std::set_union
Defined in header <algorithm>
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template< class InputIt1, class InputIt2, class OutputIt > OutputIt set_union( InputIt1 first1, InputIt1 last1, |
(1) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3 > |
(2) | (since C++17) |
template< class InputIt1, class InputIt2, class OutputIt, class Compare > |
(3) | (constexpr since C++20) |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, |
(4) | (since C++17) |
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.
[
first1,
last1)
or [
first2,
last2)
is not sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.[
first1,
last1)
or [
first2,
last2)
is not sorted with respect to comp, the behavior is undefined.
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. |
(until C++20) |
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. |
(since C++20) |
If the output range overlaps with [
first1,
last1)
or [
first2,
last2)
, the behavior is undefined.
Contents |
[edit] 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. |
comp | - | 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); 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) |
Type requirements | ||
-InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
| ||
-ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.
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-OutputIt must meet the requirements of LegacyOutputIterator.
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-Compare must meet the requirements of Compare.
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[edit] Return value
Iterator past the end of the constructed range.
[edit] Complexity
Given N
1 as std::distance(first1, last1) and N
2 as std::distance(first2, last2):
1+N
2)-1 applications of the comparison function comp.
[edit] 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.
[edit] Possible implementation
set_union (1) |
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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 (3) |
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); } |
[edit] 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.
[edit] Example
#include <algorithm> #include <iostream> #include <iterator> #include <vector> void println(const std::vector<int>& 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); }
Output:
1 2 3 4 5 6 7 1 2 3 4 5 5 5 6 7
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 291 | C++98 | it was unspecified how to handle equivalent elements in the input ranges | specified |
[edit] See also
returns true if one sequence is a subsequence of another (function template) | |
merges two sorted ranges (function template) | |
computes the difference between two sets (function template) | |
computes the intersection of two sets (function template) | |
computes the symmetric difference between two sets (function template) | |
(C++20) |
computes the union of two sets (niebloid) |