Question

I have the next piece of code, one iVar with this property retained and released in it’s class dealloc method. The iVar is used in 2 methods and continually change the value but
sometimes when I use the value is corrupted. Why is that?

.h

@interface ChatController : NSObject <ASIHTTPRequestDelegate>{
NSTimer *timer;
NSString *_idLastMessageFromServer;
}

@property(nonatomic, retain)NSString *idLastMessageFromServer;
@end

.m

@implementation ChatController

@synthesize idLastMessageFromServer = _idLastMessageFromServer;

- (void)initLoopTimer{
timer = [NSTimer timerWithTimeInterval:5 target:self selector:@selector(update:) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
}

- (void)update:(id)sender{
ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:CONSTANT_YYYY];
[request setDelegate:self];
[request addPostValue:_idLastMessageFromServer forKey:CONSTANT_XXX];
[request setDidFinishSelector:@selector(requestUpdateFinish:)];
[request startAsynchronous];
}

- (void)requestUpdateFinish:(ASIHTTPRequest *)request{
NSString *response = [request responseString];
if(response && response.length){
    if(![response isEqualToString:CHAT_RESPONSE_NO_MESSAGES]){
        NSArray *array = [response componentsSeparatedByString:CHAT_PARSE_RESPONSE];
        if(array && [array count] == 2){
            **_idLastMessageFromServer = [array objectAtIndex:0];**
        }
     }
   }
}

But when the loop calls the method update:, it crashes in this line of code

[request addPostValue:_idLastMessageFromServer forKey:CONSTANT_XXX];

with EXC_BAD_ACCESS message, but why?

Answer 1

By using _idLastMessageFromServer instead of self.idLastMessageFromServer, you are not retaining the string. The allows the retain count to drop to zero which deallocates the object. At that point you have a reference to bad memory, hence the app crashes.

Don’t use iVars directly unless you have a good reason (like -init or -dealloc). Use the property instead.

 [request addPostValue:self.idLastMessageFromServer forKey:CONSTANT_XXX];

and

self.idLastMessageFromServer = [array objectAtIndex:0];

---

I’ll add a bit more of a detailed explanation about properties.

self.idLastMessageFromServer when used to read the value of the property calls an auto generated method -idLastMessageFromServer. This method will look something like:

- (NSString *)idLastMessageFromServer
{
    return _idLastMessageFromServer;
}

self.idLastMessageFromServer when used to set the value of the property calls an auto generated method -setIdLastMessageFromServer:. This method will look something like:

- (void)setIdLastMessageFromServer:(NSString *)idLastMessageFromServer
{
    if (_idLastMessageFromServer != idLastMessageFromServer) {
        [_idLastMessageFromServer release];
        _idLastMessageFromServer = idLastMessageFromServer;
        [_idLastMessageFromServer retain];
    }
}

One final note: be sure to release _idLastMessageFromServer in your -dealloc method. Something like:

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

---

More details about properties and iVars.

Properties (like self.idLastMessageFromServer) are simply a easy way to handle getter and setter methods. They cannot hold data because they are methods. iVars (like _idLastMessageFromServer) are a pointer to a location in memory. They cannot control access and maintain state because they are simply a pointer.

Properties and iVars work together.

The line @property(nonatomic, retain) NSString *idLastMessageFromServer; says that somewhere in my implementation, my class will have a getter and setter for the property idLastMessageFromServer.

The line @synthesize idLastMessageFromServer = _idLastMessageFromServer; auto generates the getter and setter methods for idLastMessageFromServer using the iVar _idLastMessageFromServer.

In short, the property controls access to the iVar; the iVar is the storage location for the property.