C++ Constants and aggregates inside function body vs. outside

Question

Consider the following piece of code:

#include <iostream>
using namespace std;

int main()
{
    int x = 3;
    const int i[] = { 1, 2, 3, 4 };
    float f[i[3]]; 
    struct S { int i, j; };
    const S s[] = { { 1, 2 }, { 3, 4 } };
    double d[s[1].j];
}

It runs without error. However, the following:

#include <iostream>
using namespace std;


int x = 3;
const int i[] = { 1, 2, 3, 4 };
float f[i[3]]; // error: array bound is not an integer constant before ']' token|
struct S { int i, j; };
const S s[] = { { 1, 2 }, { 3, 4 } };
double d[s[1].j]; // error: array bound is not an integer constant before ']' token|

int main()
{
    return 0;
}

Does not, as it gets the errors that are highlighted as comments. Can anyone explain to me why that is?

Answer 1

More than likely, the reason the compiler allows it within the function is due to a compiler extension: variable-length arrays. They allow arrays declared inside of functions to have non-constexpr lengths. But it only works inside of functions, not at global scope.