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.