Runtime type info for accessing std::variant data

Types are not values that you can store in a data member.

In order to access the std::variant data based on its type, you have several options:

1. Use std::get with a specific type. If you know it at compile-time you can use it plainly. If you want to check the type at runtime before the call to std::get, you can use std::holds_alternative.
2. Use std::visit with a generic lambda. Will be resolved in runtime.
3. Use std::visit with a lambda per variant type. This is useful if you need separate logic per type (will be also resolved in runtime).

#include <variant>
#include <string>
#include <iostream>

class A {
public:
    A(const int a) { va = a; }
    A(const std::string& a) { va = a; }
    std::variant<int, std::string> va{};
};

void A_visitor_generic_lambda(A const& a) {
    std::visit([](auto&& arg) { std::cout << arg << std::endl; }, a.va);
}

template<class... Ts>
struct overloaded : Ts... { using Ts::operator()...; };
// explicit deduction guide (not needed as of C++20)
template<class... Ts>
overloaded(Ts...)->overloaded<Ts...>;

void A_visitor_separate_lambdas(A const& a) {
    std::visit(overloaded{
                 [](int arg) { std::cout << arg << std::endl; },
                 [](std::string const& arg) { std::cout << arg << std::endl; } },
               a.va);
}

int main() {
     const A a_obj{ 1 };
     const A b_obj{ "abc" };

    // 1. Use `std::get` with a specific type, after checking the type with `std::holds_alternative`:
    if (std::holds_alternative<int>(a_obj.va)) {
        std::cout << std::get<int>(a_obj.va) << std::endl;
    }
    if (std::holds_alternative<std::string>(b_obj.va)) {
        std::cout << std::get<std::string>(b_obj.va) << std::endl;
    }

    // 2. Use `std::visit` with a generic labmda:
    A_visitor_generic_lambda(a_obj);
    A_visitor_generic_lambda(b_obj);

    // 3. Use `std::visit` with a separate overload lambdas:
    A_visitor_separate_lambdas(a_obj);
    A_visitor_separate_lambdas(b_obj);
}

Output:

1
abc
1
abc
1
abc

Live demo