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std::set_difference

Defined in header <algorithm>
(1) 
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2, OutputIt d_first );
 (until C++20) 
template< class InputIt1, class InputIt2, class OutputIt >
constexpr OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                                   InputIt2 first2, InputIt2 last2,
                                   OutputIt d_first );
 (since C++20) 
template< class ExecutionPolicy, class ForwardIt1,
          class ForwardIt2, class ForwardIt3 >
ForwardIt3 set_difference( ExecutionPolicy&& policy,
                           ForwardIt1 first1, ForwardIt1 last1,
                           ForwardIt2 first2, ForwardIt2 last2,
                           ForwardIt3 d_first );
 (2) (since C++17)
(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 );
 (until C++20) 
template< class InputIt1, class InputIt2, class OutputIt, class Compare >
constexpr OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                                   InputIt2 first2, InputIt2 last2,
                                   OutputIt d_first, Compare comp );
 (since C++20) 
template< class ExecutionPolicy, class ForwardIt1,
          class ForwardIt2, class ForwardIt3, class Compare >
ForwardIt3 set_difference( ExecutionPolicy&& policy,
                           ForwardIt1 first1, ForwardIt1 last1,
                           ForwardIt2 first2, ForwardIt2 last2,
                           ForwardIt3 d_first, Compare comp );
 (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.

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>> is true.

 (until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

 (since 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, last1 - the range of elements to examine
first2, last2 - the range of elements to search for
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) 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.
-OutputIt must meet the requirements of LegacyOutputIterator.
-ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.

Return value

Iterator past the end of the constructed range.

Complexity

Given M and N as std::distance(first1, last1) and std::distance(first2, last2) respectively:

1,2) at most 2·(M + N) - 1 comparisons using operator<.
3,4) at most 2·(M + N) - 1 applications of the predicate p.

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.

Possible implementation

set_difference (1) 
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;
}

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
template<typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> const& v)
{
    os << '{';
    for (auto n{v.size()}; auto const& 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}

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

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
(niebloid)

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