void merge( list& other );
void merge( list&& other );
(2) (since C++11)
template < class Compare > 
void merge( list& other, Compare comp );
template < class Compare > 
void merge( 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.


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

Return value



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


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


#include <iostream>
#include <list>
std::ostream& operator<<(std::ostream& ostr, const std::list<int>& list)
    for (const auto &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};
    std::cout << "list1: " << list1 << '\n';
    std::cout << "list2: " << list2 << '\n';
    std::cout << "merged:" << list1 << '\n';


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 to the same object was not specified specified as no-op

See also

moves elements from another list
(public member function)
merges two sorted ranges
(function template)
merges two ordered ranges in-place
(function template)
merges two sorted ranges
merges two ordered ranges in-place

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.