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.