# 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 ); | (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 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 std::less <T>()(until C++14) std::less <>()(since C++14).
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

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. ​

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 <forward_list>

    std::ostream& operator<<(std::ostream& ostr, const std::forward_list<int>& list)
    {
        for (const int 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

### 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 2045 | C++11 | O(1) node moving could not be guaranteed if get_allocator() != other.get_allocator() | the behavior is undefined in this case
LWG 3088 | C++11 | the effect when *this and other refer to the same object was not specified operator< could misbehave for pointer elements | specified as no-op implementation-defined strict total order used

### See also

splice_after | moves elements from another `forward_list` (public member function)
merge | merges two sorted ranges (function template)
inplace_merge | merges two ordered ranges in-place (function template)
ranges::merge (C++20) | merges two sorted ranges (algorithm function object)
ranges::inplace_merge (C++20) | merges two ordered ranges in-place (algorithm function object)

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