std::visit

#include <iostream>
#include <variant>
int main() {
    std::variant<int, double> v = 2.5;
    double d = std::visit([](auto x){ return x * 2.0; }, v);
    std::cout << d << "\n";
}

#include <iostream>
#include <string>
#include <variant>
#include <type_traits>
int main() {
    std::variant<int, std::string> v = 5;
    std::visit([](const auto& x){
        using T = std::decay_t<decltype(x)>;
        if constexpr (std::is_same_v<T, int>) std::cout << "int " << x << "\n";
        else std::cout << "str " << x << "\n";
    }, v);
}

#include <iostream>
#include <string>
#include <variant>
struct Printer {
    void operator()(int i) const { std::cout << "int " << i << "\n"; }
    void operator()(const std::string& s) const { std::cout << "str " << s << "\n"; }
};
int main() {
    std::variant<int, std::string> v = std::string("yo");
    std::visit(Printer{}, v);
}

#include <iostream>
#include <string>
#include <variant>
template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
int main() {
    std::variant<int, std::string> v = 8;
    std::visit(overloaded{
        [](int i){ std::cout << "i=" << i << "\n"; },
        [](const std::string& s){ std::cout << "s=" << s << "\n"; }
    }, v);
}

#include <iostream>
#include <variant>
int main() {
    std::variant<int, double> v = 10;
    std::visit([](auto& x){ x += 1; }, v);
    std::cout << std::get<int>(v) << "\n";
}

#include <iostream>
#include <variant>
int main() {
    std::variant<int, double> a = 2;
    std::variant<int, double> b = 3.0;
    auto sum = std::visit([](auto x, auto y){ return x + y; }, a, b);
    std::cout << sum << "\n";
}