It will work.
From Paragraphs 20.7.1.2.3/8-9 of the C++11 Standard about the unique_ptr<> class template:
unique_ptr& operator=(nullptr_t) noexcept;Effects:
reset().Postcondition:
get() == nullptr
This means that the definition of class template unique_ptr<> includes an overload of operator = that accepts a value of type nullptr_t (such as nullptr) as its right hand side; the paragraph also specifies that assigning nullptr to a unique_ptr is equivalent to resetting the unique_ptr.
Thus, after this assignment, your A object will be destroyed.