std::map swap() method

od C++17

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

do C++17

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

Exchanges the contents of the container with those of other. Does not invoke any move, copy, or swap operations on individual elements.

Invalidation

The past-the-end iterator is invalidated.

All other iterators and references remain valid.

The Compare objects must be Swappable, and they are exchanged using unqualified call to non-member swap.

Parameters

• other - container to exchange the contents with

Return value

(none)

Exceptions

od C++17

noexcept specification:

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

do C++17

Any exception thrown by the swap of the Compare objects.

Complexity

Constant - O(1).

Example

Main.cpp

#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