std::ranges::set_difference, std::ranges::set_difference_result

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 set_difference_result<I1, O>
set_difference( I1 first1, S1 last1, I2 first2, S2 last2,
O result, Comp comp = {},

Proj1 proj1 = {}, Proj2 proj2 = {} );

(1) (since C++20)

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 set_difference_result<ranges::borrowed_iterator_t <R1>, O>
set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},

Proj1 proj1 = {}, Proj2 proj2 = {} );

(2) (since C++20)

Helper types

template< class I, class O >
using set_difference_result = ranges::in_out_result <I, O>;

(3) (since 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 algorithm function objects (informally known as 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.

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 {};

[edit] 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} }