std::merge

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

template< class InputIt1, class InputIt2,
          class OutputIt, class Compare >
OutputIt merge( InputIt1 first1, InputIt1 last1,
                InputIt2 first2, InputIt2 last2,
                OutputIt d_first, Compare comp );

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class ForwardIt3 >
ForwardIt3 merge( ExecutionPolicy&& policy,
                  ForwardIt1 first1, ForwardIt1 last1,
                  ForwardIt2 first2, ForwardIt2 last2,
                  ForwardIt3 d_first );

template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2,
          class ForwardIt3, class Compare >
ForwardIt3 merge( ExecutionPolicy&& policy,
                  ForwardIt1 first1, ForwardIt1 last1,
                  ForwardIt2 first2, ForwardIt2 last2,
                  ForwardIt3 d_first, Compare comp );

Constructs a hypothetical sorted sequence containing all elements from two sorted source ranges [first1, last1) and [first2, last2). Copies all elements of the sequence to the destination range beginning at d_first.

For each group of equivalent elements in the sequence, any pair of elements from the group has the following order:

• If the two elements are from the same source range, the order between them in the sequence follows the original order between them in the source range.
• Otherwise, the element from the first source range is always placed before the element from the second source range.

If the destination range overlaps with any of the two source ranges, the behavior is undefined.

Parameters

Return value

The past-the-end iterator of the destination range.

Complexity

Given

• \(\scriptsize N_1\)N1 as std::distance(first1, last1),
• \(\scriptsize N_2\)N2 as std::distance(first2, last2):

Exceptions

Notes

This algorithm performs a similar task as std::set_union does. Both consume two sorted source ranges and produce a sorted output with elements from both inputs. The difference between these two algorithms is with handling values from both source ranges which compare equivalent (see notes on LessThanComparable).

If any equivalent values appeared n1 times in the first range and n2 times in the second, std::merge would output all n1 + n2 occurrences whereas std::set_union would output std::max(n1, n2) ones only. So std::merge outputs exactly std::distance(first1, last1) + std::distance(first2, last2) values and std::set_union may produce fewer.

Possible implementation

See also the implementations in libstdc++ and libc++.

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);
}

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);
}

Example

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

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)));
    
    std::print("Originally:\nv1: {}\nv2: {}\n", v1, v2);
    
    std::sort(v1.begin(), v1.end());
    std::sort(v2.begin(), v2.end());
    
    std::print("After sorting:\nv1: {}\nv2: {}\n", v1, v2);
    
    // merge
    std::vector<int> dst;
    std::merge(v1.begin(), v1.end(), v2.begin(), v2.end(), std::back_inserter(dst));
    
    std::print("After merging:\ndst: {}", dst);
}

Originally:
v1: [3, 4, 9, 5, 9, 9, 9, 7, 3, 2]
v2: [6, 2, 9, 0, 9, 4, 0, 1, 9, 1]
After sorting:
v1: [2, 3, 3, 4, 5, 7, 9, 9, 9, 9]
v2: [0, 0, 1, 1, 2, 4, 6, 9, 9, 9]
After merging:
dst: [0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 6, 7, 9, 9, 9, 9, 9, 9, 9]

Defect reports

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

See also