std::forward_list swap() method

od C++17

// Non const version only
void swap( forward_list& other ) noexcept(/* see below */);

od C++11

// Non const version only
void swap( forward_list& other );

Exchanges the contents of the container with those of other.
Does not invoke any move, copy, or swap operations on individual elements.

important

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.

od C++11

If std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true. then the allocators are exchanged using an unqualified call to non-member swap.
Otherwise, they are not swapped.

Undefined Behavior

If get_allocator() != other.get_allocator(), the behavior is undefined.

Parameters

• other - the container to exchange the contents with

Return value

(none)

Complexity

Constant - O(1).

Exceptions

od C++17

noexcept(std::allocator_traits<Allocator>::is_always_equal::value)

do C++17

none

Example

Main.cpp

#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