In dynamic allocation of arrays at runtime, when the array is declared before getting the array length, I encountered a behavior that could not explain.
In the following code, the user provides the length of the array, say 5 and then provides five space-separated numbers as input, say 4 3 7 2 1. By printing length after the for loop we see that it changes.
Here is the code.

#include <cstdio>
#include <iostream>
using namespace std;


int main() {
    int length;
    int ar[length];

    cout << "Enter the length of array: ";
     
    cin >> length;
    cout << "--- length before: " << length << endl;

    cout << "Provide " << length << " integer values separated by space and then press Enter." << endl;
    
    for (int i = 0; i<length; i++)
    {
        cin >> ar[i];
        cout << "i: " << i << " - ar[i]: " << ar[i] << endl;
    }
        
    cout << "--- length after: " << length << endl;
    

    return 0;
}

And the output with input 4 3 7 2 1:

Enter the length of array: 5
--- length before: 5
Provide 5 integer values separated by space and then press Enter.
4 3 7 2 1
i: 0 - ar[i]: 4
i: 3 - ar[i]: -1
--- length after: 4

After trying out different inputs, I found that length is changed to the first input integer, here 4. The value of i in the loop also jumps based on the given input.
I know the array has to be declared after cin but any explanations for the behavior?