unordered_set clear() function in C++ STL

The unordered_set::clear() function is a built-in function in C++ STL which is used to clear an unordered_set container. That is, this function removes all of the elements from an unordered_set and empties it. All of the iterators, pointers, and references to the container are invalidated. This reduces the size of the container to zero.

Syntax:  

unordered_set_name.clear()

Parameter: This function does not accepts any parameter.
Return Value: This function does not returns any value.
Below programs illustrate the unordered_set::clear() function:
Program 1:  

CPP

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

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<int> sampleSet;

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

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

    // clear the set
    sampleSet.clear();

    // size after clearing
    cout << "\nSize of set after clearing elements: "
         << sampleSet.size();

    return 0;
}

Output

sampleSet contains: 25 20 15 5 10 
Size of set after clearing elements: 0

Program 2: 

CPP

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

#include <iostream>
#include <unordered_set>

using namespace std;

int main()
{

    unordered_set<string> sampleSet;

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

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

    // clear the set
    sampleSet.clear();

    // size after clearing
    cout << "\nSize of set after clearing elements: "
         << sampleSet.size();

    return 0;
}

Output

sampleSet contains: For Geeks GeeksforGeeks Computer Science Portal Welcome To 
Size of set after clearing elements: 0

Time complexity – Linear O(N)