struct A { double x; };
struct B { double y; };

const int size = 3;

void sum1(A const * in, B & out) {
   for (int i = 0; i != size; ++i)
       out.y += in[i].x;
}

void sum2(A const * in, A & out) {
   for (int i = 0; i != size; ++i)
       out.x += in[i].x;
}

void sum3(A const * in, double & out) {
   for (int i = 0; i != size; ++i)
       out += in[i].x;
}

int test1() {
   A a[size] = { { 1 }, { 2 }, { 3 }};
   B& out = reinterpret_cast<B&>(a[1]);
   sum1(a, out);
   return out.y;
}

int test2() {
   A a[size] = { { 1 }, { 2 }, { 3 }};
   A& out = a[1];
   sum2(a, out);
   return out.x;
}

int test3() {
   A a[size] = { { 1 }, { 2 }, { 3 }};
   double& out = a[1].x;
   sum3(a, out);
   return out;
}