std::unordered_map swap() method

since C++17

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

until C++17

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

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.

Invalidation

The past-the-end iterator is invalidated.

The Hash and KeyEqual objects must be Swappable, and they are exchanged using unqualified calls 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.

Undefined behaviour

Otherwise, they are not swapped (and if get_allocator() != other.get_allocator(), the behavior is undefined).

Parameters

• other - container to exchange the contents with

Return value

(none)

Complexity

Constant.

Exceptions

since C++17

noexcept specification:

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

until C++17

Any exception thrown by the swap of the Hash or KeyEqual objects.

Example

Main.cpp

#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