c++ - Get pointer to object from pointer to some member

Question

Suppose there's a structure

struct Thing {
  int a;
  bool b;
};

and I get a pointer to member b of that structure, say as parameter of some function:

void some_function (bool * ptr) {
  Thing * thing = /* ?? */;
}

How do I get a pointer to the containing object? Most importantly: Without violating some rule in the standard, that is I want standard defined behaviour, not undefined nor implementation defined behaviour.

As side note: I know that this circumvents type safety.

Answer 1

If you are sure that the pointer is really pointing to the member b in the structure, like if someone did

Thing t;
some_function(&t.b);

Then you should be able to use the offsetof macro to get a pointer to the structure:

std::size_t offset = offsetof(Thing, b);
Thing* thing = reinterpret_cast<Thing*>(reinterpret_cast<char*>(ptr) - offset);

Note that if the pointer ptr doesn't actually point to the Thing::b member, then the above code will lead to undefined behavior if you use the pointer thing.