std::visit

#include <variant>
#include <iostream>
#include <type_traits>
#include <iomanip>
#include <vector>
 
 
template<class T> struct always_false : std::false_type {};
 
using var_t = std::variant<int, long, double, std::string>;
 
template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 
int main() {
    std::vector<var_t> vec = {10, 15l, 1.5, "hello"};
    for(auto& v: vec) {
        // void visitor, only called for side-effects
        std::visit([](auto&& arg){std::cout << arg;}, v);
 
        // value-returning visitor. A common idiom is to return another variant
        var_t w = std::visit([](auto&& arg) -> var_t {return arg + arg;}, v);
 
        std::cout << ". After doubling, variant holds ";
        // type-matching visitor: can also be a class with 4 overloaded operator()'s
        std::visit([](auto&& arg) {
            using T = std::decay_t<decltype(arg)>;
            if constexpr (std::is_same_v<T, int>)
                std::cout << "int with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, long>)
                std::cout << "long with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, double>)
                std::cout << "double with value " << arg << '\n';
            else if constexpr (std::is_same_v<T, std::string>)
                std::cout << "std::string with value " << std::quoted(arg) << '\n';
            else 
                static_assert(always_false<T>::value, "non-exhaustive visitor!");
        }, w);
    }
 
    for (auto& v: vec) {
        std::visit(overloaded {
            [](int arg) { std::cout << arg << ' '; },
            [](long arg) { std::cout << arg << ' '; },
            [](double arg) { std::cout << arg << ' '; },
            [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; },
        }, v);
    }
}

10. After doubling, variant holds int with value 20
15. After doubling, variant holds long with value 30
1.5. After doubling, variant holds double with value 3
hello. After doubling, variant holds std::string with value "hellohello"
10 15 1.5 "hello"

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