Calling a method on an uninitialized object (null

问题:

1. What is the normal behavior in Objective-C if you call a method on an object (pointer) that is nil (maybe because someone forgot to initialize it)? Shouldn't it generate some kind of an error (segmentation fault, null pointer exception...)?
2. If this is normal behavior, is there a way of changing this behavior (by configuring the compiler) so that the program raises some kind of error / exception at runtime?

To make it more clear what I am talking about, here's an example.

Having this class:

@interface Person : NSObject {

    NSString *name;

}

@property (nonatomic, retain) NSString *name;

- (void)sayHi;

@end

with this implementation:

@implementation Person

@synthesize name;

- (void)dealloc {
    [name release];
    [super dealloc];
}

- (void)sayHi {
    NSLog(@"Hello");
    NSLog(@"My name is %@.", name);
}

@end

Somewhere in the program I do this:

Person *person = nil;
//person = [[Person alloc] init]; // let's say I comment this line
person.name = @"Mike";            // shouldn't I get an error here?
[person sayHi];                   // and here
[person release];                 // and here

回答1:

A message sent to a nil object is perfectly acceptable in Objective-C, it's treated as a no-op. There is no way to flag it as an error because it's not an error, in fact it can be a very useful feature of the language.

From the docs:

Sending Messages to nil

In Objective-C, it is valid to send a message to nil—it simply has no effect at runtime. There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid:

• If the method returns an object, then a message sent to nil returns 0 (nil), for example:

  Person *motherInLaw = [[aPerson spouse] mother];

  If aPerson’s spouse is nil, then mother is sent to nil and the method returns nil.

• 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 data structure. 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.

回答2:

From Greg Parker's site:

If running LLVM Compiler 3.0 (Xcode 4.2) or later

Messages to nil with return type | return
Integers up to 64 bits           | 0
Floating-point up to long double | 0.0
Pointers                         | nil
Structs                          | {0}
Any _Complex type                | {0, 0}

回答3:

One thing you should be clear on is that in Objective-C, you don't call a method on an object, you send a message to an object. The runtime will find the method and call it.

Since the first versions of Objective-C, a message to nil has always been a safe no-op that returns nil. There's a lot of code that depends on this behavior.