Question

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

Answer 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.