Back in July of 1995, Adam Moscowitz posed this question to Dennis Ritchie:

I teach C programming classes, and I'm often asked why C has a bitwise exclusive-OR operator but not a logical one. Is this simply an historical accident, or was there a specific reason it was omitted? Also, is there any reason why the C9x committee shouldn't consider adding it to the standard?

Dennis replied:

I've often been asked this in person, and it comes up on netnews too. Since I formulated a mildly coherent answer, I'm sending it along to Steve in case a suitably edited version is suitable for the C FAQ.


There are both historical and practical reasons why there is no ^^ operator.

The practical is: there's not much use for the operator. The main point of && and || is to take advantage of their short-circuit evaluation not only for efficiency reasons, but more often for expressiveness and correctness. For example, in

	if (cond1() && cond2()) ...
it is often important that cond1() is done first, because if it's false cond2() may not be well defined, or because cond1() is cheap and cond2() is expensive. Syntax to handle the situation has made it into lots of languages; compare Ada's `and then'.

By contrast, an ^^ operator would always force evaluation of both arms of the expression, so there's no efficiency gain. Furthermore, situations in which ^^ is really called for are pretty rare, though examples can be created. These situations get rarer and stranger as you stack up the operator--

	if (cond1() ^^ cond2() ^^ cond3() ^^ ...) ...
does the consequent exactly when an odd number of the condx()s are true. By contrast, the && and || analogs remain fairly plausible and useful.

Historical: C's predecessors (B and BCPL) had only the bitwise versions of | & ^. They also had a special rule, namely that in an statement like

	if (a & b) ...
the `&', being at the `top level' of the expression occurring in `truth-value context' (inside the if()) was interpreted just like C's &&. Similarly for |. But not so for ^.

One of the early bits of C evolution was the creation of separate && and || operators. This was better than the special rule, which was hard to explain.

In other words, the whole question arises because of the particular method of symmetry-breaking that C chose. Suppose I had reacted to the situation of BCPL and B taking their notion of

	if (a & b) ...

and had people say

	if (a) andif (b) ...
(and similarly with `orif'). One can make a case that this kind of syntax is better than && and ||. If it had happened, would people be asking for `xorif'? Probably not.

My guess is that &,&&; |,||; ^,^^ is a false symmetry. But it's one that people seem to want, and, though it's not much help, adding it wouldn't do much harm.