std::list::merge(3)

NAME
std::list::merge - std::list::merge

Synopsis
   void merge( list& other ); (1)
   void merge( list&& other ); (2) (since C++11)
   template< class Compare > (3)
   void merge( list& other, Compare comp );
   template< class Compare > (4) (since C++11)
   void merge( list&& other, Compare comp );
   The function does nothing if other refers to the same object as *this.
   Otherwise, merges other into *this. Both lists should be sorted. No elements are
   copied, and the container other becomes empty after the merge. This operation is
   stable: for equivalent elements in the two lists, the elements from *this always
   precede the elements from other, and the order of equivalent elements of *this and
   other does not change.
   No iterators or references become invalidated. The pointers and references to the
   elements moved from *this, as well as the iterators referring to these elements,
   will refer to the same elements of *this, instead of other.
   1,2) Elements are compared using operator<.
   3,4) Elements are compared using comp.
   If *this or other is not sorted with respected to the corresponding comparator, or
   get_allocator() != other.get_allocator(), the behavior is undefined.

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
           list<T,&nbsp;Allocator>::const_iterator can be dereferenced and then
           implicitly converted to both of them.

Type requirements
   -
   Compare must meet the requirements of Compare.

Return value
   (none)

Exceptions
   If an exception is thrown for any reason, these functions have no effect (strong
   exception safety guarantee). Except if the exception comes from a comparison.

Complexity
   If other refers to the same object as *this, no comparisons are performed.
   Otherwise, given \(\scriptsize N\)N as std::distance(begin(), end()) and
   \(\scriptsize R\)R as std::distance(other.begin(), other.end()):
   1,2) At most \(\scriptsize N+R-1\)N+R-1 comparisons using operator<.
   3,4) At most \(\scriptsize N+R-1\)N+R-1 applications of the comparison function
   comp.

Example
// Run this code
 #include <iostream>
 #include <list>
 std::ostream& operator<<(std::ostream& ostr, const std::list<int>& list)
 {
     for (const int i : list)
         ostr << ' ' << i;
     return ostr;
 }
 int main()
 {
     std::list<int> list1 = {5, 9, 1, 3, 3};
     std::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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 300 C++98 the effect when *this and other refer specified as no-op
                       to the same object was not specified
   LWG 1207 C++98 it was unclear whether iterators and/or keep valid
                       references will be invalidated
                       O(1) node moving could not be guaranteed if the behavior is
   LWG 1215 C++98 get_allocator() != other.get_allocator() undefined in this
                        case

See also
   splice moves elements from another list
                        (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)