C++: What is the difference between std::multiset and std::set?

multiset — allows duplicates. set — unique elements only.

std::set:

• Unique elements.
• Duplicates ignored.

1std::set<int> s;
2s.insert(1);
3s.insert(1);  // Ignored
4s.size();  // 1

std::multiset:

• Allows duplicates.
• Stores all occurrences.

1std::multiset<int> ms;
2ms.insert(1);
3ms.insert(1);  // Stored
4ms.size();  // 2

Use multiset when:

• Need to count occurrences.
• Need to store duplicates.