CPP array memory allocation

Question

Here I am declaring array length before taking input for n. At the time of array declaration n=0.

#include <iostream>
using namespace std;

int main() {
    int n;
    int sum=0;
    int arr[n]={};
    cin >> n;

    for(int i=0;i<n;i++) {
        cin >> arr[i];
        cout << arr[i];
    }
    return 0;
}

for below input 6 1 2 3 4 5 6

I am getting this output 1234.

Can someone please explain the reason?

You invoked *undefined behavior*. Determining the reason for this specific result will require detailed information of your environment. — MikeCAT, Aug 23 '20 at 15:03
@MikeCAT trying to run code on hackerrank platform. https://www.hackerrank.com/challenges/simple-array-sum/problem — Katha patel, Aug 23 '20 at 15:06
Even though some compilers allow `int arr[n]={};`, it's not valid in the standard C++ as the length must be known at compile-time. You should be using `std::vector` instead. — HolyBlackCat, Aug 23 '20 at 15:06
@Kathapatel You got lucky. `n` is uninitialized and could have any other value, and reading from it causes undefined behavior. — HolyBlackCat, Aug 23 '20 at 15:07
@HolyBlackCat yes, I am new to CPP and trying to review someone else's code. Not able to justify why getting this output with this code. — Katha patel, Aug 23 '20 at 15:08

score 2 · Accepted Answer · answered Aug 23 '20 at 15:11

Here I am declaring array length before taking input for n.

Yes, but that's no valid C++. It's a compiler specific extension. See Why aren't variable-length arrays part of the C++ standard?

At the time of array declaration n=0.

That's wrong. n has not been initialized, so it contains garbage data (which could be 0). Reading n invokes undefined behaviour.

Even if n would be 0, the loop would access the array out of bounds and undefined behaviour is invoked again.

Discussing the output of a program that invokes undefined behaviour is pointless - anything might happen. The only reasonbale thing is to avoid UB. Some good practices to do this:

Always initialize variables to a reasonable value
use std::vector instead of plain arrays/VLAs.

CPP array memory allocation

1 Answers1