Pointer being freed was not allocated?

Question

Tell me what the 3rd line is doing please.

int main(){

int *p = new int[3];

*p++=0; // What's this line doing?

delete p;

return 0;   
}

You should be more concerned about the line that follows that one. — chris, Jan 16 '13 at 04:18
What's it doing? It is invoking the dreaded undefined behavior. Prepare for a high risk of nasal demons. — dmckee --- ex-moderator kitten, Jan 16 '13 at 04:20

score 3 · Accepted Answer · edited May 23 '17 at 12:14

3

*p++=0; means this:

In other words — you have incremented the pointer and what you then passing to delete is not the same as was returned by operator new[].

As @FredLarson has also mentioned, you have to use delete [] p; in order to delete an array.

Also, I'd recommend you read up on pointers, pointer arithmetics and pre-/post-increment. Pick a book from our Definitive C++ Book Guide and List.

edited May 23 '17 at 12:14

Community

answered Jan 16 '13 at 04:22

1

And even if the pointer value were correct, it should be `delete [] p;`. – Fred Larson Jan 16 '13 at 04:23
Thanks a lot. And you are right. I don't understand the sequence of increment and assign here... – Arch1tect Jan 16 '13 at 04:30
1

Well, now you went that far. 8v) I'm off to bed myself. – Fred Larson Jan 16 '13 at 04:30
@Arch1tect: This is post-increment. It means — increment after using the variable. So the increment is guaranteed to happen __after__ dereferencing. – Jan 16 '13 at 04:31
@VladLazarenko totally undertand now. Thank you so much and every one else. :) – Arch1tect Jan 16 '13 at 04:37

score 1 · Answer 2 · answered Jan 16 '13 at 06:02

1

The first element in the array is set to 0 and p is advanced by one to point to the second element.

delete p; // this has undefined behaviour

Use delete [] p; instead.

answered Jan 16 '13 at 06:02

Yang Zhang

score 0 · Answer 3 · answered Jan 16 '13 at 04:22

0

You are setting p[0] to 0 and advancing the pointer to p[1]. What are you trying to do?

answered Jan 16 '13 at 04:22

robert_difalco

3 Answers3