Delete on already deleted object : behavior?

Question

I am wondering what will hapen if I try to do a delete on a pointer that is already deleted, or may have not been allocated ? I've read two things : first, that delete operator will do some checkings and we do not need to check if the pointer is null ; and then, I read that it can lead to unknown behaviors..

I'm asking it, because I use some personal objects that contains Qt objects attributes ; I think that Qt delete all widgets associated when we close the window, but I'm not pretty sure and still : if the soft crash before the window's close, we have to delete all objects manually.

So, what would be the best solution ? Something like that ?

if( my_object )
    delete my_object;

Can it avoid dangerous behaviours ?

Answer 1

delete on an already deleted non-null pointer is undefined behavior - your program will likely crash. You can safely use delete on a null pointer - it will yield a no-op.

So the real problem is not delete on a null pointer. The real problem is here:

 ptr = new Something();
 otherPtr = ptr;
 delete ptr;
 delete otherPtr;

This can happen if you have several pointers to the same object and it is quite dangerous. The possible solutions are:

• use smart pointers (no delete in your code) or
• only have one designated pointer for controlling each object lifetime and delete at exactly the right time.

Answer 2

if( my_object )
    delete my_object;

is redundant. delete on a NULL pointer does nothing. This is guaranteed by the standard.

delete on a pointer that was already deleted causes undefined behavior. That's why you should always remember to assign your pointers to NULL after you delete them:

delete p;
p = NULL;

EDIT: As per the comments, I feel I should specify this. If you have multiple pointers to the same object, the assignment to NULL won't make the delete safe. Regardless, it's better to use smart pointers.

Answer 3

Please note that deleting a pointer does not set it to NULL.

int* i = new int;
*i = 42;
delete i;
delete i; // oops! i is still pointing to the same memory, but it has been deleted already

Deleting a null pointer doesn't do anything, deleting an already deleted object will result in undefined behaviour.

Answer 4

the right way is:

if( my_object )
{
    delete my_object;
    my_object = NULL;
}

because, calling twice the way it was before will call delete on a deleted pointer.