# Man Page set_union.3

```
Standard C++ Library
Copyright 1998, Rogue Wave Software, Inc.

```

## NAME

```     set_union

- A basic set operation for constructing a sorted union.

```

## SYNOPSIS

```     #include <algorithm>
template <class InputIterator1, class InputIterator2,
class OutputIterator>
OutputIterator
set_union (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
template <class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator
set_union (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);

```

## DESCRIPTION

```     The_set_union algorithm constructs a  sorted  union  of  the
elements from the two ranges. It returns the end of the con-
structed range. set_union is stable, which means that if  an
element  is  present  in both ranges, the one from the first
range is copied. The result of set_union is undefined if the
result  range  overlaps  with either of the original ranges.
Note that set_union does not merge the two sorted sequences.
If an element is present in both sequences, only the element
from the first sequence is copied to result. (Use the  merge
algorithm to create an ordered merge of two sorted sequences
that contains all the elements from both sequences.)

set_union assumes that the sequences are  sorted  using  the
default  comparison operator less than (<), unless an alter-
native comparison operator (comp) is provided.

```

## COMPLEXITY

```     At most ((last1 -  first1)  +  (last2  -  first2))  *  2  -1
comparisons are performed.

```

## EXAMPLE

```     //
// set_unin.cpp
//
#include <algorithm>
#include <set>
#include <iostream>
using namespace std;

int main()
{

//Initialize some sets
int a2[6]  = {2,4,6,8,10,12};
int a3[4]  = {3,5,7,8};
set<int, less<int> >  even(a2+0, a2+6),
result, small(a3+0,a3+4);
//Create an insert_iterator for result
insert_iterator<set<int, less<int> > >
res_ins(result, result.begin());
//Demonstrate set_union
cout << "The result of:" << endl << "{";
copy(small.begin(),small.end(),
ostream_iterator<int,char>(cout," "));
cout << "} union {";
copy(even.begin(),even.end(),
ostream_iterator<int,char>(cout," "));
cout << "} =" << endl << "{";
set_union(small.begin(), small.end(),
even.begin(), even.end(), res_ins);
copy(result.begin(),result.end(),
ostream_iterator<int,char>(cout," "));
cout << "}" << endl << endl;
return 0;
}

Program Output

The result of:
{3 5 7 8 } union {2 4 6 8 10 12 } =
{2 3 4 5 6 7 8 10 12 }

```

## WARNINGS

```     If your compiler does not support default  template  parame-
ters,  then  you  always need to supply the Compare template
argument and the Allocator template argument. For  instance,
you need to write:

set<int, less<int>, allocator<int> >

set<int>

If your compiler does not support namespaces,  then  you  do
not need the using declaration for std.

```

```     includes,     set,     set_intersection,     set_difference,
set_symmetric_difference

```