NAME¶

std::set_union - std::set_union

Synopsis¶

Defined in header <algorithm>
template< class InputIt1, class InputIt2, class
OutputIt >

OutputIt set_union( InputIt1 first1, InputIt1 (until C++20)
last1,
InputIt2 first2, InputIt2 last2,

OutputIt d_first );
template< class InputIt1, class InputIt2, class
OutputIt >

constexpr OutputIt set_union( InputIt1 first1, (since C++20)
InputIt1 last1,
InputIt2 first2, InputIt2 last2,

OutputIt d_first );
template< class ExecutionPolicy, class
ForwardIt1, class ForwardIt2, class ForwardIt3 >

ForwardIt3 set_union( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,

ForwardIt3 d_first );
template< class InputIt1, class InputIt2, (1)

class OutputIt, class Compare >
OutputIt set_union( InputIt1 first1, InputIt1 (until C++20)
last1,
InputIt2 first2, InputIt2 last2,

OutputIt d_first, Compare comp );
template< class InputIt1, class InputIt2,

class OutputIt, class Compare >
constexpr OutputIt set_union( InputIt1 first1, (since C++20)
InputIt1 last1, (3)
InputIt2 first2, InputIt2 last2,

OutputIt d_first, Compare comp );
template< class ExecutionPolicy,

class ForwardIt1, class ForwardIt2,
class ForwardIt3, class Compare >
ForwardIt3 set_union( ExecutionPolicy&& policy, (4) (since C++17)
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,

ForwardIt3 d_first, Compare comp );

Constructs a sorted union beginning at d_first consisting of the set of elements
present in one or both sorted ranges [first1, last1) and [first2, last2).

If some element is found m times in [first1, last1) and n times in [first2, last2),
then all m elements will be copied from [first1, last1) to d_first, preserving
order, and then exactly std::max(n-m, 0) elements will be copied from [first2,
last2) to d_first, also preserving order.

The resulting range cannot overlap with either of the input ranges.

1) Elements are compared using operator< and the ranges must be sorted with respect
to the same.
3) Elements are compared using the given binary comparison function comp and the
ranges must be sorted with respect to the same.
2,4) Same as (1,3), but executed according to policy. These overloads do not
participate in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

Parameters¶

first1, last1 - the first input sorted range
first2, last2 - the second input sorted range
d_first - the beginning of the output range
policy - the execution policy to use. See execution policy for details.
comparison function object (i.e. an object that satisfies the
requirements of Compare) which returns true if the first argument
is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the
following:

bool cmp(const Type1 &a, const Type2 &b);
comp -
While the signature does not need to have const &, the function must
not modify the objects passed to it and must be able to accept all
values of type (possibly const) Type1 and Type2 regardless of value
category (thus, Type1 & is not allowed
, nor is Type1 unless for Type1 a move is equivalent to a copy
(since C++11)).
The types Type1 and Type2 must be such that objects of types
InputIt1 and InputIt2 can be dereferenced and then implicitly
converted to both Type1 and Type2.

Type requirements¶

-
InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
-
ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
LegacyForwardIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.

Return value¶

Iterator past the end of the constructed range.

Complexity¶

At most \(\scriptsize 2\cdot(N_1+N_2)-1\)2·(N
1+N
2)-1 comparisons, where \(\scriptsize N_1\)N
1 and \(\scriptsize N_2\)N
2 are std::distance(first1, last1) and std::distance(first2, last2), respectively.

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Notes¶

This algorithm performs a similar task as std::merge does. Both consume two sorted
input ranges and produce a sorted output with elements from both inputs. The
difference between these two algorithms is with handling values from both input
ranges which compare equivalent (see notes on LessThanComparable). If any equivalent
values appeared n times in the first range and m times in the second, std::merge
would output all n+m occurrences whereas std::set_union would output std::max(n, m)
ones only. So std::merge outputs exactly std::distance(first1, last1) +
std::distance(first2, last2) values and std::set_union may produce fewer.

Possible implementation¶

First version¶

template<class InputIt1, class InputIt2, class OutputIt> OutputIt set_union(InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2,
OutputIt d_first) {
for (; first1 != last1; ++d_first) {
if (first2 == last2)
return std::copy(first1, last1, d_first);
if (*first2 < *first1) {
*d_first = *first2++;
} else {
*d_first = *first1;
if (!(*first1 < *first2))
++first2;
++first1;
}
}
return std::copy(first2, last2, d_first); }

Second version¶

template<class InputIt1, class InputIt2,
class OutputIt, class Compare> OutputIt set_union(InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2,
OutputIt d_first, Compare comp) {
for (; first1 != last1; ++d_first) {
if (first2 == last2) {
// Finished range 2, include the rest of range 1:
return std::copy(first1, last1, d_first);
}
if (comp(*first2, *first1)) {
*d_first = *first2++;
} else {
*d_first = *first1;
if (!comp(*first1, *first2)) { // Equivalent => don't need to include *first2.
++first2;
}
++first1;
}
}
// Finished range 1, include the rest of range 2:
return std::copy(first2, last2, d_first); }

Example¶

Example with vectors :

// Run this code

#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>

int main()
{
{
std::vector<int> v1 = {1, 2, 3, 4, 5};
std::vector<int> v2 = { 3, 4, 5, 6, 7};
std::vector<int> dest1;

std::set_union(v1.begin(), v1.end(),
v2.begin(), v2.end(),
std::back_inserter(dest1));

for (const auto &i : dest1) {
std::cout << i << ' ';
}
std::cout << '\n';
}
{
std::vector<int> v1 = {1, 2, 3, 4, 5, 5, 5};
std::vector<int> v2 = { 3, 4, 5, 6, 7};
std::vector<int> dest1;