std::swap(std::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)

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

Run this code

#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)

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
Views
span (C++20)

Member functions
unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iterators
unordered_map::beginunordered_map::cbegin
unordered_map::endunordered_map::cend
Capacity
unordered_map::empty
unordered_map::size
unordered_map::max_size
Modifiers
unordered_map::clear
unordered_map::insert
unordered_map::insert_or_assign (C++17)
unordered_map::emplace
unordered_map::emplace_hint
unordered_map::try_emplace (C++17)
unordered_map::erase
unordered_map::swap
unordered_map::extract (C++17)
unordered_map::merge (C++17)
Lookup
unordered_map::at
unordered_map::operator[]
unordered_map::count
unordered_map::find
unordered_map::contains (C++20)
unordered_map::equal_range
Bucket interface
unordered_map::begin(size_type)unordered_map::cbegin(size_type)
unordered_map::end(size_type)unordered_map::cend(size_type)
unordered_map::bucket_count
unordered_map::max_bucket_count
unordered_map::bucket_size
unordered_map::bucket
Hash policy
unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Observers
unordered_map::hash_function
unordered_map::key_eq
Non-member functions
operator==operator!= (until C++20)
std::swap
erase_if (C++20)
Deduction guides(C++17)