Program 19.5 Why retain an NSString when it holds a constant string object?

[fav321]

   

Code: (Objective-C)

 -(id) initWithCoder: (NSCoder *) decoder  
    {  
       name = [[decoder decodeObjectforKey: @"AddressCardName"] retain];  
       email = [[decoder decodeObjectforKey: @"AddressCardEmail"] retain];  
      
       return self;  
    }  

Since the instance variables 'name' and 'email' are NSStrings why are we worried about retaining them?

Here is a line from chapter 17...

Constant strings use no reference-counting mechanism because they can never be
released.This is why when the retainCount message is sent to myStr1, it returns a value
of 0xfffffffffffffff.

Any ideas?

[skochan]

The values that are returned by the decoder methods are autoreleased and so must be retained (this is critical for an iOS app).   By the time these values have been archived and unarchived they're no longer "constant" string objects.

Cheers,

Steve

[fav321]

Thanks Steve.

I was under the understanding that an NSString object was always a Constant Character String but after some testing I've realized that is not the case.

As in the following code example.

Code: (Objective-C)

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
NSString *str1 = @"String One";
NSString *str2 = [NSString stringWithCString: (char *)"hello"];
NSString *str3 = [[NSString alloc] initWithString: @"String 3"];
NSString *str4 = [NSString stringWithFormat: @"%s and %i", (char *)"Happy New Year ", 2012]; 

NSLog(@"str1: Retain Count: %lx", [str1 retainCount]);
NSLog(@"str2: Retain Count: %lx", [str2 retainCount]);
NSLog(@"str3: Retain Count: %lx", [str3 retainCount]);
NSLog(@"str4: Retain Count: %lx", [str4 retainCount]);

[pool drain];
    return 0;
}

Produces the following output:

[Session started at 2012-01-06 15:02:51 -0600.]
2012-01-06 15:02:51.537 Practice[4069:10b] str1: Retain Count: 7fffffff
2012-01-06 15:02:51.559 Practice[4069:10b] str2: Retain Count: 1
2012-01-06 15:02:51.568 Practice[4069:10b] str3: Retain Count: 7fffffff
2012-01-06 15:02:51.571 Practice[4069:10b] str4: Retain Count: 1


So depending how the NSString object was created will determine whether or not it is treated as a Constant Character String Object.

It is interesting to note that even though str3 was created using alloc and init that it still doesn't need to be released.

Thanks for the help. Great book by the way.

[skochan]

Any alloc'ed object should still be released.  You are relying on the underlying implementation details, which are not documented and are subject to change.  So it's safest to always follow the rule of releasing any object you create with alloc, copy, mutableCopy, or new.

Cheers,

Steve