Logical XOR operator in C++?

Question

Is there such a thing? It is the first time I encountered a practical need for it, but I don\'t see one listed in Stroustrup. I intend to write:

// Detect wh         


        

Answer 1

Proper manual logical XOR implementation depends on how closely you want to mimic the general behavior of other logical operators (|| and &&) with your XOR. There are two important things about these operators: 1) they guarantee short-circuit evaluation, 2) they introduce a sequence point, 3) they evaluate their operands only once.

XOR evaluation, as you understand, cannot be short-circuited since the result always depends on both operands. So 1 is out of question. But what about 2? If you don't care about 2, then with normalized (i.e. bool) values operator != does the job of XOR in terms of the result. And the operands can be easily normalized with unary !, if necessary. Thus !A != !B implements the proper XOR in that regard.

But if you care about the extra sequence point though, neither != nor bitwise ^ is the proper way to implement XOR. One possible way to do XOR(a, b) correctly might look as follows

a ? !b : b

This is actually as close as you can get to making a homemade XOR "similar" to || and &&. This will only work, of course, if you implement your XOR as a macro. A function won't do, since the sequencing will not apply to function's arguments.

Someone might say though, that the only reason of having a sequence point at each && and || is to support the short-circuited evaluation, and thus XOR does not need one. This makes sense, actually. Yet, it is worth considering having a XOR with a sequence point in the middle. For example, the following expression

++x > 1 && x < 5

has defined behavior and specificed result in C/C++ (with regard to sequencing at least). So, one might reasonably expect the same from user-defined logical XOR, as in

XOR(++x > 1, x < 5)

while a !=-based XOR doesn't have this property.