std::ranges::set_difference,std::ranges::set_difference_result(3)

NAME
std::ranges::set_difference,std::ranges::set_difference_result - std::ranges::set_difference,std::ranges::set_difference_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity > (since
   requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2> (1) C++20)
   constexpr set_difference_result<I1, O>
       set_difference( I1 first1, S1 last1, I2 first2, S2 last2,
                       O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {} );
   template< ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity >
   requires std::mergeable<ranges::iterator_t<R1>, (since
   ranges::iterator_t<R2>, (2) C++20)
                        O, Comp, Proj1, Proj2>
   constexpr set_difference_result<ranges::borrowed_iterator_t<R1>, O>
       set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},
                       Proj1 proj1 = {}, Proj2 proj2 = {} );

Helper types
   template< class I, class O > (3) (since
   using set_difference_result = ranges::in_out_result<I, O>; C++20)
   Copies the elements from the sorted input range [first1, last1) which are not found
   in the sorted input range [first2, last2) to the output range beginning at result.
   The behavior is undefined if
     * the input ranges are not sorted with respect to comp and proj1 or proj2,
       respectively, or
     * the resulting range overlaps with either of the input ranges.
   1) Elements are compared using the given binary comparison function comp.
   2) Same as (1), but uses r1 as the first range and r2 as the second range, as if
   using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as
   first2, and ranges::end(r2) as last2.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first1, last1 - iterator-sentinel pair denoting the first sorted input range
   first2, last2 - iterator-sentinel pair denoting the second sorted input range
   r1 - the first sorted input range
   r2 - the second sorted input range
   result - the beginning of the output range
   comp - comparator to apply to the projected elements
   proj1 - projection to apply to the elements in the first range
   proj2 - projection to apply to the elements in the second range

Return value
   {last1, result_last}, where result_last is the end of the constructed range.

Complexity
   At most \(\scriptsize 2\cdot(N_1+N_2)-1\)2·(N
   1+N
   2)-1 comparisons and applications of each projection, where \(\scriptsize N_1\)N
   1 and \(\scriptsize N_2\)N
   2 are ranges::distance(first1, last1) and ranges::distance(first2, last2),
   respectively.

Possible implementation
struct set_difference_fn {
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_difference_result<I1, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        while (!(first1 == last1 or first2 == last2))
        {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
            {
                *result = *first1;
                ++first1;
                ++result;
            }
            else if (std::invoke(comp, std::invoke(proj2, *first2),
                        std::invoke(proj1, *first1)))
                ++first2;
            else
            {
                ++first1;
                ++first2;
            }
        }
        return ranges::copy(std::move(first1), std::move(last1), std::move(result));
    }
    template<ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                        O, Comp, Proj1, Proj2>
    constexpr ranges::set_difference_result<ranges::borrowed_iterator_t<R1>, O>
        operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    } }; inline constexpr set_difference_fn set_difference {};

Example
// Run this code
 #include <algorithm>
 #include <cassert>
 #include <iostream>
 #include <iterator>
 #include <string_view>
 #include <vector>
 auto print = [](const auto& v, std::string_view end = "")
 {
     std::cout << "{ ";
     for (auto n{v.size()}; auto i : v)
         std::cout << i << (--n ? ", " : " ");
     std::cout << "} " << end;
 };
 struct Order // a struct with some very interesting data
 {
     int order_id{};
     friend std::ostream& operator<<(std::ostream& os, const Order& ord)
     {
         return os << '{' << ord.order_id << '}';
     }
 };
 int main()
 {
     const auto v1 = {1, 2, 5, 5, 5, 9};
     const auto v2 = {2, 5, 7};
     std::vector<int> diff{};
     std::ranges::set_difference(v1, v2, std::back_inserter(diff));
     print(v1, "∖ ");
     print(v2, "= ");
     print(diff, "\n\n");
     // We want to know which orders "cut" between old and new states:
     const std::vector<Order> old_orders{{1}, {2}, {5}, {9}};
     const std::vector<Order> new_orders{{2}, {5}, {7}};
     std::vector<Order> cut_orders(old_orders.size() + new_orders.size());
     auto [old_orders_end, cut_orders_last] =
         std::ranges::set_difference(old_orders, new_orders,
                        cut_orders.begin(), {},
                        &Order::order_id, &Order::order_id);
     assert(old_orders_end == old_orders.end());
     std::cout << "old orders = ";
     print(old_orders, "\n");
     std::cout << "new orders = ";
     print(new_orders, "\n");
     std::cout << "cut orders = ";
     print(cut_orders, "\n");
     cut_orders.erase(cut_orders_last, end(cut_orders));
     std::cout << "cut orders = ";
     print(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}, {0}, {0}, {0}, {0}, {0} }
 cut orders = { {1}, {9} }

See also
   ranges::set_union computes the union of two sets
   (C++20) (niebloid)
   ranges::set_intersection computes the intersection of two sets
   (C++20) (niebloid)
   ranges::set_symmetric_difference computes the symmetric difference between two sets
   (C++20) (niebloid)
   ranges::includes returns true if one sequence is a subsequence of
   (C++20) another
                        (niebloid)
   set_difference computes the difference between two sets
                        (function template)