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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_unordered_set_empty.htm below:

C++ Unordered Set Empty

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.

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

bool empty() const noexcept;

This function does not accepts any parameter.

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

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;
}

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

first is not empty
second is empty

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;
}

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

Is unordered_set empty? true

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;
}

Following is the output of the above code −

unordered_set contains following elements: 
50
40
30
20
10

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 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

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