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

Question

Consider the following piece of code:

struct S {     union     {         int arr1[10];         char arr2[sizeof(arr1)];     }; }; 

It compiles successfully with gcc 4.9.2 in c++03 and c++11 mode. However when I change S to be a template like so:

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

I get the following error output:

error: int S<10ul>::<anonymous union>::arr1 [10]’ is inaccessible

int arr1[N];

error: within this context

char arr2[sizeof(arr1)];

Clang compiles both versions only in c++11 mode. I am curious what is the correct behavior here. Maybe should I explicitly state that arr2 size is sizeof(int) * N?

Answer 1

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.

Answer 2

You can do this:

template <size_t N> class S { private:     union     {         int arr1[N];         char arr2[N*sizeof(int)];     }; };