C++17 auto [a, b] = expr; unpacks a pair, tuple, array or plain struct into named variables in one line.

What can be unpacked

C++

#include <iostream>
#include <map>
#include <string>
#include <tuple>

struct Point { int x; int y; };

std::tuple<int, double, std::string> record() { return {7, 9.5, "ok"}; }

int main() {
    auto [x, y] = Point{3, 4};                  // aggregate struct: by member order
    std::cout << x + y << '\n';

    int arr[3] = {10, 20, 30};
    auto [a, b, c] = arr;                        // C array
    std::cout << a << b << c << '\n';

    auto [id, score, status] = record();         // tuple
    std::cout << id << ' ' << score << ' ' << status << '\n';

    std::map<std::string, int> ages{{"ada", 36}, {"bob", 25}};
    for (const auto& [name, age] : ages)         // pairs in a map
        std::cout << name << '=' << age << ' ';
    std::cout << '\n';

    if (auto [it, inserted] = ages.insert({"ada", 99}); !inserted)
        std::cout << "ada already " << it->second << '\n';
}

Output

7
102030
7 9.5 ok
ada=36 bob=25
ada already 36

Copy or reference?

C++

#include <iostream>
#include <utility>
int main() {
    std::pair<int, int> p{1, 2};
    auto [a, b] = p;      // binds to a hidden *copy* of p
    a = 100;
    auto& [c, d] = p;     // binds to p itself
    c = 50;
    std::cout << p.first << ' ' << p.second << '\n';
}

Output

50 2

auto [..] — copy of the whole object; auto& [..] — refer to the original; const auto& [..] — read-only view.
The number of names must match exactly; you can't skip one (use a dummy name like _ or [[maybe_unused]]).
For your own classes with private members, enable it by specialising std::tuple_size/std::tuple_element and providing get<N> (advanced).

How would you swap-return two values from a function in modern C++?