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 the case of p_, there is no need to set it equal to null in the destructor, but it can be a useful defensive mechanism. Imagine a case where you have a bug and something still holds a pointer to a B object after it has been deleted:

If it trys to delete the B object, p_ being null will cause that second delete to be innocuous rather than a heap corruptor.

If it trys to call a method on the B object, p_ being null will cause a crash immediately. If p_ is still the old value, the results are undefined and it may be hard to track down the cause of the resulting crash.

Answer 2

In your second example there's no reason to set p_ to null whatsoever, specifically because it is done in the destructor, meaning that the lifetime of p_ will end immediately after that.

Moreover, there's no point in comparing p_ to null before calling delete, since delete expression performs this check internally. In your specific artificial example, the destructor should simply contain delete p_ and noting else. No if, no setting p_ to null.

Answer 3

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.