C++ Unordered_set::clear() function

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The C++ std::unordered_set::clear() function is used to erase all the elements from the unordered_set container. This function doesn't make any changes if the current unordered_set is empty otherwise, it will erase all the elements from the unordered_set. when this function is invoked the size() function return a zero. The return type of this function is void, which implies that it does not return any value.

Syntax

Following is the syntax of the C++ std::unordered_set::clear() function −

void clear();

Parameters

• It does not accept any parameter.

Return Value

This function does not return any value.

Example 1

Let's look at the following example, where we are going to use the clear() function and observing the output.

#include<iostream>
#include<string>
#include<unordered_set>
using namespace std;

int main () {
   //create a unordered_set
   unordered_set<int> u_set = {1, 2, 3, 4, 5};
   cout<<"Contents of the u_set before the clear operation are: "<<endl;
   for(int n : u_set){
      cout<<n<<endl;
   }
   //using the clear() function
   u_set.clear();
   cout<<"Size of the u_set after the clear operation: "<<u_set.size();
}

Output

If we run the above code it will generate the following output −

Contents of the u_set before the clear operation are: 
5
4
3
2
1
Size of the u_set after the clear operation: 0

Example 2

Consider another scenario, where we are going to use the cler() function the unordered_set of type char.

#include<iostream>
#include<string>
#include<unordered_set>
using namespace std;

int main () {
   //create a unordered_set
   unordered_set<char> char_set = {'A', 'B', 'C', 'D', 'E'};
   cout<<"Contents of the char_set before the clear operation are: "<<endl;
   for(char c : char_set){
      cout<<c<<endl;
   }
   //using the clear() function
   char_set.clear();
   cout<<"Size of the char_set after the clear operation: "<<char_set.size();
}

Output

Following is the output of the above code −

Contents of the char_set before the clear operation are: 
E
D
C
B
A
Size of the char_set after the clear operation: 0

Example 3

Following is the another example of using the clear() function on the unordered_set of the type string.

#include<iostream>
#include<string>
#include<unordered_set>
using namespace std;

int main () {
   //create a unordered_set
   unordered_set<string> str_set = {"Rahul", "Mukesh", "Dinesh", "Raja"};
   cout<<"Contents of the str_set before the clear operation are: "<<endl;
   for(string s : str_set){
      cout<<s<<endl;
   }
   //using the clear() function
   str_set.clear();
   cout<<"Size of the str_set after the clear operation: "<<str_set.size();
}

Output

Output of the above code is as follows −

Contents of the str_set before the clear operation are: 
Dinesh
Mukesh
Raja
Rahul
Size of the str_set after the clear operation: 0