Question

const int num = 16;
struct inputs{
       double X1[num];
       double X2[num];
};

Gives me an error:

error: variably modified ‘X1’ at file scope

The same was true for ‘X2’.

But I remember the above is fine for C++, the above is fine (I may be mistaken for C++).

Can anybody clarify this for me?

Answer 1

Yes, there’s a difference. In C, a const variable still isn’t treated as a true compile-time constant (officially, it’s not allowed a part of a constant expression), so this isn’t allowed. Note, however, that although C doesn’t require that the compiler allow it, the standard does give permission for an implementation to accept other forms of constant expressions, so it’s free to accept it if it chooses.

In C++, a const variable is treated as a constant, so it is allowed.

Interestingly, roughly the reverse is true when you use a value passed as a function parameter:

void f(int size) { 
    int array[size];
}

This is allowed in C, but not in C++. This is a variably modified array; the error message you’re getting is basically trying to tell you that these are only allowed inside functions.