std::unordered_map<Key,T,Hash,KeyEqual,Allocator>:: swap

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. Hash and KeyEqual must be Swappable , and objects of these types 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 non-member swap . Otherwise, they are not swapped (and if get_allocator ( ) ! = other. get_allocator ( ) , the behavior is undefined).

Parameters

Exceptions

Complexity

Constant.

Example

#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 (const auto& 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.
}

──────── 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