C++: What is std::visit?

1std::variant<int, std::string, double> v = "hello";
2
3std::visit([](auto&& arg) {
4    using T = std::decay_t<decltype(arg)>;
5    if constexpr (std::is_same_v<T, int>) {
6        std::cout << "int: " << arg;
7    } else if constexpr (std::is_same_v<T, std::string>) {
8        std::cout << "string: " << arg;
9    } else {
10        std::cout << "double: " << arg;
11    }
12}, v);

1template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
2template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
3
4std::visit(overloaded{
5    [](int arg) { /* int */ },
6    [](const std::string& arg) { /* string */ },
7    [](double arg) { /* double */ }
8}, v);