Mutually dependant using definitions

Question

I have some mutually dependent template instances. Normally I'd just forward declare them but I don't see how this is possible. Here is an example

#include <tuple>
#include <memory>

using Tuple = std::tuple<int,TupleContainer>;
using TupleContainer = std::unique_ptr<Tuple>;

int main()
{
    return 0;
}

Can't write Tuple first due to needing TupleContainer, can't write TupleContainer first due to needing Tuple.

How can I forward declare one of the using definitions?

Possible duplicate of [How to define a recursive type?](https://stackoverflow.com/questions/25563661/how-to-define-a-recursive-type) — cpplearner, Jan 18 '18 at 05:51
boost can possibly do what you want. Here's an example https://stackoverflow.com/a/45898763/817643 — StoryTeller - Unslander Monica, Jan 18 '18 at 05:52

score 2 · Accepted Answer · answered Jan 18 '18 at 08:54

I managed to do it by using a thin wrapper class around std::tuple and using a forward declaration.

#include <tuple>
#include <memory>

struct Tuple;
using TupleContainer = std::unique_ptr<Tuple>;

struct Tuple : public std::tuple<int,TupleContainer>{
    using std::tuple<int,TupleContainer>::tuple;
};

int main()
{
    return 0;
}

score -3 · Answer 2 · edited Jan 18 '18 at 13:28

-3

You need a real pointer somewhere, just like if you were to try and have a recursive structure definition it fails because it wouldn't be able to stop.

using my_tuple = std::tuple<int, std::tuple<int>*>;

edited Jan 18 '18 at 13:28

Bo Persson

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answered Jan 18 '18 at 06:03

SoronelHaetir

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Mutually dependant using definitions

2 Answers2