Do I need to nullify a member variable in the destructor?

Question

Why one would want to explicitly clear the a vector member variable (of on in a dtor (please see the code below). what are the benefits of clearing the vector, even though i

Answer 1

In class A, there is absolutely no reason to .clear() the vector-type member variable in the destructor. The vector destructor will .clear() the vector when it is called.

In class B, the cleanup code can simply be written as:

delete p_;

There is no need to test whether p_ != NULL first because delete NULL; is defined to be a no-op. There is also no need to set p_ = NULL after you've deleted it because p_ can no longer be legitimately accessed after the object of which it is a member is destroyed.

That said, you should rarely need to use delete in C++ code. You should prefer to use Scope-Bound Resource Management (SBRM, also called Resource Acquisition Is Initialization) to manage resource lifetimes automatically.

In this case, you could use a smart pointer. boost::scoped_ptr and std::unique_ptr (from C++0x) are both good choices. Neither of them should have any overhead compared to using a raw pointer. In addition, they both suppress generation of the implicitly declared copy constructor and copy assignment operator, which is usually what you want when you have a member variable that is a pointer to a dynamically allocated object.