Why is it recommended to set a pointer to null after deleting it? [duplicate]

Question

int* ptr = new int();
delete ptr; 
ptr = 0; // or null

My book is telling me that it is good practice to set a pointer to null or 0 after deleting what it points to. I'm not understanding why. Could someone give me a scenario in which this could cause a problem?

Answer 1

Just so that you know the pointer does not point to anything anymore, and will fail if conditions and other boolean checks:

   delete ptr;
   ptr = NULL;
   if(ptr)
     *ptr = 2;

This code will run perfectly fine, although it would cause memory corruption if the pointer was not set to NULL.

Answer 2

That way, if you accidentally use ptr again later in your program, it causes a crash immediately rather than causing a hard-to-find bug later in the program.