std::decay an zero length array

Question

I am dealing with some legacy C structures - where we have zero length array. I think it is not valid, but we have to live with it. I was writing a macro, and I want to decay array to pointer type using std::decay.

But if I have a zero length array -

struct data {
   key[0]; <<
};

std::decay<decltype(data::key)> doesnt decay to pointer type. I am using this as a function return type, and it complains -

GCC Error:

error: 'function' declared as function returning an array

It works fine if its an array of length >= 1

Answer 1

We could let the compiler's type checker, instead of template substitution, to do the decay for us:

#include <type_traits>

template <typename T>
T* as_ptr(T* x) { return x; }

template <typename T>
using DecayToPointer = decltype(as_ptr(std::declval<T>()));


int main() {
    static_assert(std::is_same<DecayToPointer<int[0]>, int*>::value, "");
    static_assert(std::is_same<DecayToPointer<int[1]>, int*>::value, "");
    static_assert(std::is_same<DecayToPointer<int[]>, int*>::value, "");
}