Variant return type

factory.cpp

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

struct A { };
struct B { };
struct C { };

using Object = std::variant<A, B, C>;

Object factory(int i) {
    switch (i) {
    case 0:
        return A { };
    case 1:
        return B { };
    default:
        return C { };
    }
}

void use_object(A) {
    std::cout << "A\n";
}
void use_object(B) {
    std::cout << "B\n";
}
void use_object(C) {
    std::cout << "C\n";
}

bool parse_number(char const* arg, int& num) {
    char const* begin = arg;
    char const* end = begin + std::strlen(arg);

    num = 0;
    for (; begin != end; ++begin) {
        int n = *begin - '0';
        if (n > 9 || n < 0) {
            return false;
        }

        num = (num * 10) + n;
    }

    return true;
}

int main(int argc, char const** argv) {
    int n;
    if (argc < 2 || !parse_number(argv[1], n)) {
        std::cerr << "Please provide a positive number!\n";
        exit(1);
    }

    std::visit(
        [](auto&& obj) { use_object(std::move(obj)); },
        factory(n)
    );
}