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

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.