Question

I’ve never seen this in any language, but I was wondering if this is possible using some trick that I don’t know.

Let’s say that I have a function like

struct A {
  // some members and methods ...
  some_t t;
  // more members ...
};   


void test(some_t& x) { // a reference to avoid copying a new some_t
    // obtain the A instance if x is the t member of an A 
    // or throw an error if x is not the t member of an A
    ...
    // do something
}

Would it be possible to obtain the instance of A whose member t is x ?

Answer 1

If you know that you have a reference to the t member of some A instance, you can get the instance using container_of, e.g. A* pa = container_of(&x, A, t);.

Verifying that the resulting pointer actually is an A is technically possible if and only if A has virtual members, unfortunately there’s no portable method to check.

You can achieve something similar, however, using multiple inheritance and dynamic_cast, which allows cross-casting between subobjects.