ios - Objective C NSString* property retain count oddity

Question

I have the following example class:

Test.h:

@interface Test : UIButton {
    NSString *value;
}
- (id)initWithValue:(NSString *)newValue;
@property(copy) NSString *value;

Test.m:

@implementation Test
@synthesize value;
- (id)initWithValue:(NSString *)newValue {
    [super init];   
    NSLog(@"before nil value has retain count of %d", [value retainCount]);
    value = nil;
    NSLog(@"on nil value has retain count of %d", [value retainCount]);
    value = newValue;
    NSLog(@"after init value has retain count of %d", [value retainCount]);
    return self;
}

Which produces the following output:

2008-12-31 09:31:41.755 Concentration[18604:20b] before nil value has retain count of 0
2008-12-31 09:31:41.756 Concentration[18604:20b] on nil value has retain count of 0
2008-12-31 09:31:41.757 Concentration[18604:20b] after init value has retain count of 2147483647

I am calling it like:

Test *test = [[Test alloc] initWithValue:@"some text"];

Shouldn't value have a retain count of 1? What am I missing?

Thanks for your help.

Answer 1

Don't look at retain counts. They're not useful and will only mislead you — you can't be certain that nothing else is retaining an object, that an object you get from somewhere isn't shared.

Instead, concentrate on object ownership and follow the Cocoa memory management rules to the letter. That way your memory management will be correct no matter what optimizations Cocoa may be doing behind the scenes for you. (For example, implementing -copy as just -retain for immutable objects.)

Furthermore, it's critical to understand the difference between properties of your objects and instance variables within your objects. In your question's code, you are assigning a value to an instance variable. That instance variable is just that: a variable. Assigning to it will behave like any other variable assignment. To use the property, you must use either dot syntax or bracket syntax to actually invoke the property's setter method:

self.value = newValue;     // this is exactly equivalent to the next line
[self setValue:newValue];  // this is exactly equivalent to the previous line

The code generated for the dot syntax and the bracket syntax is identical, and neither will access the instance variable directly.