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

NAME
     std::set_intersection - std::set_intersection

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

	OutputIt set_intersection( 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_intersection( ExecutionPolicy&& policy,
				     ForwardIt1  first1,	    (2)   (since
     C++17)
	ForwardIt1 last1,
				     ForwardIt2 first2,
	ForwardIt2 last2,

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

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

				     ForwardIt3 d_first, Compare
	comp );

	Constructs  a  sorted  range beginning at d_first consisting of elements
     that are found
	in both sorted ranges [first1, last1) and [first2, last2).

	If [first1, last1) contains m elements that are equivalent to each other
     and
	[first2, last2) contains n elements that are  equivalent  to  them,  the
     first
	std::min(m,  n) elements will be copied from [first1, last1) to the out-
     put 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 first range of elements to examine
	first2, last2 - the second range of elements to examine
	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_intersection (1) template<class  In-
     putIt1,  class InputIt2, class OutputIt> OutputIt set_intersection(InputIt1
     first1, InputIt1 last1,
			       InputIt2   first2,   InputIt2   last2,	OutputIt
     d_first) {
	 while (first1 != last1 && first2 != last2)
	 {
	     if (*first1 < *first2)
		 ++first1;
	     else
	     {
		 if (!(*first2 < *first1))
		     *d_first++  = *first1++; // *first1 and *first2 are equiva-
     lent.
		 ++first2;
	     }
	 }
	 return d_first; }
					set_intersection (3) template<class  In-
     putIt1,  class InputIt2, class OutputIt, class Compare> OutputIt set_inter-
     section(InputIt1 first1, InputIt1 last1,
			       InputIt2   first2,   InputIt2   last2,	OutputIt
     d_first, Compare comp) {
	 while (first1 != last1 && first2 != last2)
	 {
	     if (comp(*first1, *first2))
		 ++first1;
	     else
	     {
		 if (!comp(*first2, *first1))
		     *d_first++  = *first1++; // *first1 and *first2 are equiva-
     lent.
		 ++first2;
	     }
	 }
	 return d_first; }

Example
     // Run this code

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

      int main()
      {
	  std::vector<int> v1{7, 2, 3, 4, 5, 6, 7, 8};
	  std::vector<int> v2{5, 7, 9, 7};
	  std::sort(v1.begin(), v1.end());
	  std::sort(v2.begin(), v2.end());

	  std::vector<int> v_intersection;
	  std::set_intersection(v1.begin(), v1.end(), v2.begin(), v2.end(),
				std::back_inserter(v_intersection));

	  for (int n : v_intersection)
	      std::cout << n << ' ';
	  std::cout << '\n';
      }

Output:
      5 7 7

	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
	set_union		 computes the union of two sets
				 (function template)
	ranges::set_intersection computes the intersection of two sets
	(C++20) 		 (niebloid)

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