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std::set_difference(3)	       C++ Standard Libary	  std::set_difference(3)

NAME
     std::set_difference - std::set_difference

Synopsis
	Defined in header <algorithm>
	template< class InputIt1, class InputIt2, class OutputIt
	>

	OutputIt set_difference( InputIt1 first1, InputIt1
	last1,							  (1) (constexpr
     since C++20)
				 InputIt2 first2, InputIt2
	last2,

				 OutputIt d_first );
	template< class ExecutionPolicy,

		  class ForwardIt1, class ForwardIt2, class
	ForwardIt3 >
	ForwardIt3 set_difference( ExecutionPolicy&& policy,
				   ForwardIt1  first1,	ForwardIt1  (2)   (since
     C++17)
	last1,
				   ForwardIt2 first2, ForwardIt2
	last2,

				   ForwardIt3 d_first );
	template< class InputIt1, class InputIt2,

		  class OutputIt, class Compare >
	OutputIt set_difference( InputIt1 first1, InputIt1
	last1,							  (3) (constexpr
     since C++20)
				 InputIt2 first2, InputIt2
	last2,

				 OutputIt d_first, Compare comp
	);
	template< class ExecutionPolicy,

		  class ForwardIt1, class ForwardIt2,
		  class ForwardIt3, class Compare >
	ForwardIt3 set_difference( ExecutionPolicy&& policy,
				   ForwardIt1  first1,	ForwardIt1  (4)   (since
     C++17)
	last1,
				   ForwardIt2 first2, ForwardIt2
	last2,

				   ForwardIt3 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 out-
     put 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 fi-
     nal
	std::max(m - n, 0) elements will be copied from [first1, last1)  to  the
     output range,
	preserving order.

	1) If [first1, last1) or [first2, last2) is not sorted with respect to
	operator<
	(until C++20)
	std::less{}
	(since C++20), the behavior is undefined.
	3)  If	[first1, last1) or [first2, last2) is not sorted with respect to
     comp, the
	behavior is undefined.
	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 the output range overlaps with [first1, last1)  or  [first2,  last2),
     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.
			comparison function object (i.e. an object  that  satis-
     fies 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);
	comp	      -
			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  regard-
     less 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 im-
     plicitly
			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.
	-
	Compare must meet the requirements of Compare.

Return value
	Iterator past the end of the constructed range.

Complexity
	Given \(\scriptsize N_1\)N
	1 as std::distance(first1, last1) and \(\scriptsize N_2\)N
	2 as std::distance(first2, last2):

	1,2) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2a(N
	1+N
	2)-1 comparisons using
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	3,4) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2a(N
	1+N
	2)-1 applications of the comparison function comp.

Exceptions
	The overloads with a template parameter named ExecutionPolicy report er-
     rors 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-de-
     fined.
	  * 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 <vector>

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

      struct Order // a struct with very 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 << " a " << v2 << " == " << diff << "\n\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.or-
     der_id; });

	  std::cout << "old orders: " << old_orders << '\n'
		    << "new orders: " << new_orders << '\n'
		    << "cut orders: " << cut_orders << '\n';
      }

Output:
      {1, 2, 5, 5, 5, 9} a {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	retroac-
     tively to
	previously published C++ standards.

	  DR	Applied to		Behavior as published		    Cor-
     rect behavior
	LWG   291  C++98       it  was	unspecified  how  to  handle  equivalent
     specified
			   elements in the input ranges

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	 computes the difference between two sets
	(C++20) 		 (niebloid)

http://cppreference.com 	   2024.06.10		  std::set_difference(3)
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