object-C class property should add "self" while assign its value?

Question

I am new here, and already searched related articles like "Is “self” necessary?" and "Setting an Objective-C class property without using a self reference" However i still can't get a clear answer which can explain my case.

I have a simple class and my UI has two textfield and one button, here is the code:

@interface testViewController : UIViewController {
    NSString       *teststring_A;
    NSString       *teststring_B;
    IBOutlet UITextField *textfield_1;
    IBOutlet UITextField *textfield_2;
}
@property (nonatomic, retain) NSString *teststring_A;
@property (nonatomic, retain) NSString *teststring_B;
- (IBAction)action1:(id)sender;
- (IBAction)action2:(id)sender;
@end

@implementation testViewController
@synthesize teststring_A;
@synthesize teststring_B;

- (void)dealloc {
    [super dealloc];
}
- (IBAction)action1:sender
{
    teststring_A = textfield_1.text ;
    NSLog(@"teststring_A in action 1 is : %@\n", teststring_A);

    teststring_B = textfield_2.text ;
    NSLog(@"teststring_B in action 1 is : %@\n", teststring_B);
}
- (IBAction)action2:(id)sender
{
 NSLog(@"teststring_A in action 2 is : %@\n", teststring_A);
 NSLog(@"teststring_B in action 2 is : %@\n", teststring_B);
}
 

the output is :

2010-11-19 15:32:14.827 test[419:207] teststring_A in action 1 is : 123
2010-11-19 15:32:14.829 test[419:207] teststring_B in action 1 is : 456
2010-11-19 15:32:14.927 test[419:207] teststring_A in action 2 is : 123
2010-11-19 15:32:14.929 test[419:207] teststring_B in action 2 is : {(
    >
)}

 

And when click button, it triggers action1 first then action2. My problem is... in action2, the value of teststring_B becomes incorrect, sometimes the application even crashes. What confuses me is (1) why is the value of teststring_A correct??? (2) teststring_B is assigned by textfield_2.text which is not created with 'alloc', so suppose the pointer should exist all the time. then why teststring_B's value becomes incorrect in action2 ??? (3) in dealloc, I should release teststring_A and teststring_B, right? (i think so )

All I know is if I add 'self.', like 'self.teststring_B = textfield_2.text;' then there won't be problem. the value will be correct. So I would like to know the technical reason.

Answer 1

You are confusing variables and properties. Properties are backed by variables, but in reality they are methods.

Here, you define a property named teststring_B which retains anything that gets assigned to it (and releases the old value). The equivalent methods would look like this (simplified):

- (NSString *)teststring_B {
    // Return the content of the variable teststring_B.
    return teststring_B;
}

- (void)setTeststring_B:(NSString *)val {
    // Retain new value.
    [val retain];
    // Release old value in variable teststring_B
    [teststring_B release];
    // Assign the new, retained value to variable teststring_B
    teststring_B = val;
}

You can now use the property in two ways: either with [self setTeststring_B:foo]; or with self.teststring_B = foo;. The important point is that the later is just a convenient way of writing, the compiler will translate it into the first form, that is the compiler will turn the self.foo = bar; lines into [self setFoo:bar];.

Now that we have this explained, on to your crash: you've got a string value which is most likely autoreleased. Now you just plain assign it to the variable teststring_B, not the property. And you forgot to retain the value. The property would have retained that value for you.

Now the assigned value was autoreleased (it didn't know you've got a variable still pointing to it) and later a new object came to life at the exact same memory location (if you're lucky). In any case, the teststring_B variable is now not pointing to the text as you thought it would, but to some random object (or to garbage).

There are two ways to fix this:

// First retain, then release; it might be the same object
// and if you would release it first its retain count might
// drop to 0 and get cleaned up before you can retain it again.
NSString *tmp = [textfield_2.text retain];
[teststring_B release];
teststring_B = tmp;

// Better !
self.teststring_B = textfield_2.text;

Answer 2

Accessing the variable directly without using the self will not retain it. So when you accessing it later the variable got auto-released and makes your application crash.

so you can write

1) [self setTeststring_B:textfield_2.text]; or
2) the dot syntax self.teststring_B = textfield_2.text; or
3) teststring_b = [textfield_2.text retain]

Answer 3

What you are doing now is simple assignment. That can cause a crash if the object teststring_A or teststring_B point to is deallocated; this is also called a dangling reference.

The reason only simple assignment is happening is because you aren't accessing the setters through the @property semantics; you could get a retain on those NSString objects by doing self.teststring_A = textfield_1.text instead.

However, you should be using copy with NSString properties. See: NSString property: copy or retain?

In other words, you want this:

@property (nonatomic, copy) NSString *teststring_A;
@property (nonatomic, copy) NSString *teststring_B;

and this:

self.teststring_A = textfield_1.text ;
self.teststring_B = textfield_1.text ;