elements in std::map (insert, emplace and operator []) for details.