std::forward_list::merge(3)

💅 NAME 💅

std::forward_list::merge — std::forward_list::merge (yes bestie, the name is the name, iconic). Welcome to std::forward_list::merge(3), a C++ Standard Libary page, and we are about to make your merge era make sense ✨💖

---

✨ Synopsis ✨

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

Okay so the function literally does nothing if other refers to the same object as *this — like, no notes, no comparisons, nothing 💀

Otherwise, it 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. It’s giving stable merge energy 👑

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 respect to the corresponding comparator, or get_allocator() != other.get_allocator(), the behavior is undefined. Yes, undefined — not “probably fine,” bestie 😭💀

---

🎀 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,&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) — literally nothing, not even a little guy.

---

😭 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 N as std::distance(begin(), end()) and R as std::distance(other.begin(), other.end()):

• 1,2) At most N+R-1 comparisons using operator<.
• 3,4) At most N+R-1 applications of the comparison function comp.

---

🧚 Example 🧚

Run this code, bestie 🏃‍♀️💨:

#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.

• LWG 2045, applied to C++11 — as published: O(1) node moving could not be guaranteed if get_allocator() != other.get_allocator(). Correct behavior: the behavior is undefined in this case.

---

👑 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 (niebloid).
• ranges::inplace_merge (C++20) — merges two ordered ranges in-place (niebloid).

---

that’s the whole page, bestie 💖 source: cppreference.com, version/date: 2024.06.10, manual: std::forward_list::merge(3). mwah 💋✨