Perform an assignment and an operation using a ternary operator + && in Objective-C?

Question

In the name of ternary bliss (and for a disdain of the verbose)... I am hoping, and am somewhat surprised that....

BOOL isItOpen = YES;
isItOpen = (isItOpen ? YES : NO); // yes, dumbie, it's open.

works fine… but that…

isItOpen = (isItOpen ? [it close] && NO : [it open] && YES);

results in Invalid operands to binary expression ('void' and 'int')

I can't seem to track down a simple yes or no as to whether one can conditionally chain operations with && (or || ), like one does in say, BASH or PHP. I tried various combinations of & and && arrangements, to no avail.. as I am a C idiot... but if this "way of doing it" is NOT possible, linguistically… is there another - that is as concise? (ie, no if s involed?)

Answer 1

The C (and by extension, C++ and Objective-C ¹ ) operators form expressions ; they're designed to evaluate to a value, rather than control program flow.

So whilst ?: , && and || all offer short-circuit evaluation of their arguments, you can't use them to conditionally call arbitrary functions; ² you should use traditional control-flow constructs (ie if ) for that.

You could use the little-known comma operator to achieve this, but I strongly recommend that you don't , because it's highly unidiomatic, and difficult to read. eg:

isItOpen = condition ? (func1(), NO) : (func2(), YES);

---

1. _{Actually, I don't know Objective-C. For all know, it might be possible!}
2. _{And by "arbitrary", I mean functions that return void , or a type that's not implicitly convertible to bool in the case of && and || , or a non-matching type in the case of ?: .}

Answer 2

Your code would work fine as long as the close and open methods you are executing on it return boolean values. Otherwise, no cigar.

Answer 3

If you want to use the ternary operator, you can do like this:

isItOpen ? ([it close], isItOpen = NO) : ([it open], isItOpen = YES);

Or:

isItOpen ? [it close] : [it open];
isItOpen = !isItOpen;

But this is not a good programming style and you should avoid it.

The following code is much more readable (at least by a C/C++/Objective-C programmer):

if (isItOpen)
    [it close];
else
    [it open];
isItOpen = !isItOpen;

In order of preference, I recommend you use the third version of the code, then the second, then the first.

Answer 4

The difference you are experiencing is due to Objective-C having:

(a) true procedures (void functions/methods) which return no value; and (b) a stronger type system than PHP

In your example the primary problem is (a) - you are calling methods which return nothing , and nothing is not a boolean value.

In PHP functions always return something , a function defined as returning void is actually defined as returning a "useless" value. However PHP will convert just about anything to anything (and does so inconsistently, for added "fun"), so a "useless" value has a boolean value - though what that is probably depends on the phase of the moon ;-) This feature does mean that you can reliably chain a "void" function after one which returns a value - <expr convertible to boolean> && <"void" function> will work in PHP (but the resulting boolean value is arbitrary). The same thing will not work in Objective-C (do not try to fix it with the comma operator, there are hidden traps with that operator).

So provided you stick with functions/methods which return either a boolean, or a type implicitly or explicitly convertible to boolean (eg for pointer types nil is false, other values true; for integral types 0 is false, everything else true) you can "conditionally chain" operations. Whether you should do this is a different question...

PS If you want to be confusing, this is short:

(isItOpen = !isItOpen) ? [it open] : [it close];

which will make most folks do a double-take ;-)