std::forward_list::merge(3)

NAME
std::forward_list::merge - std::forward_list::merge

Synopsis
   void merge( forward_list& other ); (1) (since C++11)
   void merge( forward_list&& other ); (1) (since C++11)
   template <class Compare> (2) (since C++11)
   void merge( forward_list& other, Compare comp );
   template <class Compare> (2) (since C++11)
   void merge( forward_list&& other, Compare comp );
   Merges two sorted lists into one. The lists should be sorted into ascending order.
   No elements are copied. The container other becomes empty after the operation. The
   function does nothing if other refers to the same object as *this. If
   get_allocator() != other.get_allocator(), the behavior is undefined. No iterators or
   references become invalidated, except that the iterators of moved elements now refer
   into *this, not into other. The first version uses operator< to compare the
   elements, the second version uses the given comparison function comp.
   This operation is stable: for equivalent elements in the two lists, the elements
   from *this shall always precede the elements from other, and the order of equivalent
   elements of *this and other does not change.

Parameters
   other - another container to merge
           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);
   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 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 an object of type
           forward_list<T,Allocator>::const_iterator can be dereferenced and then
           implicitly converted to both of them.

Return value
   (none)

Exceptions
   If an exception is thrown, this function has no effect (strong exception guarantee),
   except if the exception comes from the comparison function.

Complexity
   At most std::distance(begin(), end()) + std::distance(other.begin(), other.end()) -
   1 comparisons.

Example
// Run this code
 #include <iostream>
 #include <forward_list>
 std::ostream& operator<<(std::ostream& ostr, const std::forward_list<int>& list)
 {
     for (const auto &i : list) {
         ostr << ' ' << i;
     }
     return ostr;
 }
 int main()
 {
     std::forward_list<int> list1 = { 5,9,1,3,3 };
     std::forward_list<int> list2 = { 8,7,2,3,4,4 };
     list1.sort();
     list2.sort();
     std::cout << "list1: " << list1 << '\n';
     std::cout << "list2: " << list2 << '\n';
     list1.merge(list2);
     std::cout << "merged: " << list1 << '\n';
 }

Output:
 list1: 1 3 3 5 9
 list2: 2 3 4 4 7 8
 merged: 1 2 3 3 3 4 4 5 7 8 9

See also
   splice_after moves elements from another forward_list
   (C++11) (public member function)
   merge merges two sorted ranges
                        (function template)
   inplace_merge merges two ordered ranges in-place
                        (function template)
   ranges::merge merges two sorted ranges
   (C++20) (niebloid)
   ranges::inplace_merge merges two ordered ranges in-place
   (C++20) (niebloid)