Revision 2.52

Mike Pinkerton
Greg Miller
Dave MacLachlan
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Objective-C is a very dynamic, object-oriented extension of C. It's designed to be easy to use and read, while enabling sophisticated object-oriented design. It is the primary development language for new applications on Mac OS X and the iPhone.

Cocoa is one of the main application frameworks on Mac OS X. It is a collection of Objective-C classes that provide for rapid development of full-featured Mac OS X applications.

Apple has already written a very good, and widely accepted, coding guide for Objective-C. Google has also written a similar guide for C++. This Objective-C guide aims to be a very natural combination of Apple's and Google's general recommendations. So, before reading this guide, please make sure you've read:

Note that all things that are banned in Google's C++ guide are also banned in Objective-C++, unless explicitly noted in this document.

The purpose of this document is to describe the Objective-C (and Objective-C++) coding guidelines and practices that should be used for all Mac OS X code. Many of these guidelines have evolved and been proven over time on other projects and teams. Open-source projects developed by Google conform to the requirements in this guide.

Google has already released open-source code that conforms to these guidelines as part of the Google Toolbox for Mac project (abbreviated GTM throughout this document). Code meant to be shared across different projects is a good candidate to be included in this repository.

Note that this guide is not an Objective-C tutorial. We assume that the reader is familiar with the language. If you are new to Objective-C or need a refresher, please read The Objective-C Programming Language .

They say an example is worth a thousand words so let's start off with an example that should give you a feel for the style, spacing, naming, etc.

An example header file, demonstrating the correct commenting and spacing for an @interface declaration

#import <Foundation/Foundation.h> // A sample class demonstrating good Objective-C style. All interfaces, // categories, and protocols (read: all top-level declarations in a header) // MUST be commented. Comments must also be adjacent to the object they're // documenting. // // (no blank line between this comment and the interface) @interface Foo : NSObject { @private NSString *_bar; NSString *_bam; } // Returns an autoreleased instance of Foo. See -initWithBar: for details // about |bar|. + (id)fooWithBar:(NSString *)bar; // Designated initializer. |bar| is a thing that represents a thing that // does a thing. - (id)initWithBar:(NSString *)bar; // Gets and sets |_bar|. - (NSString *)bar; - (void)setBar:(NSString *)bar; // Does some work with |blah| and returns YES if the work was completed // successfully, and NO otherwise. - (BOOL)doWorkWithBlah:(NSString *)blah; @end

An example source file, demonstrating the correct commenting and spacing for the @implementation of an interface. It also includes the reference implementations for important methods like getters and setters, init, and dealloc.

#import "Foo.h" @implementation Foo + (id)fooWithBar:(NSString *)bar { return [[[self alloc] initWithBar:bar] autorelease]; } // Must always override super's designated initializer. - (id)init { return [self initWithBar:nil]; } - (id)initWithBar:(NSString *)bar { if ((self = [super init])) { _bar = [bar copy]; _bam = [[NSString alloc] initWithFormat:@"hi %d", 3]; } return self; } - (void)dealloc { [_bar release]; [_bam release]; [super dealloc]; } - (NSString *)bar { return _bar; } - (void)setBar:(NSString *)bar { [_bar autorelease]; _bar = [bar copy]; } - (BOOL)doWorkWithBlah:(NSString *)blah { // ... return NO; } @end

Blank lines before and after @interface, @implementation, and @end are optional. If your @interface declares instance variables, a blank line should come after the closing brace (}).

Unless an interface or implementation is very short, such as when declaring a handful of private methods or a bridge class, adding blank lines usually helps readability.

Use only spaces, and indent 2 spaces at a time.

We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.

Each line of text in your code should try to be at most 80 characters long.

Strive to keep your code within 80 columns. We realize that Objective C is a verbose language and in some cases it may be more readable to extend slightly beyond 80 columns, but this should definitely be the exception and not commonplace.

If a reviewer asks that you reformat a line because they feel it can be fit in 80 columns and still be readable, you should do so.

We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.

You can make violations easier to spot in Xcode by going to Xcode > Preferences > Text Editing > Show page guide.

One space should be used between the - or + and the return type, and no spacing in the parameter list except between parameters.

Methods should look like this:

- (void)doSomethingWithString:(NSString *)theString { ... }

The spacing before the asterisk is optional. When adding new code, be consistent with the surrounding file's style.

If you have too many parameters to fit on one line, giving each its own line is preferred. If multiple lines are used, align each using the colon before the parameter.

- (void)doSomethingWith:(GTMFoo *)theFoo rect:(NSRect)theRect interval:(float)theInterval { ... }

When the first keyword is shorter than the others, indent the later lines by at least four spaces, maintaining colon alignment:

- (void)short:(GTMFoo *)theFoo longKeyword:(NSRect)theRect evenLongerKeyword:(float)theInterval error:(NSError **)theError { ... }
Method invocations should be formatted much like method declarations. When there's a choice of formatting styles, follow the convention already used in a given source file.

Invocations should have all arguments on one line:

[myObject doFooWith:arg1 name:arg2 error:arg3];

or have one argument per line, with colons aligned:

[myObject doFooWith:arg1 name:arg2 error:arg3];

Don't use any of these styles:

[myObject doFooWith:arg1 name:arg2 // some lines with >1 arg error:arg3]; [myObject doFooWith:arg1 name:arg2 error:arg3]; [myObject doFooWith:arg1 name:arg2 // aligning keywords instead of colons error:arg3];

As with declarations and definitions, when the first keyword is shorter than the others, indent the later lines by at least four spaces, maintaining colon alignment:

[myObj short:arg1 longKeyword:arg2 evenLongerKeyword:arg3 error:arg4];
The @public and @private access modifiers should be indented by 1 space.

This is similar to public, private, and protected in C++.

@interface MyClass : NSObject { @public ... @private ... } @end
Format exceptions with each @ label on its own line and a space between the @ label and the opening brace ({), as well as between the @catch and the caught object declaration.

If you must use Obj-C exceptions, format them as follows. However, see Avoid Throwing Exceptions for reasons why you should not be using exceptions.

@try { foo(); } @catch (NSException *ex) { bar(ex); } @finally { baz(); }
There should not be a space between the type identifier and the name of the protocol encased in angle brackets.

This applies to class declarations, instance variables, and method declarations. For example:

@interface MyProtocoledClass : NSObject<NSWindowDelegate> { @private id<MyFancyDelegate> _delegate; } - (void)setDelegate:(id<MyFancyDelegate>)aDelegate; @end
Blocks are preferred to the target-selector pattern when creating callbacks, as it makes code easier to read. Code inside blocks should be indented four spaces.

There are several appropriate style rules, depending on how long the block is:

  • If the block can fit on one line, no wrapping is necessary.
  • If it has to wrap, the closing brace should line up with the first character of the line on which the block is declared.
  • Code within the block should be indented four spaces.
  • If the block is large, e.g. more than 20 lines, it is recommended to move it out-of-line into a local variable.
  • If the block takes no parameters, there are no spaces between the characters ^{. If the block takes parameters, there is no space between the ^( characters, but there is one space between the ) { characters.
  • Two space indents inside blocks are also allowed, but should only be used when it's consistent with the rest of the project's code.
// The entire block fits on one line. [operation setCompletionBlock:^{ [self onOperationDone]; }]; // The block can be put on a new line, indented four spaces, with the // closing brace aligned with the first character of the line on which // block was declared. [operation setCompletionBlock:^{ [self.delegate newDataAvailable]; }]; // Using a block with a C API follows the same alignment and spacing // rules as with Objective-C. dispatch_async(_fileIOQueue, ^{ NSString* path = [self sessionFilePath]; if (path) { // ... } }); // An example where the parameter wraps and the block declaration fits // on the same line. Note the spacing of |^(SessionWindow *window) {| // compared to |^{| above. [[SessionService sharedService] loadWindowWithCompletionBlock:^(SessionWindow *window) { if (window) { [self windowDidLoad:window]; } else { [self errorLoadingWindow]; } }]; // An example where the parameter wraps and the block declaration does // not fit on the same line as the name. [[SessionService sharedService] loadWindowWithCompletionBlock: ^(SessionWindow *window) { if (window) { [self windowDidLoad:window]; } else { [self errorLoadingWindow]; } }]; // Large blocks can be declared out-of-line. void (^largeBlock)(void) = ^{ // ... }; [_operationQueue addOperationWithBlock:largeBlock];

Naming rules are very important in maintainable code. Objective-C method names tend to be very long, but this has the benefit that a block of code can almost read like prose, thus rendering many comments unnecessary.

When writing pure Objective-C code, we mostly follow standard Objective-C naming rules. These naming guidelines may differ significantly from those outlined in the C++ style guide. For example, Google's C++ style guide recommends the use of underscores between words in variable names, whereas this guide recommends the use of intercaps, which is standard in the Objective-C community.

Any class, category, method, or variable name may use all capitals for initialisms within the name. This follows Apple's standard of using all capitals within a name for initialisms such as URL, TIFF, and EXIF.

When writing Objective-C++, however, things are not so cut and dry. Many projects need to implement cross-platform C++ APIs with some Objective-C or Cocoa, or bridge between a C++ back-end and a native Cocoa front-end. This leads to situations where the two guides are directly at odds.

Our solution is that the style follows that of the method/function being implemented. If you're in an @implementation block, use the Objective-C naming rules. If you're implementing a method for a C++ class, use the C++ naming rules. This avoids the situation where instance variable and local variable naming rules are mixed within a single function, which would be a serious detriment to readability.

File names should reflect the name of the class implementation that they contain—including case. Follow the convention that your project uses.

File extensions should be as follows:

.h C/C++/Objective-C header file
.m Objective-C implementation file
.mm Objective-C++ implementation file
.cc Pure C++ implementation file
.c C implementation file

File names for categories should include the name of the class being extended, e.g. GTMNSString+Utils.h or GTMNSTextView+Autocomplete.h

Within a source file, Objective-C++ follows the style of the function/method you're implementing.

In order to minimize clashes between the differing naming styles when mixing Cocoa/Objective-C and C++, follow the style of the method being implemented. If you're in an @implementation block, use the Objective-C naming rules. If you're implementing a method for a C++ class, use the C++ naming rules.

// file: cross_platform_header.h class CrossPlatformAPI { public: ... int DoSomethingPlatformSpecific(); // impl on each platform private: int an_instance_var_; }; // file: mac_implementation.mm #include "cross_platform_header.h" // A typical Objective-C class, using Objective-C naming. @interface MyDelegate : NSObject { @private int _instanceVar; CrossPlatformAPI* _backEndObject; } - (void)respondToSomething:(id)something; @end @implementation MyDelegate - (void)respondToSomething:(id)something { // bridge from Cocoa through our C++ backend _instanceVar = _backEndObject->DoSomethingPlatformSpecific(); NSString* tempString = [NSString stringWithInt:_instanceVar]; NSLog(@"%@", tempString); } @end // The platform-specific implementation of the C++ class, using // C++ naming. int CrossPlatformAPI::DoSomethingPlatformSpecific() { NSString* temp_string = [NSString stringWithInt:an_instance_var_]; NSLog(@"%@", temp_string); return [temp_string intValue]; }
Class names (along with category and protocol names) should start as uppercase and use mixed case to delimit words.

In application-level code, prefixes on class names should generally be avoided. Having every single class with same prefix impairs readability for no benefit. When designing code to be shared across multiple applications, prefixes are acceptable and recommended (e.g. GTMSendMessage).

Category names should start with a 2 or 3 character prefix identifying the category as part of a project or open for general use. The category name should incorporate the name of the class it's extending.

For example, if we want to create a category on NSString for parsing, we would put the category in a file named GTMNSString+Parsing.h, and the category itself would be named GTMStringParsingAdditions (yes, we know the file name and the category name do not match, but this file could have many separate categories related to parsing). Methods in that category should share the prefix (gtm_myCategoryMethodOnAString:) in order to prevent collisions in Objective-C which only has a single namespace. If the code isn't meant to be shared and/or doesn't run in a different address-space, the method naming isn't quite as important.

There should be a single space between the class name and the opening parenthesis of the category.

// Extending a framework class: @interface NSString (GTMStringParsingAdditions) - (NSString *)gtm_foobarString; @end // Making your methods and properties private: @interface FoobarViewController () @property(nonatomic, retain) NSView *dongleView; - (void)performLayout; @end
Method names should start as lowercase and then use mixed case. Each named parameter should also start as lowercase.

The method name should read like a sentence if possible, meaning you should choose parameter names that flow with the method name. (e.g. convertPoint:fromRect: or replaceCharactersInRange:withString:). See Apple's Guide to Naming Methods for more details.

Accessor methods should be named the same as the variable they're "getting", but they should not be prefixed with the word "get". For example: - (id)getDelegate; // AVOID - (id)delegate; // GOOD

This is for Objective-C methods only. C++ method names and functions continue to follow the rules set in the C++ style guide.

Variables names start with a lowercase and use mixed case to delimit words. Instance variables have leading underscores. For example: myLocalVariable, _myInstanceVariable.

Do not use Hungarian notation for syntactic attributes, such as the static type of a variable (int or pointer). Give as descriptive a name as possible, within reason. Don't worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. For example:

int w; int nerr; int nCompConns; tix = [[NSMutableArray alloc] init]; obj = [someObject object]; p = [network port]; int numErrors; int numCompletedConnections; tickets = [[NSMutableArray alloc] init]; userInfo = [someObject object]; port = [network port];

Instance variables are mixed case and should be prefixed with an underscore e.g. _usernameTextField. Note that historically the convention was to put the underscore at the end of the name, and projects may opt to continue using trailing underscores in new code in order to maintain consistency within their codebase (see the Historical Notes section). It is recommended you leave old code as-is, unless doing so would create inconsistency within a class.

Constant names (#defines, enums, const local variables, etc.) should start with a lowercase k and then use mixed case to delimit words, i.e. kInvalidHandle, kWritePerm.

Though a pain to write, they are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names and then trying to explain them through comments.

When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous—the next one may be you!

Remember that all of the rules and conventions listed in the C++ Style Guide are in effect here, with a few additional points, below.

A file may optionally start with a description of its contents.

Every file should contain the following items, in order:

  • license boilerplate if neccessary. Choose the appropriate boilerplate for the license used by the project (e.g. Apache 2.0, BSD, LGPL, GPL).
  • a basic description of the contents of the file if necessary.

If you make significant changes to a file with an author line, consider deleting the author line since revision history already provides a more detailed and accurate record of authorship.

Every interface, category, and protocol declaration should have an accompanying comment describing its purpose and how it fits into the larger picture. // A delegate for NSApplication to handle notifications about app // launch and shutdown. Owned by the main app controller. @interface MyAppDelegate : NSObject { ... } @end

If you have already described an interface in detail in the comments at the top of your file feel free to simply state "See comment at top of file for a complete description", but be sure to have some sort of comment.

Additionally, each method in the public interface should have a comment explaining its function, arguments, return value, and any side effects.

Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.

Use vertical bars to quote variable names and symbols in comments rather than quotes or naming the symbol inline.

This helps eliminate ambiguity, especially when the symbol is a common word that might make the sentence read like it was poorly constructed. E.g. for a symbol "count":

// Sometimes we need |count| to be less than zero.

or when quoting something which already contains quotes

// Remember to call |StringWithoutSpaces("foo bar baz")|
Make the pointer ownership model as explicit as possible when it falls outside the most common Objective-C usage idioms.

Instance variables which are pointers to objects derived from NSObject are presumed to be retained, and should be either commented as weak or declared with the __weak lifetime qualifier when applicable. Similarly, declared properties must specify an assign property attribute if they are not retained by the class. An exception is instance variables labeled as IBOutlets in desktop Mac software, which are presumed to not be retained.

Where instance variables are pointers to Core Foundation, C++, and other non-Objective-C objects, they should always be declared with the __strong or __weak type modifiers to indicate which pointers are and are not retained. Core Foundation and other non-Objective-C object pointers require explicit memory management, even when building for automatic reference counting or garbage collection. When the __weak type modifier is not allowed (e.g. C++ member variables when compiled under clang), a comment should be used instead.

Be mindful that support for automatic C++ objects encapsulated in Objective-C objects is disabled by default, as described here.

Examples of strong and weak declarations: @interface MyDelegate : NSObject { @private IBOutlet NSButton *_okButton; // normal NSControl; implicitly weak on Mac only AnObjcObject* _doohickey; // my doohickey __weak MyObjcParent *_parent; // so we can send msgs back (owns me) // non-NSObject pointers... __strong CWackyCPPClass *_wacky; // some cross-platform object __strong CFDictionaryRef *_dict; } @property(strong, nonatomic) NSString *doohickey; @property(weak, nonatomic) NSString *parent; @end

Object ownership and lifetime are explicit when using ARC, so no additional comments are required.

Instance variables should be marked @private when they are declared in a header file. @interface MyClass : NSObject { @private id _myInstanceVariable; } // public accessors, setter takes ownership - (id)myInstanceVariable; - (void)setMyInstanceVariable:(id)theVar; @end Comment and clearly identify your designated initializer.

It is important for those who might be subclassing your class that the designated initializer be clearly identified. That way, they only need to subclass a single initializer (of potentially several) to guarantee their subclass' initializer is called. It also helps those debugging your class in the future understand the flow of initialization code if they need to step through it.

When writing a subclass that requires an init... method, make sure you override the superclass' designated initializer.

If you fail to override the superclass' designated initializer, your initializer may not be called in all cases, leading to subtle and very difficult to find bugs.

It is strongly recommended and typical practice to place overridden methods of NSObject at the top of an @implementation. This commonly applies (but is not limited) to the init..., copyWithZone:, and dealloc methods. init... methods should be grouped together, followed by the copyWithZone: method, and finally the dealloc method. Don't initialize variables to 0 or nil in the init method; it's redundant.

All memory for a newly allocated object is initialized to 0 (except for isa), so don't clutter up the init method by re-initializing variables to 0 or nil.

Do not invoke the NSObject class method new, nor override it in a subclass. Instead, use alloc and init methods to instantiate retained objects.

Modern Objective-C code explicitly calls alloc and an init method to create and retain an object. As the new class method is rarely used, it makes reviewing code for correct memory management more difficult.

Keep your class simple; avoid "kitchen-sink" APIs. If a method doesn't need to be public, don't make it so. Use a private category to prevent cluttering the public header.

Unlike C++, Objective-C doesn't have a way to differentiate between public and private methods—everything is public. As a result, avoid placing methods in the public API unless they are actually expected to be used by a consumer of the class. This helps reduce the likelihood they'll be called when you're not expecting it. This includes methods that are being overridden from the parent class. For internal implementation methods, use a category defined in the implementation file as opposed to adding them to the public header.

#import "GTMFoo.h" @interface GTMFoo (PrivateDelegateHandling) - (NSString *)doSomethingWithDelegate; // Declare private method @end @implementation GTMFoo(PrivateDelegateHandling) ... - (NSString *)doSomethingWithDelegate { // Implement this method } ... @end

Before Objective-C 2.0, if you declare a method in the private @interface, but forget to implement it in the main @implementation, the compiler will not object. (This is because you don't implement these private methods in a separate category.) The solution is to put the functions within an @implementation that specifies the category.

If you are using Objective-C 2.0, you should instead declare your private category using a class extension, for example:

@interface GMFoo () { ... }

which will guarantee that the declared methods are implemented in the @implementation section by issuing a compiler warning if they are not.

Again, "private" methods are not really private. You could accidentally override a superclass's "private" method, thus making a very difficult bug to squash. In general, private methods should have a fairly unique name that will prevent subclasses from unintentionally overriding them.

Finally, Objective-C categories are a great way to segment a large @implementation section into more understandable chunks and to add new, application-specific functionality to the most appropriate class. For example, instead of adding "middle truncation" code to a random object in your app, make a new category on NSString).

#import Objective-C/Objective-C++ headers, and #include C/C++ headers.

Choose between #import and #include based on the language of the header that you are including.

  • When including a header that uses Objective-C or Objective-C++, use #import.
  • When including a standard C or C++ header, use #include. The header should provide its own #define guard.

Some Objective-C headers lack #define guards, and expect to be included only by #import. As Objective-C headers may only be included in Objective-C source files and other Objective-C headers, using #import across the board is appropriate.

Standard C and C++ headers without any Objective-C in them can expect to be included by ordinary C and C++ files. Since there is no #import in standard C or C++, such files will be included by #include in those cases. Using #include for them in Objective-C source files as well means that these headers will always be included with the same semantics.

This rule helps avoid inadvertent errors in cross-platform projects. A Mac developer introducing a new C or C++ header might forget to add #define guards, which would not cause problems on the Mac if the new header were included with #import, but would break builds on other platforms where #include is used. Being consistent by using #include on all platforms means that compilation is more likely to succeed everywhere or fail everywhere, and avoids the frustration of files working only on some platforms.

#import <Cocoa/Cocoa.h> #include <CoreFoundation/CoreFoundation.h> #import "GTMFoo.h" #include "base/basictypes.h"
Include root frameworks over individual files.

While it may seem tempting to include individual system headers from a framework such as Cocoa or Foundation, in fact it's less work on the compiler if you include the top-level root framework. The root framework is generally pre-compiled and can be loaded much more quickly. In addition, remember to use #import rather than #include for Objective-C frameworks.

#import <Foundation/Foundation.h> // good #import <Foundation/NSArray.h> // avoid #import <Foundation/NSString.h> ...
When creating new temporary objects, autorelease them on the same line as you create them rather than a separate release later in the same method.

While ever so slightly slower, this prevents someone from accidentally removing the release or inserting a return before it and introducing a memory leak. E.g.:

// AVOID (unless you have a compelling performance reason) MyController* controller = [[MyController alloc] init]; // ... code here that might return ... [controller release]; // BETTER MyController* controller = [[[MyController alloc] init] autorelease];
Assignment of objects follows the autorelease then retain pattern.

When assigning a new object to a variable, one must first release the old object to avoid a memory leak. There are several "correct" ways to handle this. We've chosen the "autorelease then retain" approach because it's less prone to error. Be aware in tight loops it can fill up the autorelease pool, and may be slightly less efficient, but we feel the tradeoffs are acceptable.

- (void)setFoo:(GMFoo *)aFoo { [_foo autorelease]; // Won't dealloc if |_foo| == |aFoo| _foo = [aFoo retain]; }
Instance subclasses may be in an inconsistent state during init and dealloc method execution, so code in those methods should avoid invoking accessors.

Subclasses have not yet been initialized or have already deallocated when init and dealloc methods execute, making accessor methods potentially unreliable. Whenever practical, directly assign to and release ivars in those methods rather than rely on accessors.

- (id)init { self = [super init]; if (self) { _bar = [[NSMutableString alloc] init]; // good } return self; } - (void)dealloc { [_bar release]; // good [super dealloc]; } - (id)init { self = [super init]; if (self) { self.bar = [NSMutableString string]; // avoid } return self; } - (void)dealloc { self.bar = nil; // avoid [super dealloc]; }
dealloc should process instance variables in the same order the @interface declares them, so it is easier for a reviewer to verify.

A code reviewer checking a new or revised dealloc implementation needs to make sure that every retained instance variable gets released.

To simplify reviewing dealloc, order the code so that the retained instance variables get released in the same order that they are declared in the @interface. If dealloc invokes other methods that release instance variables, add comments describing what instance variables those methods handle.

Setters taking an NSString, should always copy the string it accepts.

Never just retain the string. This avoids the caller changing it under you without your knowledge. Don't assume that because you're accepting an NSString that it's not actually an NSMutableString.

- (void)setFoo:(NSString *)aFoo { [_foo autorelease]; _foo = [aFoo copy]; }
Don't @throw Objective-C exceptions, but you should be prepared to catch them from third-party or OS calls.

We do compile with -fobjc-exceptions (mainly so we get @synchronized), but we don't @throw. Use of @try, @catch, and @finally are allowed when required to properly use 3rd party code or libraries. If you do use them please document exactly which methods you expect to throw.

Do not use the NS_DURING, NS_HANDLER, NS_ENDHANDLER, NS_VALUERETURN and NS_VOIDRETURN macros unless you are writing code that needs to run on Mac OS X 10.2 or before.

Also be aware when writing Objective-C++ code that stack based objects are not cleaned up when you throw an Objective-C exception. Example:

class exceptiontest { public: exceptiontest() { NSLog(@"Created"); } ~exceptiontest() { NSLog(@"Destroyed"); } }; void foo() { exceptiontest a; NSException *exception = [NSException exceptionWithName:@"foo" reason:@"bar" userInfo:nil]; @throw exception; } int main(int argc, char *argv[]) { GMAutoreleasePool pool; @try { foo(); } @catch(NSException *ex) { NSLog(@"exception raised"); } return 0; }

will give you:

2006-09-28 12:34:29.244 exceptiontest[23661] Created 2006-09-28 12:34:29.244 exceptiontest[23661] exception raised

Note that the destructor for a never got called. This is a major concern for stack based smartptrs such as shared_ptr and linked_ptr, as well as any STL objects that you may want to use. Therefore it pains us to say that if you must use exceptions in your Objective-C++ code, use C++ exceptions whenever possible. You should never re-throw an Objective-C exception, nor are stack based C++ objects (such as std::string, std::vector etc.) allowed in the body of any @try, @catch, or @finally blocks.

Use nil checks for logic flow only.

Use nil checks for logic flow of the application, not for crash prevention. Sending a message to a nil object is handled by the Objective-C runtime. If the method has no return result, you're good to go. However if there is one, there may be differences based on runtime architecture, return size, and OS X version (see Apple's documentation for specifics).

Note that this is very different from checking C/C++ pointers against NULL, which the runtime does not handle and will cause your application to crash. You still need to make sure you do not dereference a NULL pointer.

Be careful when converting general integral values to BOOL. Avoid comparing directly with YES.

BOOL is defined as a signed char in Objective-C which means that it can have values other than YES (1) and NO (0). Do not cast or convert general integral values directly to BOOL. Common mistakes include casting or converting an array's size, a pointer value, or the result of a bitwise logic operation to a BOOL which, depending on the value of the last byte of the integral result, could still result in a NO value. When converting a general integral value to a BOOL use ternery operators to return a YES or NO value.

You can safely interchange and convert BOOL, _Bool and bool (see C++ Std 4.7.4, 4.12 and C99 Std 6.3.1.2). You cannot safely interchange BOOL and Boolean so treat Booleans as a general integral value as discussed above. Only use BOOL in Objective C method signatures.

Using logical operators (&&, || and !) with BOOL is also valid and will return values that can be safely converted to BOOL without the need for a ternery operator.

- (BOOL)isBold { return [self fontTraits] & NSFontBoldTrait; } - (BOOL)isValid { return [self stringValue]; } - (BOOL)isBold { return ([self fontTraits] & NSFontBoldTrait) ? YES : NO; } - (BOOL)isValid { return [self stringValue] != nil; } - (BOOL)isEnabled { return [self isValid] && [self isBold]; }

Also, don't directly compare BOOL variables directly with YES. Not only is it harder to read for those well-versed in C, the first point above demonstrates that return values may not always be what you expect.

BOOL great = [foo isGreat]; if (great == YES) // ...be great! BOOL great = [foo isGreat]; if (great) // ...be great!
Use of the @property directive is preferred, with the following caveat: properties are an Objective-C 2.0 feature which will limit your code to running on the iPhone and Mac OS X 10.5 (Leopard) and higher. Dot notation is allowed only for access to a declared @property.

A property's associated instance variable's name must conform to the leading _ requirement. The property's name should be the same as its associated instance variable without the leading _. The optional space between the @property and the opening parenthesis should be omitted, as seen in the examples.

@interface MyClass : NSObject @property(copy, nonatomic) NSString *name; @end @implementation MyClass // No code required for auto-synthesis, else use: // @synthesize name = _name; @end

A property's declaration must come immediately after the instance variable block of a class interface. A property's definition (if not using automatic synthesis) must come immediately after the @implementation block in a class definition. They are indented at the same level as the @interface or @implementation statements that they are enclosed in.

@interface MyClass : NSObject { @private NSString *_name; } @property(copy, nonatomic) NSString *name; @end @implementation MyClass @synthesize name = _name; - (id)init { ... } @end

NSString properties should always be declared with the copy attribute.

This logically follows from the requirement that setters for NSStrings always must use copy instead of retain.

Be aware of the overhead of properties. By default, all synthesized setters and getters are atomic. This gives each set and get calls a substantial amount of synchronization overhead. Declare your properties nonatomic unless you require atomicity.

Dot notation is idiomatic style for Objective-C 2.0. It may be used when doing simple operations to get and set a @property of an object, but should not be used to invoke other object behavior.

NSString *oldName = myObject.name; myObject.name = @"Alice"; NSArray *array = [[NSArray arrayWithObject:@"hello"] retain]; NSUInteger numberOfItems = array.count; // not a property array.release; // not a property
Omit the empty set of braces on interfaces that do not declare any instance variables. @interface MyClass : NSObject // Does a lot of stuff - (void)fooBarBam; @end @interface MyClass : NSObject { } // Does a lot of stuff - (void)fooBarBam; @end

Use of automatically synthesized instance variables is preferred. Code that must support earlier versions of the compiler toolchain (Xcode 4.3 or earlier or when compiling with GCC) or is using properties inherited from a protocol should prefer the @synthesize directive.

// Header file @protocol Thingy @property(nonatomic, copy) NSString *widgetName; @end @interface Foo : NSObject<Thingy> // A guy walks into a bar. @property(nonatomic, copy) NSString *bar; @end // Implementation file @interface Foo () @property(nonatomic, retain) NSArray *baz; @end @implementation Foo @synthesize widgetName = _widgetName; @end

Automatically synthesized instance variables take the form of the property's name prefixed with an underscore and so typically conform to the required variable naming style. If your property name is unusual, or you are otherwise unable to use automatically synthesized instance variables, use of the @synthesize directive is preferred, with the instance variable name specified explicitly (as @synthesize does not add a leading underscore by default).

For projects that use Xcode 4.2 or later and will run only on 64-bit Mac OS X 10.7 and iOS 5.0 and later, ARC is preferred. Use manual reference counting when supporting earlier environments where zeroing weak pointers are not available.

Classes that require ARC should include a preprocessor directive to prevent compilation using manual reference counting.

Ownership qualifiers like __unsafe_unretained and __weak should precede variable names. Specifying __strong for variables is not required since it is the default. Properties, on the other hand, should always specify the strong keyword rather than relying on the compiler default.

Files that are compiled using ARC need to have preprocessor directives to prevent compilation without ARC. See the code snippet below for details.

Example of an implementation file enforcing ARC style. Note that declaring instance variables in the @implementation is permitted when using ARC. #if !defined(__has_feature) || !__has_feature(objc_arc) #error "This file requires ARC support." #endif #import "Foo.h" @implementation Foo { Bar* __weak _bar; Baz* __unsafe_unretained _baz; } // ... @end

For projects that use Xcode 4.4 or later with clang, the use of NSNumber literals is allowed. Note however that this will limit the portability of your code to other toolchains.

NSNumber literals are used just like Objective C string literals. Boxing is used when necessary. Code using NSNumber literals can be deployed on any iOS/MacOS system. NSNumber *fortyTwo = @42; NSNumber *piOverTwo = @(M_PI / 2); enum { kMyEnum = 2; }; NSNumber *myEnum = @(kMyEnum);

Delegate objects should not be retained.

A class that implements the delegate pattern should:

  1. Have an instance variable named _delegate to reference the delegate.
  2. Thus, the accessor methods should be named delegate and setDelegate:.
  3. The _delegate object should not be retained.

Separate the model from the view. Separate the controller from the view and the model. Use @protocols for callback APIs.

  • Separate model from view: don't build assumptions about the presentation into the model or data source. Keep the interface between the data source and the presentation abstract. Don't give the model knowledge of its view. (A good rule of thumb is to ask yourself if it's possible to have multiple presentations, with different states, on a single instance of your data source.)
  • Separate controller from view and model: don't put all of the "business logic" into view-related classes; this makes the code very unusable. Make controller classes to host this code, but ensure that the controller classes don't make too many assumptions about the presentation.
  • Define callback APIs with @protocol, using @optional if not all the methods are required. (Exception: when using Objective-C 1.0, @optional isn't available, so use a category to define an "informal protocol".)

Trailing underscores were once preferred for instance variable names.

Our style guide used to have a rule saying that instance variables should be named with a trailing underscore, similar to the naming of member variables in C++. This was changed to leading underscores to be consistent with the broader Objective-C community, to better follow Apple's official guidelines, and to allow for use of new compiler features like automatic instance variable synthesis. New projects are strongly encouraged to use leading underscores. Existing projects may continue to use trailing underscores in new code to maintain consistency with the rest of their codebase.


Revision 2.52

Mike Pinkerton
Greg Miller
Dave MacLachlan