Translating Swift enums with associated values to C++

Question

In Swift you can have an enum type with associated values.

enum Thing {
   case num(Int)
   case two(String, Double)
   case other
}
var t: Thing = .num(123)
t = .two("a", 6.022)
t = .other

From what I'm reading, you can can do a similar thing in C++ by using std::variant. It has less syntactic sugar.

But the compiler complains if you give void to std::variant, so how would you represent the other case above? Maybe an ignored integer? Or is there better way to translate something like that enum type to C++?

std::variant<int, pair<string, double>, void>  // error
std::variant<int, pair<string, double>, int>   // okay

Compiler error from Clang:

... "variant can not have a void type as an alternative."

(If I wanted to refer to that variant as Thing I could use a typedef or wrap it in a struct.)

Use [`std::monostate`](https://en.cppreference.com/w/cpp/utility/variant/monostate) as your `void` stand-in for `std::variant`. — Eljay, Sep 14 '22 at 12:00

score 1 · Accepted Answer · answered Sep 14 '22 at 12:16

1

Because void cannot be instantiated, you'll need some stand-in type to represent void.

You could make your own, struct void_t{};, or you could use the std::monostate which is provided in C++17 and later.

answered Sep 14 '22 at 12:16

Eljay

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Translating Swift enums with associated values to C++

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