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What do atomic and nonatomic mean in property declarations?

@property(nonatomic, retain) UITextField *userName;

@property(atomic, retain) UITextField *userName;

@property(retain) UITextField *userName;

What is the functional difference between these 3?

Answered By: bbum ( 637)

The last two are identical; "atomic" is the default behavior (note that it is not actually a keyword; it is specified only by the absence of nonatomic -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

I am using ARC successfully in my project. However, I have encountered a few files (e.g., in unit tests and mock objects) where the rules of ARC are a little more fragile right now. I recall hearing that there was a way to disable ARC on a per-file basis, though I have been unable to find this option.

Is this possible? How do I disable ARC on a per-file basis?

Answered By: jaminguy ( 918)

It is possible to disable ARC for individual files by adding the -fno-objc-arc compiler flag for those files.

You add compiler flags in Targets -> Build Phases -> Compile Sources. You have to double click on the right column of the row under Compiler Flags. You can also add it to multiple files by holding the cmd button to select the files and then pressing enter to bring up the flag edit box.

I created a sample project that has an example:


See this answer for more info: Disable Automatic Reference Counting for Some Files


I don't think I fundamentally understand what an enum is, and when to use it.

For example:

typedef enum {
} ShapeType;

What is really being declared here?

Answered By: Adam Rosenfield ( 888)

Three things are being declared here: an anonymous enumerated type is declared, ShapeType is being declared a typedef for that anonymous enumeration, and the three names kCircle, kRectangle, and kOblateSpheroid are being declared as integral constants.

Let's break that down. In the simplest case, an enumeration can be declared as

enum tagname { ... };

This declares an enumeration with the tag tagname. In C and Objective-C (but not C++), any references to this must be preceded with the enum keyword. For example:

enum tagname x;  // declare x of type 'enum tagname'
tagname x;  // ERROR in C/Objective-C, OK in C++

In order to avoid having to use the enum keyword everywhere, a typedef can be created:

enum tagname { ... };
typedef enum tagname tagname;  // declare 'tagname' as a typedef for 'enum tagname'

This can be simplified into one line:

typedef enum tagname { ... } tagname;  // declare both 'enum tagname' and 'tagname'

And finally, if we don't need to be able to use enum tagname with the enum keyword, we can make the enum anonymous and only declare it with the typedef name:

typedef enum { ... } tagname;

Now, in this case, we're declaring ShapeType to be a typedef'ed name of an anonymous enumeration. ShapeType is really just an integral type, and should only be used to declare variables which hold one of the values listed in the declaration (that is, one of kCircle, kRectangle, and kOblateSpheroid). You can assign a ShapeType variable another value by casting, though, so you have to be careful when reading enum values.

Finally, kCircle, kRectangle, and kOblateSpheroid are declared as integral constants in the global namespace. Since no specific values were specified, they get assigned to consecutive integers starting with 0, so kCircle is 0, kRectangle is 1, and kOblateSpheroid is 2.

I would like to check to see if I have an Internet connection on the iPhone using the Cocoa Touch libraries.

I came up with a way to do this using an NSURL. The way I did it seems a bit unreliable (because even Google could one day be down and relying on a 3rd party seems bad) and while I could check to see for a response from some other websites if Google didn't respond, it does seem wasteful and an unnecessary overhead on my application.

- (BOOL) connectedToInternet
    NSString *URLString = [NSString stringWithContentsOfURL:[NSURL URLWithString:@""]];
    return ( URLString != NULL ) ? YES : NO;

Is what I have done bad? (Not to mention stringWithContentsOfURL is deprecated in 3.0) And if so what is a better way to accomplish this?

Answered By: iWasRobbed ( 528)

Use a simple library to do it: (ARC and GCD Compatible Reachability)

OR... Do it yourself:

1) Add SystemConfiguration framework to the project but don't worry about including it anywhere

2) Add Reachability.h and Reachability.m to the project (you can get those here)

3) Add @class Reachability; to the .h file of where you are implementing the code

4) Create a couple instances to check in the interface section of the .h file:

Reachability* internetReachable;
Reachability* hostReachable;

5) Add a method in the .h for when the network status updates:

-(void) checkNetworkStatus:(NSNotification *)notice;

6) Add #import "Reachability.h" to the .m file where you are implementing the check

7) In the .m file of where you are implementing the check, you can place this in one of the first methods called (init or viewWillAppear or viewDidLoad etc):

-(void) viewWillAppear:(BOOL)animated
    // check for internet connection
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkNetworkStatus:) name:kReachabilityChangedNotification object:nil];

    internetReachable = [[Reachability reachabilityForInternetConnection] retain];
    [internetReachable startNotifier];

    // check if a pathway to a random host exists
    hostReachable = [[Reachability reachabilityWithHostName: @""] retain];
    [hostReachable startNotifier];

    // now patiently wait for the notification

8) Set up the method for when the notification gets sent and set whatever checks or call whatever methods you may have set up (in my case, I just set a BOOL)

-(void) checkNetworkStatus:(NSNotification *)notice
    // called after network status changes
    NetworkStatus internetStatus = [internetReachable currentReachabilityStatus];
    switch (internetStatus)
        case NotReachable:
            NSLog(@"The internet is down.");
            self.internetActive = NO;

        case ReachableViaWiFi:
            NSLog(@"The internet is working via WIFI.");
            self.internetActive = YES;

        case ReachableViaWWAN:
            NSLog(@"The internet is working via WWAN.");
            self.internetActive = YES;


    NetworkStatus hostStatus = [hostReachable currentReachabilityStatus];
    switch (hostStatus)
        case NotReachable:
            NSLog(@"A gateway to the host server is down.");
            self.hostActive = NO;

        case ReachableViaWiFi:
            NSLog(@"A gateway to the host server is working via WIFI.");
            self.hostActive = YES;

        case ReachableViaWWAN:
            NSLog(@"A gateway to the host server is working via WWAN.");
            self.hostActive = YES;


9) In your dealloc or viewWillDisappear or similar method, remove yourself as an observer

-(void) viewWillDisappear:(BOOL)animated
    [[NSNotificationCenter defaultCenter] removeObserver:self];

Note: There might be an instance using viewWillDisappear where you receive a memory warning and the observer never gets unregistered so you should account for that as well.


It is to my understanding that one should use a forward-class declaration in the event ClassA needs to include a ClassB header, and ClassB needs to include a ClassA header to avoid any circular inclusions. I also understand that an #import it a simple ifndefso that an include only happens.

My inquiry is this: When does one use #import and when does one use @class? Sometimes if I use a @class declaration, I see a common compiler warning such as the following:

warning: receiver 'FooController' is a forward class and corresponding @interface may not exist.

Would really love to understand this, versus just removing the @class forward-declaration and throwing an #import in to silence the warnings the compiler is giving me.

Answered By: Ben Gottlieb ( 459)

If you see this warning:

warning: receiver 'myCoolClass' is a forward class and corresponding @interface may not exist

you need to #import the file, but you can do that in your implementation file (.m), and use the @class declaration in your header file.

@class does not (usually) remove the need to #import files, it just moves the requirement down closer to where the information is useful.

For Example

If you say @class myCoolClass, the compiler knows that it may see something like:

myCoolClass *myObject;

It doesn't have to worry about anything other than myCoolClass is a valid class, and it should reserve room for a pointer to it (really, just a pointer). Thus, in your header, @class suffices 90% of the time.

However, if you ever need to create or access myObject's members, you'll need to let the compiler know what those methods are. At this point (presumably in your implementation file), you'll need to #import "myCoolClass.h", to tell the compiler additional information beyond just "this is a class".

I'm developing a Cocoa app, and I'm using constant NSStrings as ways to store key names for my preferences.

I understand this is a good idea because it allows easy changing of keys if necessary. Plus, it's the whole 'separate your data from your logic' notion.

Anyway, is there a good way to make these constants defined once for the whole app? I'm sure that there's an easy and intelligent way, but right now my classes just redefine the ones they use.

Answered By: Barry Wark ( 532)

You should create a header file like

// Constants.h
FOUNDATION_EXPORT NSString *const MyFirstConstant;
FOUNDATION_EXPORT NSString *const MySecondConstant;

You can include this file in each file that uses the constants or in the pre-compiled header for the project.

You define these constants in a .m file like

// Constants.m
NSString *const MyFirstConstant = @"FirstConstant";
NSString *const MySecondConstant = @"SecondConstant";

Constants.m should be added to your application/framework's target so that it is linked in to the final product.

The advantage of using string constants instead of #define'd constants is that you can test for equality using pointer comparison (stringInstance == MyFirstConstant) which is much faster than string comparison ([stringInstance isEqualToString:MyFirstConstant]) (and easier to read, IMO).

I know about the HIG (which is quite handy!), but what programming practices do you use when writing Objective-C, and more specifically when using Cocoa (or CocoaTouch).

Answered By: Kendall Helmstetter Gelner ( 400)

There are a few things I have started to do that I do not think are standard:

1) With the advent of properties, I no longer use "_" to prefix "private" class variables. After all, if a variable can be accessed by other classes shouldn't there be a property for it? I always disliked the "_" prefix for making code uglier, and now I can leave it out.

2) Speaking of private things, I prefer to place private method definitions within the .m file in a class extension like so:

#import "MyClass.h"

@interface MyClass ()
- (void) someMethod;
- (void) someOtherMethod;

@implementation MyClass

Why clutter up the .h file with things outsiders should not care about? The empty () works for private categories in the .m file, and issues compile warnings if you do not implement the methods declared.

3) I have taken to putting dealloc at the top of the .m file, just below the @synthesize directives. Shouldn't what you dealloc be at the top of the list of things you want to think about in a class? That is especially true in an environment like the iPhone.

3.5) In table cells, make every element (including the cell itself) opaque for performance. That means setting the appropriate background color in everything.

3.6) When using an NSURLConnection, as a rule you may well want to implement the delegate method:

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
                  willCacheResponse:(NSCachedURLResponse *)cachedResponse
      return nil;

I find most web calls are very singular and it's more the exception than the rule you'll be wanting responses cached, especially for web service calls. Implementing the method as shown disables caching of responses.

Also of interest, are some good iPhone specific tips from Joseph Mattiello (received in an iPhone mailing list). There are more, but these were the most generally useful I thought (note that a few bits have now been slightly edited from the original to include details offered in responses):

4) Only use double precision if you have to, such as when working with CoreLocation. Make sure you end your constants in 'f' to make gcc store them as floats.

float val = someFloat * 2.2f;

This is mostly important when someFloat may actually be a double, you don't need the mixed-mode math, since you're losing precision in 'val' on storage. While floating-point numbers are supported in hardware on iPhones, it may still take more time to do double-precision arithmetic as opposed to single precision. References:

On the older phones supposedly calculations operate at the same speed but you can have more single precision components in registers than doubles, so for many calculations single precision will end up being faster.

5) Set your properties as nonatomic. They're atomic by default and upon synthesis, semaphore code will be created to prevent multi-threading problems. 99% of you probably don't need to worry about this and the code is much less bloated and more memory-efficient when set to nonatomic.

6) SQLite can be a very, very fast way to cache large data sets. A map application for instance can cache its tiles into SQLite files. The most expensive part is disk I/O. Avoid many small writes by sending BEGIN; and COMMIT; between large blocks. We use a 2 second timer for instance that resets on each new submit. When it expires, we send COMMIT; , which causes all your writes to go in one large chunk. SQLite stores transaction data to disk and doing this Begin/End wrapping avoids creation of many transaction files, grouping all of the transactions into one file.

Also, SQL will block your GUI if it's on your main thread. If you have a very long query, It's a good idea to store your queries as static objects, and run your SQL on a separate thread. Make sure to wrap anything that modifies the database for query strings in @synchronize() {} blocks. For short queries just leave things on the main thread for easier convenience.

More SQLite optimization tips are here, though the document appears out of date many of the points are probably still good;


Are there any shortcuts to (stringByAppendingString:) string concatenation in Objective-C or shortcuts for working with NSString or other objects in general?

For example, I'd like to make

NSString *myString = @"This";
NSString *test = [myString stringByAppendingString:@" is just a test"];

something more like

string myString = "This";
string test = myString + " is just a test";
Answered By: Chris Blackwell ( 157)

Two answers I can think of... neither is particularly as pleasant as just having a concatenation operator.

First, use an NSMutableString, which has an appendString method, removing some of the need for extra temp strings.

Second, use an NSArray to concatenate via the componentsJoinedByString method.


With a huge influx of newbies to Xcode, I'm sure there are lots of Xcode tips and tricks to be shared.

What are yours?

Answered By: epatel ( 343)

Switch to Header/Source File

  • Option ⌥ Command ⌘ Up Arrow ↑

  • View > Switch to Header/Source File

Switches between the .m and .h files.

  • In Xcode 4 this is ctrl Command ⌘ Up Arrow ↑

My singleton accessor method is usually some variant of:

static MyClass *gInstance = NULL;

+ (MyClass *)instance
        if (gInstance == NULL)
            gInstance = [[self alloc] init];


What could I be doing to improve this?

Answered By: Robbie Hanson ( 209)

Another option is to use the +(void)initialize method. From the documentation:

The runtime sends initialize to each class in a program exactly one time just before the class, or any class that inherits from it, is sent its first message from within the program. (Thus the method may never be invoked if the class is not used.) The runtime sends the initialize message to classes in a thread-safe manner. Superclasses receive this message before their subclasses.

So you could do something akin to this:

static MySingleton *sharedSingleton;

+ (void)initialize
    static BOOL initialized = NO;
        initialized = YES;
        sharedSingleton = [[MySingleton alloc] init];