set::emplace() in C++ STL

Sets are a type of associative containers in which each element has to be unique, because the value of the element identifies it. The value of the element cannot be modified once it is added to the set, though it is possible to remove and add the modified value of that element.

set::emplace()

This function is used to insert a new element into the set container, only if the element to be inserted is unique and does not already exists in the set.

Syntax :

setname.emplace(value)
Parameters :
The element to be inserted into the set
is passed as the parameter.
Result :
The parameter is added to the set if 
the set does not contain that element already.

Examples:

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

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

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 the same type as that of the container otherwise, an error is thrown

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

int main()
{
    set<int> myset{};
    myset.emplace(2);
    myset.emplace(6);
    myset.emplace(8);
    myset.emplace(9);
    myset.emplace(0);
    // set becomes 0, 2, 6, 8, 9

    // adding unique element

    myset.emplace(5);
    // set becomes 0, 2, 5, 6, 8, 9

    // adding element which already
    // exists there will be no
    // change in the set

    myset.emplace(2);
    // set remains 0, 2, 5, 6, 8, 9

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

Output:

0 2 5 6 8 9

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

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

    // adding unique element
    myset.emplace("GeeksForGeeks");
    // set becomes GeeksForGeeks, This, is,
    // a, computer science, portal

    // adding element which already exists
    // there will be no change in the set
    myset.emplace("is");

    // set remains GeeksForGeeks, This, is,
    // a, computer science, portal

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

Output:

GeeksForGeeks This a computer science is portal

Time Complexity : O(logn)

Application
Input an empty multi set with the following numbers and order using emplace() function and find sum of elements.

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;

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

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

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

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

Output :

36

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 set
    set<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 emplace() to insert pair in-place
    ms.insert(make_pair('b', 25));

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

    return 0;
}

Output :

 a 24
 b 25