What's with unique_ptr::get() instead of &*?

Question

I'm using unique_ptr to manage some resources for safe destruction in every circumstance, etc.

void do_something(BLOB* b);
unique_ptr<BLOB> b(new_BLOB(20));

Is &* much worse than get? e.g.

do_something(&*b);

or

do_someting(b.get());

both seem to work.

Answer 1

The operator* of std::unique_ptr has undefined behavior if no memory is owned by the unique_ptr where as get() will return nullptr.

If the unique_ptr could ever be empty I would suggest using get() otherwise there is no difference.

Answer 2

&*b is not necessarily equivalent to b.get(). In order to be sure to recieve a pointer to the managed object, you want to use get(). There are two reasons:

• Calling operator* on a unique_ptr not managing any object is undefined behaviour
• The managed object of type T may overload operator& and as such you may not recieve a valid pointer to T.

The standard defines the bevaiour of operator* of std::unique_ptr:

typename add_lvalue_reference<T>::type operator*() const;

1. Requires: get() != nullptr

2. Returns: *get()

Thus, applying operator* (from &*) is not allowed if get() returns nullptr.

You will get the return value of any overloaded operator& in the managed object if present. This can be circumvented by using std::addressof but the undefined behaviour problem persists.

std::unique_ptr<T>::get() will give you the address of the managed object or nullptr if no object is managed. It is safer to use get() and you have a predictable result.