std::unordered_map swap() method
- od C++17
- do C++17
// Non const version only
void swap( unordered_map& other ) noexcept(/* see below */);
// 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.
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.
- od 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.
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
- od C++17
- do 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)
Hash
or KeyEqual
objects.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 (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.
}
──────── 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
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