C++ size()

The size() member function of unordered_set is used to return the number of elements currently stored in the container as a size_type. If the unordered_set is empty, it returns 0.

Syntax

set_name.size();

Parameters:

This function takes no parameters.

Return value:

Returns a size_type value representing the number of elements in the unordered_set.

Example 1: Basic Usage of size()

In this example, the program inserts one element into an unordered_set and prints its size:

#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
  unordered_set<int> mySet;
  mySet.insert(10);

  cout << "Size: " << mySet.size();
  return 0;
}

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The output of this code is:

Size: 1

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Example 2: Counting Unique Elements

In this example, the program initializes an unordered_set with integers (including duplicates), prints its size, and displays its elements:

#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
  unordered_set<int> mySet {1, 2, 3, 3, 1, 3, 2, 4, 5, 7};

  cout << "There are " << mySet.size() << " elements.\n";
  cout << "The elements are: ";
  for (int ele : mySet) {
    cout << ele << " ";
  }
  return 0;
}

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The output of this code is:

There are 6 elements.
The elements are: 7 5 4 3 2 1

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Note: The order of elements may vary.

Since an unordered_set cannot contain duplicates, the code returns 6 as its size and only the unique elements in the container.

Codebyte Example

In this example, the program compares size() with sizeof() to show that element count and memory footprint are unrelated:

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size() returns the number of stored elements, while sizeof() returns the memory footprint of the container object, which does not grow with element count.