NAME

std::map::swap - std::map::swap

Synopsis

void swap( map& other ); (until C++17)
void swap( map& other ) noexcept(/* see below */); (since C++17)

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 Compare objects must be Swappable, and they are exchanged using unqualified call
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 (since
non-member swap. Otherwise, they are not swapped (and if get_allocator() != C++11)
other.get_allocator(), the behavior is undefined).

Parameters

other - container to exchange the contents with

Return value

(none)

Exceptions

Any exception thrown by the swap of the Compare objects. (until C++17)
noexcept specification:
noexcept(std::allocator_traits<Allocator>::is_always_equal::value (since C++17)
&& std::is_nothrow_swappable<Compare>::value)

Complexity

Constant.

Example

// Run this code

#include <iostream>
#include <string>
#include <utility>
#include <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::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.
}

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