Question

I have this code in C++ which is giving weird output:

#include<iostream>
using namespace std;
int main(){
    int r[15]={0};
    int n = 5;
    r[15]=20;
    cout<<n;    
}

The output should obviously be 5, but it gives me 20. Now I know r[15] is out of bounds. This code should’ve thrown an exception for trying to access r[15], shouldn’t it? However, it compiles normally with g++ and giving wrong output. I’m not able to figure out what’s causing this anomaly. Can anyone help?

Just FYI, this code is just a sample, I had to figure this bug out from a larger code which took me a lot of time which, otherwise, could’ve been saved if an exception was thrown.

Update:
I checked the following code:

#include<iostream>
using namespace std;
int main(){
    int n = 5;
    int r[15]={0};
    r[15]=20;
    cout<<n;
}

Output:
20

I checked the following code too:

#include<iostream>
using namespace std;
int main(){
    int n = 5;
    int a=5;
    int r[15]={0};
    r[15]=20;
    cout<<n<<endl<<a;
}

Output:
5
5

So if the stack explanation is correct, either of the values should’ve been modified in this case too, right? It doesn’t.

Answer 1

Since r is a 15-element array, r[14] is the last element. Therefore r[15]=20; is undefined behavior. C++ doesn’t do bounds checking so you won’t get exceptions when dealing with plain arrays.

In your case r[15]=20 happens to overwrite the stack at the precise location where n is stored.