Difference Between Set, Multiset, Unordered Set, and Unordered Multiset in C++

Here we will see what are the differences for set, multiset, unordered_set and unordered_multiset in C++. Let us see the properties of them using some example.

Set

The properties of set are like below

• Stores data in sorted order
• Stores only unique values
• We can insert or delete data, but cannot change the data
• We can remove more than one element using start and end iterator
• We can traverse using iterators
• Sets are implemented using Binary Search Tree

Now let us see an example

Example

#include <iostream>
#include <set>
using namespace std;
main() {
   int data[15] = {11, 55, 22, 66, 33, 22, 11, 44, 77, 88, 66, 99, 66, 23, 41};
   set<int> my_set;
   for(int i = 0; i<15; i++) {
      my_set.insert(data[i]);
   }
   set<int>::iterator it;
   for(it = my_set.begin(); it != my_set.end(); it++) {
      cout << "Item: " << *it << endl;
   }
}

Output

Item: 11
Item: 22
Item: 23
Item: 33
Item: 41
Item: 44
Item: 55
Item: 66
Item: 77
Item: 88
Item: 99

Multiset

The properties of set are like below −

• Stores data in sorted order
• It allows to store duplicate data
• We can remove more than one element using start and end iterator.

Now let us see an example.

Example

#include <iostream>
#include <set>
using namespace std;
main() {
   int data[15] = {11, 55, 22, 66, 33, 22, 11, 44, 77, 88, 66, 99, 66, 23, 41};
   multiset<int> my_set;
   for(int i = 0; i<15; i++) {
      my_set.insert(data[i]);
   }
   multiset<int>::iterator it;
   for(it = my_set.begin(); it != my_set.end(); it++) {
      cout << "Item: " << *it << endl;
   }
}

Output

Item: 11
Item: 11
Item: 22
Item: 22
Item: 23
Item: 33
Item: 41
Item: 44
Item: 55
Item: 66
Item: 66
Item: 66
Item: 77
Item: 88
Item: 99

Unordered Set

The properties of set are like below −

• Data can be placed in any order
• Duplicate data will be discarded
• This set is created using Hash-tables
• We can erase only one element, where the iterator is pointing

Now let us see an example.

Example

#include <iostream>
#include <unordered_set>
using namespace std;
main() {
   int data[15] = {11, 55, 22, 66, 33, 22, 11, 44, 77, 88, 66, 99, 66, 23, 41};
   unordered_set<int> my_set;
   for(int i = 0; i<15; i++) {
      my_set.insert(data[i]);
   }
   unordered_set<int>::iterator it;
   for(it = my_set.begin(); it != my_set.end(); it++) {
      cout << "Item: " << *it << endl;
   }
}

Output

Item: 11
Item: 55
Item: 22
Item: 66
Item: 33
Item: 44
Item: 77
Item: 88
Item: 99
Item: 23
Item: 41

Unordered Multiset

The properties of set are like below −

• Data can be placed in any order
• Duplicate data are allowed
• This set is created using Hash-tables
• We can erase only one element, where the iterator is pointing

Now let us see an example.

Example

#include <iostream>
#include <unordered_set>
using namespace std;
main() {
   int data[15] = {11, 55, 22, 66, 33, 22, 11, 44, 77, 88, 66, 99, 66, 23, 41};
   unordered_multiset<int> my_set;
   for(int i = 0; i<15; i++) {
      my_set.insert(data[i]);
   }
   unordered_multiset<int>::iterator it;
   for(it = my_set.begin(); it != my_set.end(); it++) {
      cout << "Item: " << *it << endl;
   }
}

Output

Item: 11
Item: 55
Item: 22
Item: 66
Item: 33
Item: 22
Item: 11
Item: 44
Item: 77
Item: 88
Item: 66
Item: 99
Item: 66
Item: 23
Item: 41