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unordered_set emplace() function in C++ STL

Last Updated : 05 Jun, 2023

The unordered_set::emplace() function is a built-in function in C++ STL which is used to insert an element in an unordered_set container. The element is inserted only if it is not already present in the container. This insertion also effectively increases the container size 1.
Syntax: 

unordered_set_name.emplace(element)

Parameter: This function accepts a single parameter element which is to be inserted in the unordered_set container.
Return Value: This function returns a pair on successful insertion. The pair consists of an iterator pointing to the newly inserted element and a boolean value True. If the element to be inserted is already present in the container then it returns a pair with an iterator pointing to the already present element and a boolean value false.
Below programs illustrate the unordered_set::emplace() function:
Program 1: 
 

// C++ program to illustrate the
// unordered_set::emplace() function

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<int> sampleSet;

    // Inserting elements
    sampleSet.emplace(5);
    sampleSet.emplace(10);
    sampleSet.emplace(15);
    sampleSet.emplace(20);
    sampleSet.emplace(25);

    // displaying all elements of sampleSet
    cout << "sampleSet contains: ";
    for (auto itr = sampleSet.begin(); itr != sampleSet.end(); itr++) {
        cout << *itr << " ";
    }

    return 0;
}

sampleSet contains: 25 5 10 15 20

Program 2: 

// C++ program to illustrate the
// unordered_set::emplace() function

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<string> sampleSet;

    // Inserting elements using
    // emplace() function
    sampleSet.emplace("Welcome");
    sampleSet.emplace("To");
    sampleSet.emplace("GeeksforGeeks");
    sampleSet.emplace("Computer Science Portal");
    sampleSet.emplace("For Geeks");

    // displaying all elements of sampleSet
    cout << "sampleSet contains: ";
    for (auto itr = sampleSet.begin(); itr != sampleSet.end(); itr++) {
        cout << *itr << " ";
    }

    return 0;
}

sampleSet contains: Welcome To GeeksforGeeks For Geeks Computer Science Portal

Time complexity: O(n) 

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