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Showing content from https://www.geeksforgeeks.org/cpp/set-vs-unordered_set-c-stl/ below:

set vs unordered_set in C++ STL

Last Updated : 11 Jul, 2025

Differences : 

                |     set             | unordered_set
---------------------------------------------------------
Ordering        | increasing  order   | no ordering
                | (by default)        |

Implementation  | Self balancing BST  | Hash Table
                | like Red-Black Tree |  

search time     | log(n)              | O(1) -> Average 
                |                     | O(n) -> Worst Case

Insertion time  | log(n) + Rebalance  | Same as search
                      
Deletion time   | log(n) + Rebalance  | Same as search

Use set when

• We need ordered data.
• We would have to print/access the data (in sorted order).
• We need predecessor/successor of elements.
• Since set is ordered, we can use functions like binary_search(), lower_bound() and upper_bound() on set elements. These functions cannot be used on unordered_set().
• See advantages of BST over Hash Table for more cases.

Use unordered_set when

• We need to keep a set of distinct elements and no ordering is required.
• We need single element access i.e. no traversal.

Examples:

set:
Input  :  1, 8, 2, 5, 3, 9
Output :  1, 2, 3, 5, 8, 9

Unordered_set:
Input  : 1, 8, 2, 5, 3, 9
Output : 9 3 1 8 2 5 

If you want to look at implementation details of set and unordered_set in c++ STL, see Set Vs Map. Set allows to traverse elements in sorted order whereas Unordered_set doesn't allow to traverse elements in sorted order. 

Implementation:

// Program to print elements of set
#include <bits/stdc++.h>
using namespace std;

int main()
{
    set<int> s;
    s.insert(5);
    s.insert(1);
    s.insert(6);
    s.insert(3);
    s.insert(7);
    s.insert(2);

    cout << "Elements of set in sorted order: \n";
    for (auto it : s)
        cout << it << " ";

    return 0;
}

Elements of set in sorted order: 
1 2 3 5 6 7 

Implementation:

// Program to print elements of set
#include <bits/stdc++.h>
using namespace std;

int main()
{
    unordered_set<int> s;
    s.insert(5);
    s.insert(1);
    s.insert(6);
    s.insert(3);
    s.insert(7);
    s.insert(2);

    cout << "Elements of unordered_set: \n";
    for (auto it : s)
        cout << it << " ";

    return 0;
}

Elements of unordered_set: 
2 7 3 6 5 1 

Predecessor/Successor in Set: Set can be modified to find predecessor or successor whereas Unordered_set doesn't allow to find predecessor/Successor. 

Implementation:

// Program to print inorder predecessor and inorder successor
#include <bits/stdc++.h>
using namespace std;

set<int> s;

void inorderPredecessor(int key)
{
    if (s.find(key) == s.end()) {
        cout << "Key doesn't exist\n";
        return;
    }

    set<int>::iterator it;
    it = s.find(key); // get iterator of key

    // If iterator is at first position
    // Then, it doesn't have predecessor
    if (it == s.begin()) {
        cout << "No predecessor\n";
        return;
    }

    --it; // get previous element
    cout << "predecessor of " << key << " is=";
    cout << *(it) << "\n";
}

void inorderSuccessor(int key)
{
    if (s.find(key) == s.end()) {
        cout << "Key doesn't exist\n";
        return;
    }

    set<int>::iterator it;
    it = s.find(key); // get iterator of key
    ++it; // get next element

    // Iterator points to NULL (Element does
    // not exist)
    if (it == s.end())
    {
        cout << "No successor\n";
        return;
    }
    cout << "successor of " << key << " is=";
    cout << *(it) << "\n";
}

int main()
{
    s.insert(1);
    s.insert(5);
    s.insert(2);
    s.insert(9);
    s.insert(8);

    inorderPredecessor(5);
    inorderPredecessor(1);
    inorderPredecessor(8);
    inorderSuccessor(5);
    inorderSuccessor(2);
    inorderSuccessor(9);

    return 0;
}

predecessor of 5 is=2
No predecessor
predecessor of 8 is=5
successor of 5 is=8
successor of 2 is=5
No successor

Let us see the differences in a tabular form -:

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