C++ union element size based on other’s element size

Your union is anonymous. Thus, the compiler will create arr1 and arr2 at class-level.

Because of this, char arr2[sizeof(arr1)]; won’t refer to arr1 properly.

Here is a workaround :

template <size_t N>
struct S
{
    union A
    {   
        int arr1[N];
        char arr2[sizeof(arr1)];
    };  
};

Compiles fine here : https://ideone.com/JcvOYg

By naming the union, we prevent the compiler from including it directly. It is then able to retrieve arr1 properly.

But it also means that arr1 and arr2 aren’t not S‘s members anymore.

Finally, Members can be defined in terms of other members but the latter has to be “findable” by the compiler.