std::set_union en C++

Unión de dos rangos ordenados 
La unión de dos conjuntos está formada por los elementos que están presentes en uno de los conjuntos, o en ambos. Los elementos del segundo rango que tienen un elemento equivalente en el primer rango no se copian al rango resultante.
Los elementos se comparan usando operator< para la primera versión y comp para la segunda. Dos elementos, a y b, se consideran equivalentes si (!(a<b) && !(b<a)) o si (!comp(a, b) && !comp(b, a)).
Los elementos de las gamas ya estarán ordenados.
1. Usando el operador predeterminado < : 

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 program to illustrate
// std :: set_union
#include <algorithm> // std::set_union, std::sort
#include <iostream> // std::cout
#include <vector> // std::vector
 
// Driver code
int main()
{
    int first[] = { 5, 10, 15, 20, 25 };
    int second[] = { 50, 40, 30, 20, 10 };
    int n = sizeof(first) / sizeof(first[0]);
 
    // Print first array
    std::cout << "First array contains :";
    for (int i = 0; i < n; i++)
        std::cout << " " << first[i];
    std::cout << "\n";
 
    // Print second array
    std::cout << "Second array contains :";
    for (int i = 0; i < n; i++)
        std::cout << " " << second[i];
    std::cout << "\n\n";
 
    std::vector<int> v(10);
    std::vector<int>::iterator it, st;
 
    std::sort(first, first + n);
    std::sort(second, second + n);
 
    // Using default function
    it = std::set_union(first, first + n, second,
                        second + n, v.begin());
 
    std::cout << "The union has " << (it - v.begin())
              << " elements:\n";
    for (st = v.begin(); st != it; ++st)
        std::cout << ' ' << *st;
    std::cout << '\n';
 
    return 0;
}

Producción: 

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. Usando la función personalizada: 
Sintaxis:  

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.

// CPP program to demonstrate use of
// std :: set_symmetric_difference
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
 
using namespace std;
 
// Driver code
int main()
{
    string first[] = { "Sachin", "Rakesh",
                       "Sandeep", "Serena" };
    string second[] = { "Vaibhav", "Sandeep",
                          "Rakesh", "Neha" };
    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;
}

Producción: 

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

Posible aplicación: se utiliza para encontrar los elementos que están presentes en un contenedor o en ambos contenedores.
1. Se puede usar para encontrar la lista de todos los estudiantes que están cursando ambas materias.  

// 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;
}

Producción: 

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. También se puede usar para encontrar la unión de dos conjuntos. 
El programa se da arriba.
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