std::set_difference
| Defined in header <algorithm>
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template< class InputIt1, class InputIt2, class OutputIt > OutputIt set_difference( 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) |
Copies the elements from the sorted range [first1, last1) which are not found in the sorted range [first2, last2) to the range beginning at d_first. The output range is also sorted.
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the final std::max(m - n, 0) elements will be copied from [first1, last1) to the output range, 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) |
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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 pair of iterators defining the first input sorted range of elements to examine |
| first2, last2 | - | the pair of iterators defining the second input sorted range of elements to search for |
| d_first | - | the beginning of the output range |
| policy | - | the execution policy to use |
| 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.
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-OutputIt must meet the requirements of LegacyOutputIterator.
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-ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.
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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 N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):
[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
ExecutionPolicyis 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_difference (1) |
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template<class InputIt1, class InputIt2, class OutputIt> OutputIt set_difference(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first) { while (first1 != last1) { if (first2 == last2) return std::copy(first1, last1, d_first); if (*first1 < *first2) *d_first++ = *first1++; else { if (! (*first2 < *first1)) ++first1; ++first2; } } return d_first; } |
| set_difference (3) |
template<class InputIt1, class InputIt2, class OutputIt, class Compare> OutputIt set_difference(InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first, Compare comp) { while (first1 != last1) { if (first2 == last2) return std::copy(first1, last1, d_first); if (comp(*first1, *first2)) *d_first++ = *first1++; else { if (!comp(*first2, *first1)) ++first1; ++first2; } } return d_first; } |
[edit] Example
#include <algorithm> #include <iostream> #include <iterator> #include <vector> template<typename T> std::ostream& operator<<(std::ostream& os, const std::vector<T>& v) { os << '{'; for (auto n{v.size()}; const auto& e : v) os << e << (--n ? ", " : ""); return os << '}'; } struct Order // a struct with very interesting data { int order_id{}; friend std::ostream& operator<<(std::ostream& os, const Order& ord) { return os << ord.order_id; } }; int main() { const std::vector<int> v1{1, 2, 5, 5, 5, 9}; const std::vector<int> v2{2, 5, 7}; std::vector<int> diff; std::set_difference(v1.begin(), v1.end(), v2.begin(), v2.end(), std::inserter(diff, diff.begin())); std::cout << v1 << " ∖ " << v2 << " == " << diff << "\n\n"; // we want to know which orders "cut" between old and new states: std::vector<Order> old_orders{{1}, {2}, {5}, {9}}; std::vector<Order> new_orders{{2}, {5}, {7}}; std::vector<Order> cut_orders; std::set_difference(old_orders.begin(), old_orders.end(), new_orders.begin(), new_orders.end(), std::back_inserter(cut_orders), [](auto& a, auto& b) { return a.order_id < b.order_id; }); std::cout << "old orders: " << old_orders << '\n' << "new orders: " << new_orders << '\n' << "cut orders: " << cut_orders << '\n'; }
Output:
{1, 2, 5, 5, 5, 9} ∖ {2, 5, 7} == {1, 5, 5, 9}
old orders: {1, 2, 5, 9}
new orders: {2, 5, 7}
cut orders: {1, 9}[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 |
|---|---|---|---|
| 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) | |
| computes the symmetric difference between two sets (function template) | |
| (C++20) |
computes the difference between two sets (algorithm function object) |
