Does Objective-C use short-circuit evaluation?

问题

I tried something along the lines of:

if(myString != nil && myString.length) { ... }

And got:

-[NSNull length]: unrecognized selector sent to instance

Does Objective-C not short-circuit after the first condition fails?

回答1:

Objective-C does support short-circuit evaluation, just like C.

It seems that in your example myString is NSNull and not nil, therefore myString != nil is true.

NSNull is a singleton and is used to represent nil where only objects are allowed, for example in an NSArray.

Btw, normally, people write if (!myString && myString.length == 0). Comparing to nil is quite ugly. Also, I'd compare the length to 0. That seems to be more clear.

回答2:

Objective-C is a strict superset of C.

Because C supports short-circuit evaluation, Objective-C does as well.

回答3:

What is NSNull defined as? If it is an object that is supposed to represent nothing, than it would not be nil. in other words, NSNull and nil aren't the same.

回答4:

If you have an NSNull somewhere, you are probably either using a JSON parser or CoreData. When a number in CoreData is not set, CoreData will give you back NSNull - possibly the same goes for NSString values in CoreData too.

Similarly, you can have empty elements in JSON returned from a server and some parsers will give you that as an NSNull object. So in both cases, you have to be careful when you are using values since the thing you thought was an NSString or NSNumber object is really NSNull.

One solution is to define a category on NSNull that simply ignores all non-understood messages sent to the object, as per the code below. Then the code you have would work because NSNull.length would return 0. You can include something like this in your project .pch file, which gets included in every single file in your project.

// NSNull+IgnoreMessages.h
@interface NSNull(IgnoreMessages) 
- (void)forwardInvocation:(NSInvocation *)anInvocation;
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector;
@end

//NSNull+IgnoreMessages.m
#import "NSNull+IgnoreMessages.h"
@implementation NSNull(IgnoreMessages)
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    if ( [self respondsToSelector:[anInvocation selector]] )
      [anInvocation invokeWithTarget:self];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
    NSMethodSignature *sig=[[NSNull class] instanceMethodSignatureForSelector:aSelector];
        // Just return some meaningless signature
    if(sig==nil)
      sig=[NSMethodSignature signatureWithObjCTypes:"@^v^c"];

    return sig;
}
@end

来源：https://stackoverflow.com/questions/2060741/does-objective-c-use-short-circuit-evaluation

标签

objective-c

short-circuiting