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

From cppreference.com

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 > 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.

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 $2\cdot(N 1 +N 2)-1$ 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} }

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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