NAME¶

std::set::swap - std::set::swap

Synopsis¶

void swap( set& other ); (until C++17)
void swap( set& 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. The past-the-end iterator is invalidated.

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

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¶

Any exception thrown by the swap of the Compare objects. (until C++17)
noexcept specification:
noexcept(std::allocator_traits<Allocator>::is_always_equal::value (since C++17)
&& std::is_nothrow_swappable<Compare>::value)

Complexity¶

Constant.

Example¶

// Run this code

#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::set<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::set<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 } { 4 5 } 2 5 1 4
{ 4 5 } { 1 2 3 } 2 5 1 4
{ 1 2 } { 3 4 } 6 9
{ 3 4 } { 1 2 } 9 6

See also¶

std::swap(std::set) specializes the std::swap algorithm
(function template)