std::forward_list::swap(3)

NAME
std::forward_list::swap - std::forward_list::swap

Synopsis
   void swap( forward_list& other ); (since C++11)
                        (until C++17)
   void swap( forward_list& other ) noexcept(/* see below */); (since C++17)
   Exchanges the contents of the container with those of other. Does not invoke any
   move, copy, or swap operations on individual elements.
   All iterators and references remain valid. It is unspecified whether an iterator
   holding the past-the-end value in this container will refer to this or the other
   container after the operation.
   If std::allocator_traits<allocator_type>::propagate_on_container_swap::value
   is true, then the allocators are exchanged using an unqualified call to (since
   non-member swap. Otherwise, they are not swapped (and if get_allocator() != C++11)
   other.get_allocator(), the behavior is undefined).

Parameters
   other - container to exchange the contents with

Return value
   (none)

Exceptions
   (none) (until C++17)
   noexcept specification: (since C++17)
   noexcept(std::allocator_traits<Allocator>::is_always_equal::value)

Complexity
   Constant.

Example
// Run this code
 #include <iostream>
 #include <forward_list>
 template<class Os, class Co> Os& operator<<(Os& os, const Co& co) {
     os << "{";
     for (auto const& i : co) { os << ' ' << i; }
     return os << " } ";
 }
 int main()
 {
     std::forward_list<int> a1{1, 2, 3}, a2{4, 5};
     auto it1 = std::next(a1.begin());
     auto it2 = std::next(a2.begin());
     int& ref1 = a1.front();
     int& ref2 = a2.front();
     std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
     a1.swap(a2);
     std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
     // Note that after swap the iterators and references stay associated with their
     // original elements, e.g. it1 that pointed to an element in 'a1' with value 2
     // still points to the same element, though this element was moved into 'a2'.
 }

Output:
 { 1 2 3 } { 4 5 } 2 5 1 4
 { 4 5 } { 1 2 3 } 2 5 1 4

See also
   std::swap(std::forward_list) specializes the std::swap algorithm
   (C++11) (function template)