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TheC++std::unordered_set::empty() function is used to return a boolean value indicating whether the unordered_set container is empty or not.

If the unordered_set is empty, then begin() is equal to end(), and the empty() function returns true; otherwise, it returns false.

Syntax

Following is the syntax for std::unordered_set::empty.

bool empty() const noexcept;

Parameters

This function does not accepts any parameter.

Return Value

This function returns true if the container size is 0, otherwise false.

Example 1

Consider the following example, where we are going to demonstrate the usage of unordered_set::empty() function.

#include <iostream>
#include <string>
#include <unordered_set>

int main () {
   std::unordered_set<std::string> first = {"sairam","krishna","mammahe"};
   std::unordered_set<std::string> second;
   std::cout << "first " << (first.empty() ? "is empty" : "is not empty" ) << std::endl;
   std::cout << "second " << (second.empty() ? "is empty" : "is not empty" ) << std::endl;
   return 0;
}

Output

Let us compile and run the above program, this will produce the following result −

first is not empty
second is empty

Example 2

Let's look at the following example, where we are going to use the empty() function to check whether the unordered_set is empty or not.

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

int main () {
   unordered_set<string> uSet;
   if(uSet.empty()==1){
      cout<<boolalpha;
      cout<<"Is unordered_set empty? "<<uSet.empty()<<endl;
   } else{
      cout<<"unordered_set contains following elements: "<<endl;
      for(auto it: uSet)
         cout<<it<<endl;
   }
   return 0;
}

Output

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

Is unordered_set empty? true

Example 3

In the following example, we are going to use the empty() function to check whether the unordered_set is empty or not and going to display the content of the unordered_set if it contains elements.

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

int main () {
   unordered_set<int> uSet{10, 20, 30, 40, 50};
   if(uSet.empty()==1){
      cout<<boolalpha;
      cout<<"Is unordered_set empty? "<<uSet.empty()<<endl;
   } else{
      cout<<"unordered_set contains following elements: "<<endl;
      for(auto it: uSet)
         cout<<it<<endl;
   }
   return 0;
}

Output

Following is the output of the above code −

unordered_set contains following elements: 
50
40
30
20
10

Example 4

Following is the another example of the usage of the empty() function and displaying the result in the boolean value.

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

int main () {
   unordered_set<string> uSet;
   
   cout<<boolalpha;
   cout<<"boolean value before the insertion of elements: "<<endl;
   cout<<"Is unordered_set empty? "<<uSet.empty()<<endl;
   uSet.insert("tutorix");
   uSet.insert("tutorialspoint");
   cout<<"boolean value after the insertion of elements: "<<endl;
   cout<<"Is unordered_set empty? "<<uSet.empty()<<endl;
   
   return 0;
}

Output

Output of the above code is as follows −

boolean value before the insertion of elements: 
Is unordered_set empty? true
boolean value after the insertion of elements: 
Is unordered_set empty? false