C++ - Recursive structure - is it possible?

Question

I'm trying to implement a recursive structure in C++, something that should look kinda like this:

typedef struct { static constexpr int foo() { return 1; } typedef struct { // not valid - I meant foo() from "type" not from "recursive_type" static constexpr int foo() { return 2 * foo(); } // ? (there should be another recursive type here) } recursive_type; } type;

and that should work like this:

static_assert(type::foo() == 1, "Nope"); static_assert(type::recursive_type::foo() == 2, "Nope"); static_assert(type::recursive_type::recursive_type::foo() == 4, "Nope");

Basically - I want recursive_type to contain structure that looks exactly like type, but its foo() returns twice the value of type's foo(). But as I have pointed in the comments, there are a couple of problems with my approach and sadly it doesn't work.

Can such a structure be declared somehow in C++, or maybe it is not possible?

You'll better use pointers, perhaps smart ones. And avoid naming two struct (in the same translation unit) with the same name. — Basile Starynkevitch
– Basile Starynkevitch, Commented Dec 2, 2015 at 13:25
Yeah, I know I could use pointers to do that, but I need an object whose semantics look like the ones I described. (So recursive_type should spawn slightly modified version of the object it is called on) — qiubit
– qiubit, Commented Dec 2, 2015 at 13:27
Did you consider switching to Ocaml. It has recursive functors... — Basile Starynkevitch
– Basile Starynkevitch, Commented Dec 2, 2015 at 13:29

Šimon Tóth · Accepted Answer · 2015-12-02 13:33:02Z

Sort of. This is the way you implement type recursion in C++.

template< int tag > struct X { static constexpr int foo() { return 2 * X<tag-1>::foo(); } }; template< > struct X<1> { static constexpr int foo() { return 1; } }; #include <iostream> using namespace std; int main() { static_assert(X<1>::foo() == 1, "Nope"); static_assert(X<2>::foo() == 2, "Nope"); static_assert(X<3>::foo() == 4, "Nope"); cout << X<10>::foo() << endl; }

skyking · Accepted Answer · 2015-12-02 13:43:43Z

Yes, borrowing from Let_Me_Be you can get the behavior you're asking for:

template< int tag > struct X { static constexpr int foo() { return 2 * X<tag-1>::foo(); } typedef X<tag+1> recursive_type; }; template< > struct X<0> { static constexpr int foo() { return 1; } typedef X<1> recursive_type; }; typedef X<0> type; static_assert(type::foo() == 1, "Nope"); static_assert(type::recursive_type::foo() == 2, "Nope"); static_assert(type::recursive_type::recursive_type::foo() == 4, "Nope");

Of course with the bonus that you could write deep recursive uses of recursive_type as X<n> instead...

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