What is the difference between `*a` and `a[0]`?

Question

Given a user-defined type A, and a pointer A* a, what is the difference between *a and a[0]?

(Though *(a+0)/a[0] are defined to be equivalent, the same is not the case for *a/a[0], where a subtle difference may cause a compilation error in certain circumstances.)

Answer 1

If A is an incomplete type, *a works, but a[0] does not, in this example:

struct A;

void foo(A& r)
{
}

void bar(A* a)
{
    foo(*a);
    foo(a[0]);   // error: invalid use of incomplete type ‘struct A’
}

That's because a[0] is equivalent to *(a+0), but you cannot add something to a pointer to an object of incomplete type (not even zero), because pointer arithmetic requires the size to be known.