Compile-time parent child relationship through templates

Question

This is mainly out of curiousity. I am wondering if there is a way to define a parent-type to child-type relationship at compile-time using templates.

template <class T>
struct parent_t {
  T *child;
};

template <class T>
struct child_t {
  T *parent;
};

It seems impossible due to infinite template recursion when trying to construct:

auto parent = new parent_t<child_t<parent_t< (... to infinity)

Answer 1

You can do it with an extra layer of indirection:

template <typename T> struct parent;
template <typename T> struct child;

struct Traits {
    using p = parent<Traits>;
    using c = child<Traits>;
};

template <typename T>
struct parent {
    typename T::c* child;
};

template <typename T>
struct child {
    typename T::p* parent;
};

int main()
{
     parent<Traits> p{nullptr};
     child<Traits> c{nullptr};

     c.parent = &p;
     p.child = &c;
}