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

NAME
     std::merge - std::merge

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

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

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

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

			  ForwardIt3 d_first, Compare comp );

	Merges	two  sorted  ranges [first1, last1) and [first2, last2) into one
     sorted range
	beginning at d_first.

	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)

	This  merge function is stable, which means that for equivalent elements
     in the
	original two ranges, the elements from the first range (preserving their
     original
	order) precede the elements from  the  second  range  (preserving  their
     original order).

	If  the  output  range overlaps with [first1, last1) or [first2, last2),
     the behavior
	is undefined.

Parameters
	first1, last1 - the first range of elements to merge
	first2, last2 - the second range of elements to merge
	d_first       - the beginning of the destination 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.
	-
	ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
	LegacyForwardIterator.
	-
	OutputIt must meet the requirements of LegacyOutputIterator.
	-
	Compare must meet the requirements of Compare.

Return value
	An output iterator to element past the last element copied.

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

	1) At most \(\scriptsize N_1+N_2-1\)N
	1+N
	2-1 comparisons using
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	2) \(\scriptsize O(N_1+N_2)\)O(N
	1+N
	2) comparisons using
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	3) At most \(\scriptsize N_1+N_2-1\)N
	1+N
	2-1 applications of the comparison function comp.
	4) \(\scriptsize O(N_1+N_2)\)O(N
	1+N
	2) 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
	See also the implementations in libstdc++ and libc++.

			       merge (1)
	template<class InputIt1, class InputIt2, class OutputIt>
	OutputIt merge(InputIt1 first1, InputIt1 last1,
		       InputIt2 first2, InputIt2 last2,
		       OutputIt d_first)
	{
	    for (; first1 != last1; ++d_first)
	    {
		if (first2 == last2)
		    return std::copy(first1, last1, d_first);

		if (*first2 < *first1)
		{
		    *d_first = *first2;
		    ++first2;
		}
		else
		{
		    *d_first = *first1;
		    ++first1;
		}
	    }
	    return std::copy(first2, last2, d_first);
	}
			       merge (3)
	template<class InputIt1, class InputIt2,
		 class OutputIt, class Compare>
	OutputIt merge(InputIt1 first1, InputIt1 last1,
		       InputIt2 first2, InputIt2 last2,
		       OutputIt d_first, Compare comp)
	{
	    for (; first1 != last1; ++d_first)
	    {
		if (first2 == last2)
		    return std::copy(first1, last1, d_first);

		if (comp(*first2, *first1))
		{
		    *d_first = *first2;
		    ++first2;
		}
		else
		{
		    *d_first = *first1;
		    ++first1;
		}
	    }
	    return std::copy(first2, last2, d_first);
	}

Notes
	This algorithm performs a similar task as std::set_union does. Both con-
     sume two
	sorted input ranges and produce a sorted output with elements from  both
     inputs. The
	difference  between  these  two  algorithms is with handling values from
     both input
	ranges which compare equivalent (see notes  on	LessThanComparable).  If
     any equivalent
	values	appeared  n  times in the first range and m times in the second,
     std::merge
	would output all n + m occurrences whereas std::set_union  would  output
     std::max(n,
	m) ones only. So std::merge outputs exactly std::distance(first1, last1)
     +
	std::distance(first2,  last2)  values  and  std::set_union  may  produce
     fewer.

Example
     // Run this code

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

      auto print = [](const auto rem, const auto& v)
      {
	  std::cout << rem;
	  std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout,  "
     "));
	  std::cout << '\n';
      };

      int main()
      {
	  // fill the vectors with random numbers
	  std::random_device rd;
	  std::mt19937 mt(rd());
	  std::uniform_int_distribution<> dis(0, 9);

	  std::vector<int> v1(10), v2(10);
	  std::generate(v1.begin(), v1.end(), std::bind(dis, std::ref(mt)));
	  std::generate(v2.begin(), v2.end(), std::bind(dis, std::ref(mt)));

	  print("Originally:\nv1: ", v1);
	  print("v2: ", v2);

	  std::sort(v1.begin(), v1.end());
	  std::sort(v2.begin(), v2.end());

	  print("After sorting:\nv1: ", v1);
	  print("v2: ", v2);

	  // merge
	  std::vector<int> dst;
	  std::merge(v1.begin(),  v1.end(),  v2.begin(), v2.end(), std::back_in-
     serter(dst));

	  print("After merging:\ndst: ", dst);
      }

Possible output:
      Originally:
      v1: 2 6 5 7 4 2 2 6 7 0
      v2: 8 3 2 5 0 1 9 6 5 0
      After sorting:
      v1: 0 2 2 2 4 5 6 6 7 7
      v2: 0 0 1 2 3 5 5 6 8 9
      After merging:
      dst: 0 0 0 1 2 2 2 2 3 4 5 5 5 6 6 6 7 7 8 9

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	  DR	Applied to	  Behavior as published        Correct behavior
	LWG 780 C++98	   the merge operation was not defined defined

See also
	inplace_merge merges two ordered ranges in-place
		      (function template)
	is_sorted     checks whether a range is sorted into ascending order
	(C++11)       (function template)
	set_union     computes the union of two sets
		      (function template)
	sort	      sorts a range into ascending order
		      (function template)
		      sorts  a	range of elements while preserving order between
     equal
	stable_sort   elements
		      (function template)
	ranges::merge merges two sorted ranges
	(C++20)       (niebloid)

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