Why nilObject isEqual nilObject return no?

Question

When I use this code:

    NSString *a = nil;
    NSString *b = nil;
    if([a isEqual:b]){
        NSLog(@"YES");
    }
    else{
        NSLog(@"NO");
    }

The console print "NO" I don't understand this behavior. Could you explain me ?

Answer 1

The rules for sending messages to nilare as follows:

(Source: https://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/Chapters/ocObjectsClasses.html#//apple_ref/doc/uid/TP30001163-CH11-SW7)

• If the method returns any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0.
• If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the struct. Other struct data types will not be filled with zeros.
• If the method returns anything other than the aforementioned value types, the return value of a message sent to nil is undefined.

(Thanks @Jim)

So for example, if you do this:

if(![nil someMessageThatAlwaysReturnsTrue]) {
   NSLog(@"Watch this.");
}

It will print out "Watch this" every time.

Answer 2

See Sending Messages to nil in The Objective-C Programming Language. When you send a message to an object that is nil and the method returns an object, the expression evaluates to nil itself, which is equivalent to 0, which is equivalent to NO.