即使我没有提到数组的大小，为什么在编译和运行代码时仍能正常工作？

Question

class array {
    public:
        int arr[];

        array() {
            arr[0] = 1;
            arr[100] = 2;
        }
};

int main() {
    array a;
    cout << a.arr[0] << a.arr[100] << endl;
    return 0;
}

I was expecting a segmentation fault on running the above code. However, it printed the correct output even though I have not mentioned array size. What is the reason for this?

Answer 1

What you get is Undefined Behavior.

Reading / Writing to unallocated memory does not automatically generate segmentation fault(s), but it is of course "bad practice" and should be avoided.

It's impossible to tell exactly what will happen with such code, where that array will be addressed or what is already there and hence - Undefined Behavior.

Note : As mentioned by @juanchopanza, the code as it is is illegal in C++ because arr is an incomplete type. Your compiler might (and obviously does) ignore this due to default settings, but a legal code that would demonstrate the same behavior is either:

class array {
    public:
        int *arr;
// ...

or

class array {
    public:
        int arr[1];
// ...