Segfault when deleting pointer

Question

I've been experiencing segfaults when running some C++ code. I've isolated the problem to a line in the program that deletes a pointer. Here's a simple example that produces the same error:

int main()
{
  int* pointer=0;
  int number = 3;

  pointer = &number;
  delete pointer;//This line causes a segmentation fault
  pointer=0;

  return 0;
}

A slight modification produces code that will work as expected:

int main()
{
  int* pointer=new int(3);

  delete pointer;//This line now works
  pointer=0;

  return 0;
}

Can someone explain why the first causes a segfault and the second does not? I know the pointer isn't invalid, since it's been assigned to the address of the number variable.

Solution 1

You should only ever delete memory that has been allocated with new. Automatic variables declared on the stack do not need to be deleted. As a rule, always match your memory allocation and deallocation types:

• Memory allocated with new should be deallocated with delete.
• Memory allocated with new [] should be deallocated with delete [].
• Memory allocated with malloc() should be deallocated with free().

The segfault is because the delete operator will attempt to put that memory back into the heap, and that relies on certain properties of the memory that don't hold true for automatic memory on the stack that didn't originate from the heap.

Solution 2

You can't use delete on anything you didn't get with new. Trying to do so will cause undefined behaviour. Your program crashed, but anything could have happened.

Solution 3

Calling delete on a pointer, deallocates the dynamically allocated memory that the pointer points to.

In the first program, pointer points to a statically allocated memory location.The variable number is an 'automatic' variable, which means that its memory is automatically managed.

On the other hand in the second program, pointer is pointing to a memory location allocated in the heap segment, which needs to be manually deallocated by calling delete.

You might find this link useful.