why this overflow is occurring?

[munnibhai]

Hello guys,
I am relatively new to C++ programming but have good experience in PHP. Anyways, I wrote this simple program to check how pointers behave in an array but there is this one little thing I did not understand. The last index on the array is generating an overflow type value when pointer is moved to it to display the index value (it should be 0 but it isnt.)

Now i dont undertand why this is happening and how can I avoid this in future?

My code and CLI screenshot is as follows

#include <iostream>
#include <iomanip>
using namespace std;
int main(){
int arr[10] = {0};
int *arrPtr;
arrPtr = arr;
cout << setw(8) << "Index" << setw(20) << "Index Address" << setw(20) << "integer Value" << setw(20) << "Adress of Value" << endl;
for(int i=0;i<=9;i++){
arrPtr++;
cout << setw(8) << i << setw(20) << arrPtr << setw(20) << *arrPtr << setw(20) << &*arrPtr
<< endl << "-----------------------------------------------------------------------------" << endl;
}
system("pause");
}

https://i.imgur.com/Y7WesYy.png

[Repeater]

You are incrementing arrPtr BEFORE you're reading from it. So you're not showing index 0-9. You're showing index 1-9, and then you're reading beyond the end of the array. You're reading whatever value happens to be off the end of the array.

[doug4]

To add on to what @Repeater said, arrays in C++ are always indexed from 0 to n-1, not 1 to n. The idiom for array access is as follows:

// set up an array of known size
const size_t ARRAY_SIZE = 10;
int array[ARRAY_SIZE];

// access elements within the array
// always loop starting at 0, while index < size
for (int i = 0; i < ARRAY_SIZE; ++i)
{
    array[i] = 0;
}

[munnibhai]

@Repeater: Thanks a lot friend. So this becomes a serious error from my side where one can manipulate to hack into it. Wow...

Anyways, using ++arrPtr is also giving off the same problem. I dont know if its buffer overlow or integer overflow because I haven't gotten to that concept clearance yet. I just read about them superficially so far.

If I increase the array size to 11 and read only 10 indices, the problem doesnt occur.
Also, at array size 11, and reading from i=0,i<=10;i++ with arrPtr++ doesnt reproduce the problem. It seems to be doing it at [10] size only :/

@doug: Thanks for the hint sir. the array size in my prog is 10 and I am reading it until loop remains<=9 so it will read from index 0 to 9 (completing all 10 indices). Putting only "<9" would end the loop at index 8. Confirmed it too by executing the given logic. Your code is array initialization in a loop with ++i (increment and read)

[Repeater]

Anyways, using ++arrPtr is also giving off the same problem.

arrPtr begins pointing at arr[0]. First read from arrPtr, THEN increment it.

This is bad:

for(int i=0;i<=9;i++)
{
  arrPtr++; //BAD
  cout << setw(8) << i << setw(20) << arrPtr << setw(20) << *arrPtr << setw(20) << &*arrPtr
<< endl << "-----------------------------------------------------------------------------" << endl;
}

This is good:

for(int i=0;i<=9;i++)
{
  cout << setw(8) << i << setw(20) << arrPtr << setw(20) << *arrPtr << setw(20) << &*arrPtr
<< endl << "-----------------------------------------------------------------------------" << endl;
  arrPtr++; // GOOD
}

[doug4]

@doug: Thanks for the hint sir. the array size in my prog is 10 and I am reading it until loop remains<=9 so it will read from index 0 to 9 (completing all 10 indices). Putting only "<9" would end the loop at index 8. Confirmed it too by executing the given logic.

You missed the point.

Sizing an array to 10 and then looping while i <= 9 will do what you want it to do, but it's not the way C++ programmers generally do it. Probably 99% of the time in this situation C++ programmers will write code to loop while i < 10. Looping while i <= 9 looks funny to C++ programmers, and you should get used to the standard, idiomatic C++ way of writing code. In your code, that would be for (int i = 0; i < 10; i++) // or ++i, no difference in this case

If you look at the example I gave you, the size of the array and the loop limit are the same value (10 in your example). I did not create a size-10 array and loop from 0 to 8.

Your code is array initialization in a loop with ++i (increment and read)

Yes, I was initializing an array in my example. I was only demonstrating the typical C++ way to loop through an array. You can obviously do whatever you want (set / get / manipulate) the array element once you isolate it in the loop. Initialization vs. reading wasn't the point of my example.

And the difference between i++ vs. ++i is meaningless in the example. In old compilers ++i was probably slightly faster than i++ because i++ had to save the original value in case it was being assigned. Someone drilled it into me to use ++i whenever the original value was not being used, so I default to that. Modern compilers should recognize this situation and provide just-as-efficient code either way. I'm sorry if I added confusion with this.

[munnibhai]

@repeater: The //GOOD logic worked. learnt a new thing with this.. read and then increment. so this is what happened. I was incrementing arrPtr before it read and it actually started from 1 and read until 10th index which wasnt there. Now I understand your first reply about reading/incrementing. first I thought you were saying about ++* or *++. :D Thanks alot sir

@doug: Wow... I am really bad with C++ programming.. To be honest, I did learn yet a new thing. Programming etiquette of C++ and the logic about having ++* or *++ being the same thing now so I wouldnt be worrying about it in future :) Thanks to you :)

[doug4]

having ++* or *++ being the same thing

In this case they are the same thing. However, they are not the same thing in general.

int a = 5;
int b = a++; // now, b = 5, a = 6
c = ++a // now c = 7, a = 7 

++a increments "a" and then returns the new value of a.
a++ saves the value of a, increments a, and then returns the previous value of a.

Google "post increment vs pre increment" for all the details.

Because the 3rd part of the for loop does not return a use the returned value, the two constructs are logically identical.

Again, I apologize for adding to the confusion by tweaking your incrementer.

Edit: reworded "Because..." statement.