Unordered Set in C++ (STL)

std::unordered_set is an STL container and its introduced in C++11. It provides a functionality of a Set i.e. it can contains the unique elements only. unordered_set stores the elements internally using a Hash Table.

We can declare an unordered_set of a particular type only i.e.

std::unordered_set<T> setOfTypeT;

Elements of Type with which an unoredered_set is declared can only be added in it. Also, all elements of Type T should be,

• Copyable or Movable
• Comparable

Above requirements are because of the reason that element added in unordered_set will act as both key and value for internal Hash Table. As it’s a key therefore it should be comparable and copy-able.

Also, all elements in the unordered_set are immutable i.e. they can not be modified once inserted. They can only be deleted after the insertion.

Header File Required is,

#include <unordered_set>

Creating and using an std::unordered_set of std::string

As all objects of type std::string is copy-able and comparable. So, let’s create an unordered_set of type std::string and use it i.e.

std::unordered_set<std::string> setOfStrs;

Here, as we have declared an unordered set of type std::string. It means it can only store unique strings. Let’s see a complete example,

#include <iostream>
#include <unordered_set>
#include <algorithm>

int main() {

	// Creating an Unoredered_set of type string
	std::unordered_set<std::string> setOfStrs;

	// Insert strings to the set
	setOfStrs.insert("First");
	setOfStrs.insert("second");
	setOfStrs.insert("third");

	// Try to Insert a duplicate string in set
	setOfStrs.insert("second");

	// Iterate Over the Unordered Set and display it
	for (std::string s : setOfStrs)
		std::cout << s << std::endl;

}

Output:

third
second
First

Here we added 4 elements in the unoredered_set but only 3 will get added, last insertion will not work because “second” is already inserted in set and unoredered_set contains only the unique elements.

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Thanks.