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.

Answer 2

X* get_ptr(bool* b){
    static typename std::aligned_storage<sizeof(X),alignof(X)>::type buffer;

    X* p=static_cast<X*>(static_cast<void*>(&buffer));
    ptrdiff_t const offset=static_cast<char*>(static_cast<void*>(&p->b))-static_cast<char*>(static_cast<void*>(&buffer));
    return static_cast<X*>(static_cast<void*>(static_cast<char*>(static_cast<void*>(b))-offset));
}

First, we create some static storage that could hold an X. Then we get the address of the X object that could exist in the buffer, and the address of the b element of that object.

Casting back to char*, we can thus get the offset of the bool within the buffer, which we can then use to adjust a pointer to a real bool back to a pointer to the containing X.

Answer 3

void some_function (bool * ptr) {
  Thing * thing = (Thing*)(((char*)ptr) - offsetof(Thing,b));
}

I think there is no UB.

Answer 4

My proposal is derived from the @Rod answer in Offset from member pointer without temporary instance, and the similar @0xbadf00d's one in Offset of pointer to member.

I started imagining a form of offset driving the implementation of a pointer to a class data member, later confirmed by the post in question and the tests i've made.

I'm not a C++ practitioner so sorry for the brevity.

#include <iostream>
#include <cstddef>

using namespace std;

struct Thing {
    int a;
    bool b;
};

template<class T, typename U>
std::ptrdiff_t member_offset(U T::* mem)
{
    return 
    ( &reinterpret_cast<const char&>( 
        reinterpret_cast<const T*>( 1 )->*mem ) 
      - reinterpret_cast<const char*>( 1 )      );
}

template<class T, typename U>
T* get_T_from_data_member_pointer (U * ptr, U T::*pU) {
  return reinterpret_cast<T*> (
      reinterpret_cast<char*>(ptr) 
    - member_offset(pU));
}

int main()
{

    Thing thing;
    thing.b = false;

    bool * ptr = &thing.b;
    bool Thing::*pb = &Thing::b;

    std::cout << "Thing object address accessed from Thing test object lvalue; value is: " 
        << &thing << "!\n";     
    std::cout << "Thing object address derived from pointer to class member; value is: " 
        << get_T_from_data_member_pointer(ptr, &Thing::b) << "!\n";    
}