Is the contents of a pointer to a unique_ptr's contents valid after the unique_ptr is moved?

Question

I've been led to understand that calling a member function on the contents of a moved-from std::unique_ptr is undefined behaviour. My question is: if I call .get() on a unique_ptr and then move it, will the original .get() pointer continue to point to the contents of the original unique pointer?

In other words,

std::unique_ptr<A> a = ...
A* a_ptr = a.get();
std::unique_ptr<A> a2 = std::move(a);
// Does *a_ptr == *a2?

I think it does, but I want to make sure.

('contents' is probably the wrong word. I mean the data you get when you dereference the pointer)

Answer 1

Merely moving the unique_ptr only changes the ownership on the pointed-to object, but does not invalidate (delete) it. The pointer pointed to by unique_ptr<>::get() will be valid as long as it hasn't been deleted. It will be deleted, for example, by the destructor of an owning unique_ptr<>. Thus:

obj*ptr = nullptr;                          // an observing pointer
{ 
  std::unique_ptr<obj> p1;
  {
    std::unique_ptr<obj> p2(new obj);       // p2 is owner
    ptr = p2.get();                         // ptr is copy of contents of p2
    /* ... */                               // ptr is valid 
    p1 = std::move(p2);                     // p1 becomes new owner
    /* ... */                               // ptr is valid but p2-> is not
  }                                         // p2 destroyed: no effect on ptr
  /* ... */                                 // ptr still valid
}                                           // p1 destroyed: object deleted
/* ... */                                   // ptr invalid!

Of course, you must never try to use a unique_ptr that has been moved from, because a moved-from unique_ptr has no contents. Thus

std::unique_ptr<obj> p1(new obj);
std::unique_ptr<obj> p2 = std::move(p1);
p1->call_member();                          // undefined behaviour