Objective C: retain vs alloc

Question

I have a singleton class with this code:

manager.h

@interface Manager : NSObject {
  NSString *jobsLimit;
  NSMutableDictionary *jobTitles;
}

@property (nonatomic, retain) NSString *jobsLimit;
@property (nonatomic, assign) NSMutableDictionary *jobTitles;


@implementation Manager

@synthesize jobsLimit;
@synthesize jobTitles;

+ (id)sharedManager {
    @synchronized(self) {
        if(shared == nil)
            shared = [[super allocWithZone:NULL] init];
    }
    return shared;
}

- (id)init {
    if (self = [super init]) {  
        jobsLimit = [[NSString alloc] initWithString:@"50"];
        jobTitles = [[NSMutableDictionary alloc] init];
    }
    return self;
}

Then in the code i'm assigning these variables like this:

 self.jobsLimit = [NSString stringWithFormat:@"%d", progressAsInt];
 [self.jobTitles addEntriesFromDictionary:anotherDictionary];


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

Now my question is this code correct? Is the assignment correct?
I'm very confused with when to use alloc and/or retain. Do I need to use alloc if my property is already retained? and If I use alloc should my property be assign?
What will be the reference count now for these variables and will they be dealloc'd/under-dealloc'd when the dealloc is called?
Also for singleton classes do I need to initialize my ivars as in the init method above or I do not have to.

I'd appreciate if someone can help me clear this confusion out and thanks in advance.

Regards,

score 2 · Answer 1 · edited May 23 '17 at 11:55

2

I guess you need a lot of this : IOS Memory Management

and a bit of : What should my Objective-C singleton look like?

good lectures !

edited May 23 '17 at 11:55

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answered Dec 21 '10 at 17:31

VdesmedT

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David Gelhar · Accepted Answer · 2010-12-21T17:41:02.100

Your code actually looks correct, but perhaps some explanation is in order, since it sounds like you're a little unsure.

When you assign to a property that has retain semantics using the "." syntax, the accessor method calls retain. The "." syntax is just shorthand for invoking the accessor method, so

self.jobsLimit = [NSString stringWithFormat:@"%d", progressAsInt];

is exactly the same as

[self setJobsLimit:[NSString stringWithFormat:@"%d", progressAsInt]];

That works out to:

create an (autoreleased) string with a numeric value
retain the string (you now own it) and assign it to jobsLimit

If, on the other hand, you assign to the iVar directly (not using the "."-accessor), the setter method is not called. For example:

jobsLimit = [[NSString alloc] initWithString:@"50"];

That is:

allocate a string (you own it), with value "50"
assign it to jobsLimit

Either way, you now own the string referred to by jobsLimit, and are responsible for eventually releasing it (e.g., in your dealloc method).

Thanks a lot David, but now the alloc'd string in "[[NSString alloc] .." will it ever gets dealloc'd since now jobsLimit is pointing to another string "[NString stringWith ..."??? — mshaaban, Dec 21 '10 at 18:55
As long as you use the accessor method (self.jobsLimit = ...) to assign, you'll be ok: the accessor method calls `release` on the old value before assigning the new one. — David Gelhar, Dec 22 '10 at 05:39

Objective C: retain vs alloc

2 Answers2