Question

Context: I’m trying to wrap my head around pointers, we just saw them a couple of weeks ago in school and while practicing today I ran into a silly? issue, it can be super straightforward to you but I have little to none programming experience.

I’ve seen quite a few questions over in SO about deleting pointers but they all seem to be related to deleting a class and not a ‘simple’ pointer (or whatever the proper term might be), here’s the code I’m trying to run:

#include <iostream>;

using namespace std;

int main() {
  int myVar,
      *myPointer;

  myVar = 8;
  myPointer = &myVar;

  cout << "delete-ing pointers " << endl;
  cout << "Memory address: " << myPointer << endl;

  // Seems I can't *just* delete it, as it triggers an error 
  delete myPointer;
  cout << "myPointer: " << myPointer << endl;
  // Error: a.out(14399) malloc: *** error for object 0x7fff61e537f4:
  // pointer being freed was not allocated
  // *** set a breakpoint in malloc_error_break to debug
  // Abort trap: 6

  // Using the new keyword befor deleting it works, but
  // does it really frees up the space? 
  myPointer = new int;
  delete myPointer;
  cout << "myPointer: " << myPointer << endl;
  // myPointer continues to store a memory address.

  // Using NULL before deleting it, seems to work. 
  myPointer = NULL;
  delete myPointer;
  cout << "myPointer: " << myPointer << endl;
  // myPointer returns 0.

}

So my questions are:

1. Why won’t the first case work? Seems the most straightforward use to use and delete a pointer? The error says the memory wasn’t allocated but ‘cout’ returned an address.
2. On the second example the error is not being triggered but doing a cout of the value of myPointer still returns a memory address?
3. Does #3 really work? Seems to work to me, the pointer is no longer storing an address, is this the proper way to delete a pointer?

Sorry for the long question, wanted to make this as clear as possible, also to reiterate, I have little programming experience, so if someone could answer this using layman’s terms, it would be greatly appreciated!

Answer 1

1 & 2

myVar = 8; //not dynamically allocated. Can't call delete on it.
myPointer = new int; //dynamically allocated, can call delete on it.

The first variable was allocated on the stack. You can call delete only on memory you allocated dynamically (on the heap) using the new operator.

3.

  myPointer = NULL;
  delete myPointer;

The above did nothing at all. You didn’t free anything, as the pointer pointed at NULL.

---

The following shouldn’t be done:

myPointer = new int;
myPointer = NULL; //leaked memory, no pointer to above int
delete myPointer; //no point at all

You pointed it at NULL, leaving behind leaked memory (the new int you allocated).
You should free the memory you were pointing at. There is no way to access that allocated new int anymore, hence memory leak.

---

The correct way:

myPointer = new int;
delete myPointer; //freed memory
myPointer = NULL; //pointed dangling ptr to NULL

---

The better way:

If you’re using C++, do not use raw pointers. Use smart pointers instead which can handle these things for you with little overhead. C++11 comes with several.