There is this little trick question that some interviewers like to ask for whatever reason:
int arr[] = {1, 2, 3};
2[arr] = 5; // does this line compile?
assert(arr[2] == 5); // does this assertion fail?
From what I can understand, a[b]
gets converted to *(a + b)
and since addition is commutative, it doesn't really matter their order, so 2[a]
is really *(2 + a)
and that works fine.
Is this guaranteed to work by C and/or C++'s specs?
Yes. 6.5.2.1 paragraph 1 (C99 standard) describes the arguments to the []
operator:
One of the expressions shall have type "pointer to object
type
", the other expression shall have integer type, and the result has type "type
".
6.5.2.1 paragraph 2 (emphasis added):
A postfix expression followed by an expression in square brackets
[]
is a subscripted designation of an element of an array object. The definition of the subscript operator[]
is thatE1[E2]
is identical to(*((E1)+(E2)))
. Because of the conversion rules that apply to the binary+
operator, ifE1
is an array object (equivalently, a pointer to the initial element of an array object) andE2
is an integer,E1[E2]
designates theE2
-th element ofE1
(counting from zero).
It says nothing requiring the order of the arguments to []
to be sane.