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) | |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3 > ForwardIt3 set_union( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, |
(2) | (since C++17) |
template< class InputIt1, class InputIt2, class OutputIt, class Compare > |
(3) | |
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class Compare > |
(4) | (since C++17) |
Constructs a sorted range beginning at d_first
consisting of all 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.
operator<
and the ranges must be sorted with respect to the same.comp
and the ranges must be sorted with respect to the same.policy
. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is trueParameters
first1, last1 | - | the first input sorted range |
first2, last2 | - | the second input sorted 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); The signature does not need to have const &, but the function object must not modify the objects passed to it. |
Type requirements | ||
-InputIt1, InputIt2 must meet the requirements of InputIterator .
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-ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of ForwardIterator .
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-OutputIt must meet the requirements of OutputIterator .
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Return value
Iterator past the end of the constructed range.
Complexity
At most 2·(N1+N2-1) comparisons, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).
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 three 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.
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 bewteen 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 less.
Possible implementation
First version |
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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); } |
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) return std::copy(first1, last1, d_first); if (comp(*first2, *first1)) { *d_first = *first2++; } else { *d_first = *first1; if (!comp(*first1, *first2)) ++first2; ++first1; } } return std::copy(first2, last2, d_first); } |
Example
Example with vectors :
#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'; }
Output:
1 2 3 4 5 6 7
See also
returns true if one set is a subset 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) |