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;   
}

Answer 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.

Answer 2

*p++=0; means this:

1. Write sizeof(int) zero bytes to an address stored in p.
2. Increment the value of p by sizeof(int).

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.

Answer 3

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