std::multiset swap() method

since C++17

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

until C++17

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

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

Invalidation

The past-the-end iterator is invalidated.

All other iterators and references remain valid.

The Compare objects must be Swappable, and they are exchanged using unqualified call to non-member swap.

since 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 behaviour

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

Parameters

• other - container to exchange the contents with

Return value

(none)

Exceptions

since C++17

noexcept specification:

noexcept(std::allocator_traits<Allocator>::is_always_equal::value
&& std::is_nothrow_swappable<Compare>::value)

until C++17

Any exception thrown by the swap of the Compare objects.

Complexity

Constant - O(1).

Example

Main.cpp

#include <functional>
#include <iostream>
#include <set>
 
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::multiset<int> a1{3, 1, 3, 2}, a2{5, 4, 5};
 
    auto it1 = std::next(a1.begin());
    auto it2 = std::next(a2.begin());
 
    const int& ref1 = *(a1.begin());
    const int& ref2 = *(a2.begin());
 
    std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
    a1.swap(a2);
    std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
 
    // Note that every iterator referring to an element in one container before the swap
    // refers to the same element in the other container after the swap. Same is true
    // for references.
 
    struct Cmp : std::less<int> {
        int id{};
        Cmp(int i) : id{i} { }
    };
 
    std::multiset<int, Cmp> s1{ {2, 2, 1, 1}, Cmp{6} }, s2{ {4, 4, 3, 3}, Cmp{9} };
 
    std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';
    s1.swap(s2);
    std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';
 
    // So, comparator objects (Cmp) are also exchanged after the swap.
}

Output

{ 1 2 3 3 } { 4 5 5 } 2 5 1 4
{ 4 5 5 } { 1 2 3 3 } 2 5 1 4
{ 1 1 2 2 } { 3 3 4 4 } 6 9
{ 3 3 4 4 } { 1 1 2 2 } 9 6