After deleting a pointer that was created using the 'new' keyword you should set it's value to NULL otherwise if you delete it again somewhere else in the code, anything could happen. This is called undefined behaviour. Also, when initializing pointers you should assign it to a NULL value if you are not using it straight away.
Quote from C++ Primer:
After deleting a pointer, the pointer becomes what is referred to as a dangling pointer. A dangling pointer is one that refers to memory that once held an object but does so no longer. A dangling pointer can be the source of program errors that are difficult to detect.
Setting the pointer to 0 (or NULL) after the object it refers to has been deleted makes it clear that the pointer points to no object.