std::set_difference() algorithm

since C++20

// (1)
template< class InputIt1, class InputIt2, class OutputIt >
constexpr OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                                   InputIt2 first2, InputIt2 last2, OutputIt d_first );

// (2)
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 );

// (3)
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 );

// (4)
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 );

since C++17

// (1)
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2, OutputIt d_first );

// (2)
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 );

// (3)
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 );

// (4)
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 );

until C++17

// (1)
template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_difference( InputIt1 first1, InputIt1 last1,
                         InputIt2 first2, InputIt2 last2, OutputIt d_first );

// (2)
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 );

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) Both ranges must be sorted with respect to operator<.

• (2) Both ranges must be sorted with respect to comp.

• (3 - 4) Same as (1) and (2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if 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)

Undefined Behaviour

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 last2	The first sorted range of elements to examine.
first2 last3	The second sorted range of elements to examine.
d_first	The beginning of the destination 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). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 an object of type RandomIt can be implicitly converted to both of them.

Type requirements

InputIt1 InputIt2	LegacyInputIterator
ForwardIt1 ForwardIt2 ForwardIt3	LegacyForwardIterator
OutputIt1	LegacyOutputIterator

Return value

Iterator past the end of the constructed range.

Complexity

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

• (1, 3) at most 2 * (M + N) - 1 comparisons using operator<.
• (2, 4) at most 2 * (M + N) - 1 comparison with comp.

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(2)

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;
}

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

template<typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> const& v)
{
    for (os << "{ "; auto const& e : v)
        os << e << ' ';
    return os << '}';
}

struct Order // a struct with some 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';

    // 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';
}

Possible 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 }