Q: I came across some ``joke'' code containing the ``expression'' 5["abcdef"] . How can this be legal C?
A: Yes, Virginia, array subscripting is commutative in C. [footnote] This curious fact follows from the pointer definition of array subscripting, namely that a[e] is identical to *((a)+(e)), for any two expressions a and e, as long as one of them is a pointer expression and one is integral. The ``proof'' looks like
a[e] *((a) + (e)) (by definition) *((e) + (a)) (by commutativity of addition) e[a] (by definition)
This unsuspected commutativity is often mentioned in C texts as if it were something to be proud of, but it finds no useful application outside of the Obfuscated C Contest (see question 20.36).
Since strings in C are arrays of char, the expression "abcdef" is perfectly legal, and evaluates to the character 'f'. You can think of it as a shorthand for
char *tmpptr = "abcdef"; ... tmpptr ...See question 20.10 for a realistic example.
Rationale Sec. 188.8.131.52
H&S Sec. 5.4.1 p. 124, Sec. 7.4.1 pp. 186-7