Accessing arrays by index[array] in C and C++

Question

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?

Answer 1

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 that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

It says nothing requiring the order of the arguments to [] to be sane.

Answer 2

In general 2[a] is identical to a[2] and this is guaranteed to be equivalent in both C and C++ (assuming no operator overloading), because as you meantioned it translates into *(2+a) or *(a+2), respectively. Because the plus operator is commutative, the two forms are equivalent.

Although the forms are equivalent, please for the sake of all that's holy (and future maintenance programmers), prefer the "a[2]" form over the other.

P.S., If you do get asked this at an interview, please do exact revenge on behalf of the C/C++ community and make sure that you ask the interviewer to list all trigraph sequences as a precondition to you giving your answer. Perhaps this will disenchant him/her from asking such (worthless, with regard to actually programming anything) questions in the future. In the odd event that the interviewer actually knows all nine of the trigraph sequences, you can always make another attempt to stomp them with a question about the destruction order of virtual base classes - a question that is just as mind bogglingly irrelevant for everyday programming.