How can I delete dynamic array pointer correctly

Question

Possible Duplicate:
C++ delete - It deletes my objects but I can still access the data?
Why doesn't delete destroy anything?

I've Created dynamic array, and I added values to the values of the array like that.

int *pArr;
pArr = new int[10];

for(int i=0;i<10;i++){
   pArr[i] = i+2;
}

delete[] pArr;  

// After deletion I can find the value of the array items.
cout << pArr[5] << endl;

As you see in the code above and in the last line, I can output the fifth element in the array without any problem . With that supposed to be the array has been removed.

You misunderstand what `delete` means. The memory isn't GONE, you've just told C++ that you aren't going to use it anymore. Then you broke your promise. — Fred Larson, Jan 11 '12 at 16:27
[Your bible wasn't touched.](http://stackoverflow.com/a/6445794/596781) — Kerrek SB, Jan 11 '12 at 16:33

score 1 · Answer 1 · answered Jan 11 '12 at 16:27

1

Once you delete[] the array and still try to access the elements of the array it is Undefined Behavior and any behavior is possible.

answered Jan 11 '12 at 16:27

Alok Save

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score 1 · Accepted Answer · answered Jan 11 '12 at 16:31

To show that the memory can be used again, consider this expansion of your code:

int *pArr;
pArr = new int[10];

for(int i=0;i<10;i++){
       pArr[i] = i+2;
}

delete[] pArr;

int *pArr2;
pArr2 = new int[10];

for(int i=0;i<10;i++){
       pArr2[i] = (2*i)+2;
}

cout << pArr[5] << endl;

That prints out 12 for me. i.e. the value from pArr2[5] actually. Or at least I should say it does for my machine with my specific compiler & version, etc. As others have pointed out it is undefined behaviour. But hopefully my example shows you at least one kind of undefined behaviour.

score 0 · Answer 3 · answered Jan 11 '12 at 16:28

0

Yes, this may work, but is not guaranteed to work. Delete [] only invalidates the memory but does not zero it. The memory you are referencing is invalidate at this point. So don't do it :)

answered Jan 11 '12 at 16:28

Tobias Schlegel

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score -1 · Answer 4 · answered Jan 11 '12 at 16:27

You did delete it correctly.

Then, you invoked Undefined Behaviour and found the memory still existed(*) and wasn't zeroed - a tool like valgrind would still show it as a bug though, which it is.

(*) By "still existed" I mean "is accessible to the process" - it could contain anything, but happens not to have been overwritten ... yet.

How can I delete dynamic array pointer correctly

4 Answers4