Get pointer to object from pointer to some member

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

I think there is no UB.

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.

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.