who tells you that? C++ implementation don't require assign the pointer to null, so you can't judge whether the deletion is succeed by compare the pointer to null. According to C++ standard, if the deletion is failed, an exception will arise, you can catch and handle it.
When you delete a pointer of an object, system will invoke the destructor of the pointed object (if there's any destructor), and de-allocate the memory space that the object used. It will not assign the pointer to null. But you shouldn't use deleted pointer any more. If so, you'll get an undefined result.