std::set_difference

 

C++

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

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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 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)

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

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>
 
auto print = [](const auto& v, std::string_view end = "")
{
    std::cout << "{ ";
    for (auto i : v) std::cout << i << ' ';
    std::cout << "} " << end;
};
 
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()));
    print(v1, "∖ ");
    print(v2, "= ");
    print(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 = "; print(old_orders, "\n");
    std::cout << "new orders = "; print(new_orders, "\n");
    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, }

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

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