Why does a for loop in c++ access a memory location in non-initialized compared to the behavior of a normal cout?

Question

I found this occurrence to be rather interesting, let me explain:

When I initialized a int array, I started to wonder how c++ handles an index with a no initialized value. When directly using cout, c++ directly outputs the values as 0. However, when inserting a for loop right afterwards, with the same purpose it instead points to the values inside the memory location, and pretends they were not initialized.

To regenerate this error, copy & paste the code below onto a compiler. Run it once without the for loop, and once with the for loop.

I am just interested to find out why this occurs.

---

#include <iostream>
using namespace std;

int main() {
    int myArray[4];
    myArray[2] = 32;
    cout << "\n Val 1: "<< myArray[0] << "\n Val 2: "<< myArray[1]<< "\n Val 3: "<< myArray[2]<< "\n Val 4: "<< myArray[3]<< "\n Val 5: "<< myArray[4];
    cout <<"\n ----------------------------";
    /*
    for(int i = 0; i < 5; i++){
        cout << "\n Val " << i << ": " << myArray[i];
    }
    */
    return 0;
}

Answer 1

You are likely witnessing the work of a (clever) optimizer:

Without the for loop, you access the array elements with a fixed constant, a constant which the optimizer can easily proove will lead to an uninitialized value, which is also never used again. As such, it can optimize away actually reading the element from the uninitialized memory, because it is perfectly entitled to use some constant instead.

With the for loop, you have a second usage of the values (through the use of a dynamic index), and the optimizer has to ensure that the undefined value you read from the array elements in the first cout is the same as the one that is later read within the loop. Obviously, it does not try to unroll the loop - after that it would know how to optimize the reads away.

In any case, whenever you access an uninitialized value, that value is undefined, it can be anything, including zero (even though you are not yet invoking undefined behavior). Whenever you use such a value for memory access (uninitialized pointer etc.), you have undefined behavior at its worst.