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

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< cpp‎ | container‎ | forward list

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

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
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 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 (auto &i : list) {
            ostr << " " << i;
        }
        return ostr;
    }

    int main()
    {
        std::forward_list<int> list1 = { 5,9,0,1,3 };
        std::forward_list<int> list2 = { 8,7,2,6,4 };

        list1.sort();
        list2.sort();
        std::cout << "list1:  " << list1 << "\n";
        std::cout << "list2:  " << list2 << "\n";
        list1.merge(list2);
        std::cout << "merged: " << list1 << "\n";
    }

Output:

    list1:   0 1 3 5 9
    list2:   2 4 6 7 8
    merged:  0 1 2 3 4 5 6 7 8 9

### See also

splice_after | moves elements from another `forward_list` (public member function)

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