Looping using pointers and arrays [duplicate]

I just got into learning pointers in C++ and I have been trying some different instances of using them to get a better understanding to them but there is some I go into that seems a little weird to me.

The code below is for looping through an array and what I assumed after it ran well that arr is just a pointer to the beginning of the array (because I could do ptr = arr;.

    int size = 3;
    int arr[size] = {50, 30, 20};
    int *ptr;
    ptr = arr;
    for (int i = 0; i < size; i++) {
        std::cout << *ptr << std::endl;
        ptr++;
    }

But When I try looping through the same array but with using arr instead of ptr without assigning it to arr it gave me an error (lvalue required as increment operand) referring arr++.

This is the Code.

    int size = 3;
    int arr[size] = {50, 30, 20};
    for (int i = 0; i < size; i++) {
        std::cout << *arr << std::endl;
        arr++;
    }

I don't understand why the first one work and the second doesn't although they are both pointers(as far as I know).