C++ unordered_multimap::swap() Function

The C++ std::unordered_multimap::swap() function is used to exchange the content of the first unordered_multimap with another. It will take place when the other container is of the same type.

This function exchanges the element or key/value pairs without actually performing any copies or moves on the individual element, allowing for constant-time execution no matter the size.

Syntax

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

void swap(unordered_multimap& n);

Parameters

  • n − It indicates the another unordered_multimap object.

Return value

This function does not returns anything.

Example 1

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

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm1 = {
      {'a', 1}, {'a', 10}, {'a', 100}, {'a', 1000}, {'a', 10000} 
   };
   unordered_multimap<char, int> umm2;
   umm1.swap(umm2);
   cout << "second unordered_multimap contains following elements after swap " << endl;
   for (auto it = umm2.begin(); it != umm2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

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

second unordered_multimap contains following elements after swap 
a = 10000
a = 1000
a = 100
a = 10
a = 1

Example 2

Consider the following example, where we are going to use the swap() function and swap the elements from one container to another container, vice-versa.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<char, int> umm1 = { {'a', 1}, {'b', 2}, {'c', 3}, {'b', 20}, {'c', 30}, };
   unordered_multimap<char, int> umm2 = { {'e', 5}, {'f', 6}, {'g', 7}, {'h', 8} };
   cout<<"umm1 contains following element before swap: "<<endl;
   for(auto & it:umm1){
      cout<<it.first<< " = "<<it.second<<endl;
   }
   cout<<"umm2 contains following element before swap: "<<endl;
   for(auto & it:umm2){
      cout<<it.first<< " = "<<it.second<<endl;
   }
   umm1.swap(umm2);
   cout << "umm1 contains following elements after swap: " << endl;
   for (auto it = umm1.begin(); it != umm1.end(); ++it)
      cout << it->first << " = " << it->second << endl;
      cout << "umm2 contains following elements after swap: " << endl;
   for (auto it = umm2.begin(); it != umm2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Following is the output of the above code −

umm1 contains following element before swap: 
c = 30
c = 3
b = 20
b = 2
a = 1
umm2 contains following element before swap: 
h = 8
g = 7
f = 6
e = 5
umm1 contains following elements after swap: 
h = 8
g = 7
f = 6
e = 5
umm2 contains following elements after swap: 
c = 30
c = 3
b = 20
b = 2
a = 1

Example 3

In the following example, we are going to use the swap() function to swap only the only even key and its value from one unordered_multimap to another unordered_multimap.

#include <iostream>
#include <unordered_map>
using namespace std;
int main(void) {
   unordered_multimap<int, int> umm1, umm2, temp;
   umm1 = {{1, 10}, {2, 20}, {1, 100}, {2, 200}, {3, 300}, {4, 40}};
   cout<<"umm1 contains following element before swap: "<<endl;
   for(auto& it:umm1){
      cout<<it.first<<" = "<<it.second<<endl;
   }
   for(auto & it:umm1){
      if(it.first%2 == 0){
         temp.insert({it.first, it.second});
      }
   }
   umm2.swap(temp);
   cout<<"umm2 contains following element before swap: "<<endl;
   for (auto it = umm2.begin(); it != umm2.end(); ++it)
      cout << it->first << " = " << it->second << endl;
   return 0;
}

Output

Output of the above code is as follows −

umm1 contains following element before swap: 
4 = 40
3 = 300
2 = 200
2 = 20
1 = 100
1 = 10
umm2 contains following element before swap: 
2 = 20
2 = 200
4 = 40
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