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std::swap(std::unordered_map)

Defined in header <unordered_map>

Defined in header `<unordered_map>`
template< class Key, class T, class Hash, class KeyEqual, class Alloc > void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs, std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs );		(since C++11) (until C++17)
template< class Key, class T, class Hash, class KeyEqual, class Alloc > void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs, std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs ) noexcept(/* see below */);		(since C++17)

template< class Key, class T, class Hash, class KeyEqual, class Alloc >
void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs,
           std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs );

(since C++11)
(until C++17)

template< class Key, class T, class Hash, class KeyEqual, class Alloc >
void swap( std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& lhs,
           std::unordered_map<Key, T, Hash, KeyEqual, Alloc>& rhs )
               noexcept(/* see below */);

(since C++17)

Specializes the std::swap algorithm for std::unordered_map. Swaps the contents of lhs and rhs. Calls lhs.swap(rhs).

Parameters

lhs, rhs

containers whose contents to swap

Return value

(none).

Complexity

Constant.

Exceptions

noexcept specification:

noexcept(noexcept(lhs.swap(rhs)))

(since C++17)

Notes

Although the overloads of std::swap for container adaptors are introduced in C++11, container adaptors can already be swapped by std::swap in C++98. Such calls to std::swap usually have linear time complexity, but better complexity may be provided.

Example

#include <algorithm>
#include <iostream>
#include <unordered_map>
 
int main()
{
    std::unordered_map<int, char> alice{{1, 'a'}, {2, 'b'}, {3, 'c'}};
    std::unordered_map<int, char> bob{{7, 'Z'}, {8, 'Y'}, {9, 'X'}, {10, 'W'}};
 
    auto print = [](std::pair<const int, char>& n)
    {
        std::cout << ' ' << n.first << ':' << n.second;
    };
 
    // Print state before swap
    std::cout << "alice:";
    std::for_each(alice.begin(), alice.end(), print);
    std::cout << "\n" "bob  :";
    std::for_each(bob.begin(), bob.end(), print);
    std::cout << '\n';
 
    std::cout << "-- SWAP\n";
    std::swap(alice, bob);
 
    // Print state after swap
    std::cout << "alice:";
    std::for_each(alice.begin(), alice.end(), print);
    std::cout << "\n" "bob  :";
    std::for_each(bob.begin(), bob.end(), print);
    std::cout << '\n';
}

Possible output:

alice: 1:a 2:b 3:c
bob  : 7:Z 8:Y 9:X 10:W
-- SWAP
alice: 7:Z 8:Y 9:X 10:W
bob  : 1:a 2:b 3:c