std::set_union in C++

Union of two sorted ranges
The union of two sets is formed by the elements that are present in either one of the sets, or in both. Elements from the second range that have an equivalent element in the first range are not copied to the resulting range.
The elements are compared using operator< for the first version, and comp for the second. Two elements, a and b are considered equivalent if (!(a<b) && !(b<a)) or if (!comp(a, b) && !comp(b, a)).
The elements in the ranges shall already be ordered.
1. Using default operator < :

Template :
OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
                          InputIterator2 first2, InputIterator2 last2,
                          OutputIterator result);

Parameters :

first1, last1
Input iterators to the initial and final positions of the first
sorted sequence. The range used is [first1, last1], which contains
all the elements between first1 and last1, including the element
pointed by first1 but not the element pointed by last1.

first2, last2
Input iterators to the initial and final positions of the second
sorted sequence. The range used is [first2, last2].

result
Output iterator to the initial position of the range where the
resulting sequence is stored.

comp
Binary function that accepts two arguments of the types pointed by
the input iterators, and returns a value convertible to bool.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

The ranges shall not overlap.

CPP

// CPP program to demonstrate use of
// std :: set_symmetric_difference
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

// Driver code
int main()
{
    string first[] = { "John", "Bob", "Mary", "Serena" };
    string second[] = { "Jim", "Mary", "John", "Bob" };
    int n = sizeof(first) / sizeof(first[0]);

    // Print students of first list
    cout << "Students in first subject :";
    for (int i = 0; i < n; i++)
        cout << " " << first[i];
    cout << "\n";

    // Print students of second list
    cout << "Students in second subject :";
    for (int i = 0; i < n; i++)
        cout << " " << second[i];
    cout << "\n\n";

    vector<string> v(10);
    vector<string>::iterator it, st;

    // Sorting both the list
    sort(first, first + n);
    sort(second, second + n);

    // Using default operator<
    it = set_union(first, first + n, second, second + n,
                   v.begin());

    cout << "Students attending both subjects are :\n";
    for (st = v.begin(); st != it; ++st)
        cout << ' ' << *st;
    cout << '\n';

    return 0;
}

Output: 

First array contains : 5 10 15 20 25
Second array contains : 50 40 30 20 10

The union has 8 elements:
 5 10 15 20 25 30 40 50

2. Using custom function :
Syntax :  

Template :
OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
                          InputIterator2 first2, InputIterator2 last2,
                          OutputIterator result, Compare comp);


Parameters :

first1, last1, first2, last2, result are same as above.

comp
Binary function that accepts two arguments of the types pointed by
the input iterators, and returns a value convertible to bool.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

The ranges shall not overlap.

Output: 

Students in first subject : Sachin Rakesh Sandeep Serena
Students in second subject : Vaibhav Sandeep Rakesh Neha

Students attending both subjects are :
Neha Rakesh Sachin Sandeep Serena Vaibhav

Possible Application : It is used to find the elements that are present in either one container or in both containers.
1. It can be used to find the list of all the students that are attending both subjects.

CPP

// CPP program to demonstrate use of
// std :: set_symmetric_difference
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

// Driver code
int main()
{
    string first[] = { "John", "Bob", "Mary", "Serena" };
    string second[] = { "Jim", "Mary", "John", "Bob" };
    int n = sizeof(first) / sizeof(first[0]);

    // Print students of first list
    cout << "Students in first subject :";
    for (int i = 0; i < n; i++)
        cout << " " << first[i];
    cout << "\n";

    // Print students of second list
    cout << "Students in second subject :";
    for (int i = 0; i < n; i++)
        cout << " " << second[i];
    cout << "\n\n";

    vector<string> v(10);
    vector<string>::iterator it, st;

    // Sorting both the list
    sort(first, first + n);
    sort(second, second + n);

    // Using default operator<
    it = set_union(first, first + n, second, second + n,
                   v.begin());

    cout << "Students attending both subjects are :\n";
    for (st = v.begin(); st != it; ++st)
        cout << ' ' << *st;
    cout << '\n';

    return 0;
}

Output: 

Students in first subject : Sachin Rakesh Sandeep Serena
Students in second subject : Vaibhav Sandeep Rakesh Neha

Students attending both subjects are :
Neha Rakesh Sachin Sandeep Serena Vaibhav

2. It can also be use to find union of two sets. 
Program is given above.