= Google Objective-C Style Guide

:sectlinks:

Revision 2.59

Mike Pinkerton +
 Greg Miller +
 Dave MacLachlan





Table of Contents

[width="100%",cols="50%,50%",]
|===================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================
a|
link:#example[Example]

 a|
a|
link:#spacing-and-formatting[Spacing And Formatting]

 a|
link:#spaces-vs-tabs[Spaces vs. Tabs] link:#line-length[Line Length] link:#method-declarations-and-definitions[Method Declarations and Definitions] link:#method-invocations[Method Invocations] link:#@public_and_@private[@public and @private] link:#exceptions[Exceptions] link:#protocols[Protocols] link:#blocks[Blocks] link:#container-literals[Container Literals]

a|
link:#naming[Naming]

 a|
link:#file-names[File Names] link:#Objective-C++[Objective-C++] link:#class-names[Class Names] link:#category-names[Category Names] link:#Objective-C_Method_Names[Objective-C Method Names] link:#variable-names[Variable Names]

a|
link:#comments[Comments]

 a|
link:#file-comments[File Comments] link:#declaration-comments[Declaration Comments] link:#implementation-comments[Implementation Comments] link:#object-ownership[Object Ownership]

a|
link:#Cocoa_and_Objective-C_Features[Cocoa and Objective-C Features]

 a|
link:#Instance_Variables_In_Headers_Should_Be_@private[Instance Variables In Headers Should Be @private] link:#identify-designated-initializer[Identify Designated Initializer] link:#override-designated-initializer[Override Designated Initializer] link:#overridden-nsobject-method-placement[Overridden NSObject Method Placement] link:#initialization[Initialization] link:#Avoid_+new[Avoid +new] link:#keep-the-public-api_simple[Keep the Public API Simple] link:#-import-and-_include[#import and #include] link:#use-root-frameworks[Use Root Frameworks] link:#prefer-to-autorelease-at_time_of_creation[Prefer To autorelease At Time of Creation] link:#autorelease-then-retain[Autorelease Then Retain] link:#avoid-accessors-during-init_and_dealloc[Avoid Accessors During init and dealloc] link:#dealloc-instance-variables-in_declaration______________________order[Dealloc Instance Variables in Declaration Order] link:#setters-copy-nsstrings[Setters copy NSStrings] link:#avoid-throwing-exceptions[Avoid Throwing Exceptions] link:#nil-checks[nil Checks] link:#bool-pitfalls[BOOL Pitfalls] link:#properties[Properties] link:#interfaces-without-instance-variables[Interfaces Without Instance Variables] link:#automatically-synthesized-instance-variables[Automatically Synthesized Instance Variables] link:#automatic-reference-counting-_arc_[Automatic Reference Counting (ARC)] link:#nsnumber-literals[NSNumber Literals]

a|
link:#cocoa-patterns[Cocoa Patterns]

 a|
link:#delegate-pattern[Delegate Pattern] link:#model-view/controller[Model/View/Controller]

a|
link:#historical-notes[Historical Notes]

 a|
link:#trailing-vs-leading-underscores[Trailing vs Leading Underscores]

|===================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================

[[Important_Note]]
== Important Note

=== Displaying Hidden Details in this Guide



This style guide contains many details that are initially hidden from view. They are marked by the triangle icon, which you see here on your left. Click it now. You should see "Hooray" appear below.

Hooray! Now you know you can expand points to get more details. Alternatively, there's an "expand all" at the top of this document.

[[Background]]
== Background

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:


* http://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/index.html[Apple's Cocoa Coding Guidelines]

* http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml[Google's Open Source C++ Style Guide]

_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 http://code.google.com/p/google-toolbox-for-mac/[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 http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html[Programming with Objective-C].

[[Example]]
== Example

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 non-trivial 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

// Returns an autoreleased instance of Foo. See -initWithBar: for details
// about |bar|.
+ (instancetype)fooWithBar:(NSString *)bar;

// Designated initializer. |bar| is a thing that represents a thing that
// does a thing.
- (instancetype)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 {
  NSString *_bar;
  NSString *_foo;
}

+ (instancetype)fooWithBar:(NSString *)bar {
  return [[[self alloc] initWithBar:bar] autorelease];
}

// Must always override super's designated initializer.
- (instancetype)init {
  return [self initWithBar:nil];
}

- (instancetype)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.

[[Spacing_And_Formatting]]
== Spacing And Formatting

=== Spaces vs. Tabs



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.

=== Line Length



The maximum line length for Objective-C and Objective-C++ files is 100 columns. Projects may opt to use an 80 column limit for consistency with the C++ style guide.

You can make violations easier to spot by enabling _Preferences > Text Editing > Page guide at column: 100_ in Xcode.

=== Method Declarations and Definitions



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



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];
----------------------------

Invocations containing inlined link:#blocks[blocks] may have their segments left-aligned at a four space indent.

=== @public and @private



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
-------------------------------

=== Exceptions



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 link:#avoid-throwing-exceptions[Avoid Throwing Exceptions] for reasons why you should not be using exceptions.

--------------------------
@try {
  foo();
}
@catch (NSException *ex) {
  bar(ex);
}
@finally {
  baz();
}
--------------------------

=== Protocols



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



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.

* Invocations containing inlined blocks may have their segments left-aligned at a four-space indent. This helps when invocations contain multiple inlined blocks.

* 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];

// An example with multiple inlined blocks in one invocation.
[myObject doSomethingWith:arg1
    firstBlock:^(Foo *a) {
        // ...
    }
    secondBlock:^(Bar *b) {
        // ...
    }];
----------------------------------------------------------------------

=== Container Literals



For projects using Xcode 4.4 or later and clang, the use of container (array and dictionary) literals is encouraged. If split across multiple lines, the contents should be indented two spaces.

If the collection fits on one line, put a single space after the opening and before the closing brackets.

------------------------------------------------------------------------------
NSArray* array = @[ [foo description], @"Another String", [bar description] ];

NSDictionary* dict = @{ NSForegroundColorAttributeName : [NSColor redColor] };
------------------------------------------------------------------------------

Not:

---------------------------------------------------------------------------
NSArray* array = @[[foo description], [bar description]];

NSDictionary* dict = @{NSForegroundColorAttributeName: [NSColor redColor]};
---------------------------------------------------------------------------

If the collection spans more than a single line, place the opening bracket on the same line as the declaration, indent the body by two spaces, and place the closing bracket on a new line that is indented to the same level as the opening bracket.

------------------------------------------------------------------------
NSArray* array = @[
  @"This",
  @"is",
  @"an",
  @"array"
];

NSDictionary* dictionary = @{
  NSFontAttributeName : [NSFont fontWithName:@"Helvetica-Bold" size:12],
  NSForegroundColorAttributeName : fontColor
};
------------------------------------------------------------------------

For dictionary literals, there should be one space before the colon and at least one space after it (to optionally align the values).

--------------------------------------------------------------------------
NSDictionary* option1 = @{
  NSFontAttributeName : [NSFont fontWithName:@"Helvetica-Bold" size:12],
  NSForegroundColorAttributeName : fontColor
};

NSDictionary* option2 = @{
  NSFontAttributeName :            [NSFont fontWithName:@"Arial" size:12],
  NSForegroundColorAttributeName : fontColor
};
--------------------------------------------------------------------------

The following are all incorrect:

-------------------------------------------------------------------
// There should be a space before the colon.
NSDictionary* wrong = @{
  AKey:       @"b",
  BLongerKey: @"c",
};

// The items should each be on a new line, or the entire expression
// should fit on one line.
NSDictionary* alsoWrong= @{ AKey : @"a",
                            BLongerKey : @"b" };

// There should be no variable space before the colon, only after.
NSDictionary* stillWrong = @{
  AKey       : @"b",
  BLongerKey : @"c",
};
-------------------------------------------------------------------

[[Naming]]
== Naming

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 http://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html[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 http://en.wikipedia.org/wiki/Initialism[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



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:

[cols=",",]
|========================================
|`.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`

=== Objective-C++



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 stringWithFormat:@"%d", _instanceVar];
  NSLog(@"%@", tempString);
}
@end

// The platform-specific implementation of the C++ class, using
// C++ naming.
int CrossPlatformAPI::DoSomethingPlatformSpecific() {
  NSString* temp_string = [NSString stringWithFormat:@"%d", an_instance_var_];
  NSLog(@"%@", temp_string);
  return [temp_string intValue];
}
------------------------------------------------------------------------------

=== Class Names



Class names (along with category and protocol names) should start as uppercase and use mixed case to delimit words.

When designing code to be shared across multiple applications, prefixes are acceptable and recommended (e.g. `GTMSendMessage`). Prefixes are also recommended for classes of large applications that depend on external libraries.

=== Category Names



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
------------------------------------------------

=== Objective-C Method Names



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 http://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingMethods.html#//apple_ref/doc/uid/20001282-BCIGIJJF[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.

=== Variable Names



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

Common Variable Names

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

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.

Constants

Constant names (#defines, enums, const local variables, etc.) should start with a lowercase k and then use mixed case to delimit words. For example:

---------------------------------------
const int kNumberOfFiles = 12;
NSString *const kUserKey = @"kUserKey";
enum DisplayTinge {
  kDisplayTingeGreen = 1,
  kDisplayTingeBlue = 2
};
---------------------------------------

Because Objective-C does not provide namespacing, constants with global scope should have an appropriate prefix to minimize the chance of name collision, typically like kClassNameFoo.

[[Comments]]
== Comments

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.

=== File Comments



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.

=== Declaration Comments



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.

=== Implementation Comments



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")|
--------------------------------------------------------

=== Object Ownership



Make the pointer ownership model as explicit as possible when it falls outside the most common Objective-C usage idioms.

Manual Reference Counting

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 `strong` and `weak` comments 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.

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...
  CWackyCPPClass *_wacky;  // Strong, some cross-platform object
  CFDictionaryRef *_dict;  // Strong
}
@property(strong, nonatomic) NSString *doohickey;
@property(weak, nonatomic) NSString *parent;
@end
---------------------------------------------------------------------------------

Automatic Reference Counting

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

[[Cocoa_and_Objective-C_Features]]
== Cocoa and Objective-C Features

=== Instance Variables In Headers Should Be @private



Instance variables should typically be declared in implementation files or auto-synthesized by properties. When ivars are declared in a header file, they should be marked `@private`.

-------------------------------
@interface MyClass : NSObject {
 @private
  id _myInstanceVariable;
}
@end
-------------------------------

=== Identify Designated Initializer



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.

=== Override Designated Initializer



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.

=== Overridden NSObject Method Placement



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 other `NSObject` methods.

Convenience class methods for creating instances may precede the `NSObject` methods.

=== Initialization



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`.

=== Avoid +new



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 the Public API Simple



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
-----------------------------------------------------------------

If you are using Objective-C 2.0, you should instead declare your private category using a http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/chapter_4_section_5.html#[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 and #include



`#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 link:cppguide.xml?showone=The__define_Guard#The__define_Guard[#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"
------------------------------------------

=== Use Root Frameworks



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>
...
----------------------------------------------

=== Prefer To autorelease At Time of Creation



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];
---------------------------------------------------------------------

=== Autorelease Then Retain



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];
}
-----------------------------------------------------------

=== Avoid Accessors During init and dealloc



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.

---------------------------------------------------
- (instancetype)init {
  self = [super init];
  if (self) {
    _bar = [[NSMutableString alloc] init];  // good
  }
  return self;
}

- (void)dealloc {
  [_bar release];                           // good
  [super dealloc];
}
---------------------------------------------------

--------------------------------------------------
- (instancetype)init {
  self = [super init];
  if (self) {
    self.bar = [NSMutableString string];  // avoid
  }
  return self;
}

- (void)dealloc {
  self.bar = nil;                         // avoid
  [super dealloc];
}
--------------------------------------------------

=== Dealloc Instance Variables in Declaration Order



`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 copy NSStrings



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];
}
---------------------------------

=== Avoid Throwing Exceptions



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.

=== nil Checks



Use `nil` checks for logic flow only.

Use `nil` pointer checks for logic flow of the application, not for preventing crashes when sending messages. With current compilers (http://www.sealiesoftware.com/blog/archive/2012/2/29/objc_explain_return_value_of_message_to_nil.html[as of LLVM 3.0/Xcode 4.2]), sending a message to `nil` reliably returns nil as a pointer, zero as an integer or floating-point value, structs initialized to 0, and `_Complex` values equal to \{0, 0}.

Note that this applies to `nil` as a message target, not as a parameter value. Individual methods may or may not safely handle `nil` parameter values.

Note too that this is distinct from checking C/C++ pointers and block 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.

=== BOOL Pitfalls



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!
---------------------------

=== Properties



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`.

Naming

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
-------------------------------------------------

Location

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;

- (instancetype)init {
  ...
}
@end
------------------------------------------

Use Copy Attribute For Strings

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`.

Atomicity

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

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
-------------------------------------------------------------

=== Interfaces Without Instance Variables



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
-------------------------------

=== Automatically Synthesized Instance Variables



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).

=== Automatic Reference Counting (ARC)



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
-------------------------------------------------------

=== NSNumber Literals



For projects that use Xcode 4.4 or later with clang, the use of http://clang.llvm.org/docs/ObjectiveCLiterals.html[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);
----------------------------------

[[Cocoa_Patterns]]
== Cocoa Patterns

=== Delegate Pattern



Delegate objects should not be retained when doing so would create a retain cycle.

A class that implements the delegate pattern should typically:

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 be weak if the class is typically retained by its delegate, such that a strong delegate would create a retain cycle.

=== Model/View/Controller



Separate the model from the view. Separate the controller from the view and the model. Use `@protocol`s 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.

[[Historical_Notes]]
== Historical Notes

=== Trailing vs Leading Underscores



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 2.59

Mike Pinkerton +
 Greg Miller +
 Dave MacLachlan +
