std::set_union() algorithm
- od C++20
- od C++17
- do C++17
// (1)
template< class InputIt1, class InputIt2, class OutputIt >
constexpr OutputIt set_union( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2, OutputIt d_first );
// (2)
template< class InputIt1, class InputIt2, class OutputIt, class Compare >
constexpr OutputIt set_union( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2,
OutputIt d_first, Compare comp );
// (3)
template< class ExecutionPolicy, class ForwardIt1,
class ForwardIt2, class ForwardIt3 >
ForwardIt3 set_union( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,
ForwardIt3 d_first );
// (4)
template< class ExecutionPolicy, class ForwardIt1,
class ForwardIt2, class ForwardIt3, class Compare >
ForwardIt3 set_union( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,
ForwardIt3 d_first, Compare comp );
// (1)
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2, OutputIt d_first );
// (2)
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 );
// (3)
template< class ExecutionPolicy, class ForwardIt1,
class ForwardIt2, class ForwardIt3 >
ForwardIt3 set_union( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,
ForwardIt3 d_first );
// (4)
template< class ExecutionPolicy, class ForwardIt1,
class ForwardIt2, class ForwardIt3, class Compare >
ForwardIt3 set_union( ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1,
ForwardIt2 first2, ForwardIt2 last2,
ForwardIt3 d_first, Compare comp );
// (1)
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
InputIt2 first2, InputIt2 last2, OutputIt d_first );
// (2)
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 );
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 [first1
; last1
) contains m
elements that are equivalent to each other and [first2
; last2
) contains n
elements
that are equivalent to them, then all m
elements will be copied from [first1
; last1
) to the output range, preserving order,
and then the final std::max(n - m, 0)
elements will be copied from [first2
; last2
) to the output range, also preserving order.
-
(1) Both ranges must be sorted with respect to
operator<
. -
(2) Both ranges must be sorted with respect to
comp
. -
(3 - 4) Same as (1) and (2), but executed according to policy.
Overload ResolutionThese overloads participate in overload resolution only if
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
istrue
. (do C++20)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
istrue
. (od C++20)
If either of the input ranges is not sorted (using operator<
or comp
, respectively) or overlaps with the output range,
the behavior is undefined
Parameters
first1 last2 | The first sorted range of elements to examine. |
first2 last3 | The second sorted range of elements to examine. |
d_first | The beginning of the destination range. |
policy | The execution policy to use. See execution policy for details. |
comp | Comparison function object (i.e. an object that satisfies the requirements of Compare). The signature of the comparison function should be equivalent to the following:
|
Type requirements
InputIt1 InputIt2 | LegacyInputIterator |
ForwardIt1 ForwardIt2 ForwardIt3 | LegacyForwardIterator |
OutputIt1 | LegacyOutputIterator |
Return value
Iterator past the end of the constructed range.
Complexity
Given M
std::distance(first1, last1) and N
as std::distance(first2, last2)
:
- (1, 3) at most 2 * (M + N) - 1 comparisons using
operator<
. - (2, 4) at most 2 * (M + N) - 1 comparison with
comp
.
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 otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Possible implementation
set_union(1)
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);
}
set_union(2)
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);
}
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.
Examples
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
void println(std::vector<int> const& v)
{
for (int i : v)
std::cout << i << ' ';
std::cout << '\n';
}
int main()
{
std::vector<int> v1, v2, dest;
v1 = {1, 2, 3, 4, 5};
v2 = { 3, 4, 5, 6, 7};
std::set_union(v1.cbegin(), v1.cend(),
v2.cbegin(), v2.cend(),
std::back_inserter(dest));
println(dest);
dest.clear();
v1 = {1, 2, 3, 4, 5, 5, 5};
v2 = { 3, 4, 5, 6, 7};
std::set_union(v1.cbegin(), v1.cend(),
v2.cbegin(), v2.cend(),
std::back_inserter(dest));
println(dest);
}
1 2 3 4 5 6 7
1 2 3 4 5 5 5 6 7
Hover to see the original license.