C++ Unordered_multimap::empty() Function

The C++ std::unordered_multimap::empty() functionis used to check whether unordered_multimap is empty or not. If the unordered_multimap is empty, then it returns true; otherwise, it returns false. This function does not modify or change the content of the container in any way.

Syntax

Following is the syntax of std::unordered_multimap::empty() function.

bool empty() const noexcept;

Parameters

This function does not accepts any parameters.

Return value

This function returns boolean values, i.e.true if unordered_multimap is empty (size zero) otherwise false.

Example 1

Let's look at the following example, where we are going to demonstrate the usage of unordered_multimap::empty() function.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm;
   if (umm.empty())
      cout << "Unordered multimap is empty." << endl;
   umm.emplace_hint(umm.begin(), 'a', 1);
   umm.emplace_hint(umm.end(), 'b', 2);
   if (!umm.empty())
      cout << "Unordered multimap is not empty." << endl;
   return 0;
}

Output

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

Unordered multimap is empty.
Unordered multimap is not empty.

Example 2

Consider the following example, where we are going to use the empty() function to check if an unordered_multimap contains any elements or not.

#include <unordered_map>
#include <iostream>
#include <utility>
using namespace std; 
int main() {
   unordered_multimap<int, int> numbers;
   cout << boolalpha;
   cout << "Initially, numbers.empty(): " << numbers.empty() << '\n';
 
   numbers.emplace(55, 110);
   numbers.insert(make_pair(12345, 115)); 
   cout << "After adding elements, numbers.empty(): " << numbers.empty() << '\n';
}

Output

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

Initially, numbers.empty(): true
After adding elements, numbers.empty(): false

Example 3

In the following example, we are going to use the empty() function and display all the elements in the container if it was not empty.

#include <unordered_map>
#include <iostream>
#include <utility>
using namespace std; 
int main() {
   unordered_multimap<string, int> marks_of_students;
   marks_of_students.insert(make_pair("Aman", 100));
   marks_of_students.insert(make_pair("Akash", 95));
   marks_of_students.insert({{"Vivek", 98},{"Aman", 92},{"Akash", 97},{"Rahul", 96}});
   
   for(auto & it: marks_of_students){
      if(marks_of_students.empty()){
         cout<<"marks of students are empty, I can't be able to fetch the data!"<<endl;
      }
      else{
         cout<<"Marks of "<<it.first<<" is "<<it.second<<endl;
      }
   }
   cout<<endl;
   return 0;
}

Output

Following is the output of the above code −

Marks of Rahul is 96
Marks of Vivek is 98
Marks of Akash is 97
Marks of Akash is 95
Marks of Aman is 92
Marks of Aman is 100