NAME¶

std::unordered_map::swap - std::unordered_map::swap

Synopsis¶

void swap( unordered_map& other ); (since C++11)
(until C++17)
void swap( unordered_map& 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 end() iterator is invalidated.

The Hash and KeyEqual objects must be Swappable, and they are exchanged using
unqualified calls 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 Hash or KeyEqual objects. (until C++17)
noexcept specification:
noexcept(std::allocator_traits<Allocator>::is_always_equal::value
(since C++17)
&& std::is_nothrow_swappable<Hash>::value

&& std::is_nothrow_swappable<key_equal>::value)

Complexity¶

Constant.

Example¶

// Run this code

#include <iostream>
#include <string>
#include <utility>
#include <unordered_map>

// print out a std::pair
template<class Os, class U, class V>
Os& operator<<(Os& os, const std::pair<U, V>& p)
{
return os << p.first << ':' << p.second;
}

// print out a container
template<class Os, class Co>
Os& operator<<(Os& os, const Co& co)
{
os << '{';
for (auto const& i : co)
os << ' ' << i;
return os << " }\n";
}

int main()
{
std::unordered_map<std::string, std::string>
m1{{"γ", "gamma"}, {"β", "beta"}, {"α", "alpha"}, {"γ", "gamma"}},
m2{{"ε", "epsilon"}, {"δ", "delta"}, {"ε", "epsilon"}};

const auto& ref = *(m1.begin());
const auto iter = std::next(m1.cbegin());

std::cout << "──────── before swap ────────\n"
<< "m1: " << m1 << "m2: " << m2 << "ref: " << ref
<< "\niter: " << *iter << '\n';

m1.swap(m2);

std::cout << "──────── after swap ────────\n"
<< "m1: " << m1 << "m2: " << m2 << "ref: " << ref
<< "\niter: " << *iter << '\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.
}

Possible output:¶

──────── before swap ────────
m1: { α:alpha β:beta γ:gamma }
m2: { δ:delta ε:epsilon }
ref: α:alpha
iter: β:beta
──────── after swap ────────
m1: { δ:delta ε:epsilon }
m2: { α:alpha β:beta γ:gamma }
ref: α:alpha
iter: β:beta

See also¶

std::swap(std::unordered_map) specializes the std::swap algorithm
(C++11) (function template)

Category:¶

* conditionally noexcept