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std::forward_list<T,Allocator>::merge

void merge( forward_list& other );	(1)	(since C++11)
void merge( forward_list&& other );	(2)	(since C++11)
template < class Compare > void merge( forward_list& other, Compare comp );	(3)	(since C++11)
template < class Compare > void merge( forward_list&& other, Compare comp );	(4)	(since C++11)

The function does nothing if other refers to the same object as *this.

Otherwise, merges two sorted lists into one. The lists should be sorted into ascending order. No elements are copied, and the container other becomes empty after the merge. 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.

If get_allocator() != other.get_allocator(), the behavior is undefined.

Parameters

other - another container to merge

comp

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);

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 a comparison.

Complexity

If other refers to the same object as *this, no comparisons are performed.

Otherwise, given M and N as std::distance(begin(), end()) and std::distance(other.begin(), other.end()) respectively:

1) at most M + N - 1 comparisons using operator<

2) at most M + N - 1 comparisons using comp

Example

#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