Objective-C && doesn't short circuit for !var - objective-c

Question

Try running this:
UIView *testView = nil;
NSLog(#"Take 1");
NSString *message = #"view doesn't exist";
if (!testView && !testView.subviews) {
message = #"this message should never appear" ;
}
NSLog(message);
NSLog(#"Take 2");
message = #"view doesn't exist";
if (testView != nil && testView.subviews != nil) {
message = #"this message should never appear" ;
}
NSLog(message);
NSLog(#"Take 3");
message = #"view doesn't exist";
if (!testView) {
if (!testView.subviews) {
message = #"this message should never appear" ;
}
}
NSLog(message);
NSLog(#"Take 4");
message = #"view doesn't exist";
if (testView != nil) {
if (testView.subviews != nil) {
message = #"this message should never appear" ;
}
}
NSLog(message);
Output I get is:
Take 1
this message should never appear
Take 2
view doesn't exist
Take 3
this message should never appear
Take 4
view doesn't exist
Why doesn't Obj-C short circuit for !testView (in Take 1)?
Why does it go into !testView when testView is clearly nil in Take 3?
Should I not be testing the function of a nil object (e.g. when I test for subviews)?

Answer 1

The output you see is correct and the short-circuit behavior is working correctly too.
When boolean expressions are evaluated the result is considered to be true if the expression is not 0 or false if it is 0. So everywhere where you have if (something) you can read this as if (something != 0). The ! operator is the negation, so if you expand it you get the following for your first case: !(testView != 0) && !(testView.subviews != 0). The double negation can be removed and you get (testView == 0) && (testView.subviews == 0) which obviously is true (nil is 0 too).
There the short-circuiting is also correctly applied, you just can't see it. To prove that you could use a little wrapper function for your tests:
id testFunc( id value ) {
NSLog(#"testFunc: %#", value );
return value;
}
And use that in your tests: if (!testFunc(testView) && !testFunc(testView.subviews))
To make it short, your assumptions about the boolean not operator ! are wrong. It goes into if (!testView) because testView is nil.

Answer 2

There's nothing strange going on here. I rewrote your code, substituting testView for nil and testView.subviews for nil (messages to nil return nil).
NSLog(#"Take 1");
NSString *message = #"if clause was false";
if (!nil && !nil) { // if(true && true)
message = #"if clause was true" ;
}
NSLog(message); // outputs "if clause was true"
NSLog(#"Take 2");
message = #"if clause was false";
if (nil != nil && nil != nil) { // if (false && false)
message = #"if clause was true" ;
}
NSLog(message); // outputs "if clause was false"
NSLog(#"Take 3");
message = #"if clause was false";
if (!nil) { // if (true)
if (!nil) { // if (true)
message = #"if clause was true" ;
}
}
NSLog(message); // outputs "if clause was true"
NSLog(#"Take 4");
message = #"if clause was false";
if (nil != nil) { // if (false)
if (nil != nil) { // if (false)
message = #"if clause was true" ;
}
}
NSLog(message); // outputs "if clause was false"
If you want to know if the view exists, you can use something like:
if (testView) {
NSLog(#"View exists a.k.a. testView != nil");
} else {
NSLog(#"View doesn't exist a.k.a. testView == nil");
}
I'd recommend just using if (testView) and if (!testView) to check for the existence of objects. This is the standard in the Objective-C community and reads more clearly.