objcguide.md

Google Objective-C Style Guide

Objective-C is a dynamic, object-oriented extension of C. It's designed to be easy to use and read, while enabling sophisticated object-oriented design. It is one of the primary development languages for applications on Apple platforms.

Apple has already written a very good, and widely accepted, Cocoa Coding Guidelines for Objective-C. Please read it in addition to this guide.

The purpose of this document is to describe the Objective-C (and Objective-C++) coding guidelines and practices. 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.

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 Programming with Objective-C.

Principles

Optimize for the reader, not the writer

Codebases often have extended lifetimes and more time is spent reading the code than writing it. We explicitly choose to optimize for the experience of our average software engineer reading, maintaining, and debugging code in our codebase rather than the ease of writing said code. For example, when something surprising or unusual is happening in a snippet of code, leaving textual hints for the reader is valuable.

Be consistent

When the style guide allows multiple options it is preferable to pick one option over mixed usage of multiple options. Using one style consistently throughout a codebase lets engineers focus on other (more important) issues. Consistency also enables better automation because consistent code allows more efficient development and operation of tools that format or refactor code. In many cases, rules that are attributed to "Be Consistent" boil down to "Just pick one and stop worrying about it"; the potential value of allowing flexibility on these points is outweighed by the cost of having people argue over them.

Be consistent with Apple SDKs

Consistency with the way Apple SDKs use Objective-C has value for the same reasons as consistency within our code base. If an Objective-C feature solves a problem that's an argument for using it. However, sometimes language features and idioms are flawed, or were just designed with assumptions that are not universal. In those cases it is appropriate to constrain or ban language features or idioms.

Style rules should pull their weight

The benefit of a style rule must be large enough to justify asking engineers to remember it. The benefit is measured relative to the codebase we would get without the rule, so a rule against a very harmful practice may still have a small benefit if people are unlikely to do it anyway. This principle mostly explains the rules we don’t have, rather than the rules we do: for example, goto contravenes many of the following principles, but is not discussed due to its extreme rarity.

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, and so on.

Here is an example header file, demonstrating the correct commenting and spacing for an @interface declaration.

// GOOD:

#import <Foundation/Foundation.h>

@class Bar;

/**
 * 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.
 */
@interface Foo : NSObject

/** The retained Bar. */
@property(nonatomic) Bar *bar;

/** The current drawing attributes. */
@property(nonatomic, copy) NSDictionary<NSString *, NSNumber *> *attributes;

/**
 * Convenience creation method.
 * See -initWithBar: for details about @c bar.
 *
 * @param bar The string for fooing.
 * @return An instance of Foo.
 */
+ (instancetype)fooWithBar:(Bar *)bar;

/**
 * Initializes and returns a Foo object using the provided Bar instance.
 *
 * @param bar A string that represents a thing that does a thing.
 */
- (instancetype)initWithBar:(Bar *)bar NS_DESIGNATED_INITIALIZER;

/**
 * Does some work with @c blah.
 *
 * @param blah
 * @return YES if the work was completed; NO otherwise.
 */
- (BOOL)doWorkWithBlah:(NSString *)blah;

@end

An example source file, demonstrating the correct commenting and spacing for the @implementation of an interface.

// GOOD:

#import "Shared/Util/Foo.h"

@implementation Foo {
  /** The string used for displaying "hi". */
  NSString *_string;
}

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

- (instancetype)init {
  // Classes with a custom designated initializer should always override
  // the superclass's designated initializer.
  return [self initWithBar:nil];
}

- (instancetype)initWithBar:(Bar *)bar {
  self = [super init];
  if (self) {
    _bar = [bar copy];
    _string = [[NSString alloc] initWithFormat:@"hi %d", 3];
    _attributes = @{
      @"color" : UIColor.blueColor,
      @"hidden" : @NO
    };
  }
  return self;
}

- (BOOL)doWorkWithBlah:(NSString *)blah {
  // Work should be done here.
  return NO;
}

@end

Naming

Names should be as descriptive as possible, within reason. Follow standard Objective-C naming rules.

Avoid non-standard abbreviations (including non-standard acronyms and initialisms). 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:

// GOOD:

// Good names.
int numberOfErrors = 0;
int completedConnectionsCount = 0;
tickets = [[NSMutableArray alloc] init];
userInfo = [someObject object];
port = [network port];
NSDate *gAppLaunchDate;

// AVOID:

// Names to avoid.
int w;
int nerr;
int nCompConns;
tix = [[NSMutableArray alloc] init];
obj = [someObject object];
p = [network port];

Any class, category, method, function, or variable name should use all capitals for acronyms and initialisms within (including at the beginning of) the name. This follows Apple's standard of using all capitals within a name for acronyms such as URL, ID, TIFF, and EXIF.

Names of C functions and typedefs should be capitalized and use camel case as appropriate for the surrounding code.

Inclusive Language

In all code, including naming and comments, use inclusive language and avoid terms that other programmers might find disrespectful or offensive (such as "master" and "slave", "blacklist" and "whitelist", or "redline"), even if the terms also have an ostensibly neutral meaning. Similarly, use gender-neutral language unless you're referring to a specific person (and using their pronouns). For example, use "they"/"them"/"their" for people of unspecified gender (even when singular), and "it"/"its" for non-people.

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:

Extension	Type
.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

Files containing code that may be shared across projects or used in a large project should have a clearly unique name, typically including the project or class prefix.

File names for categories should include the name of the class being extended, like GTMNSString+Utils.h or NSTextView+GTMAutocomplete.h

Prefixes

Prefixes are commonly required in Objective-C to avoid naming collisions in a global namespace. Classes, protocols, global functions, and global constants should generally be named with a prefix that begins with a capital letter followed by one or more capital letters or numbers.

WARNING: Apple reserves two-letter prefixes—see Conventions in Programming with Objective-C—so prefixes with a minimum of three characters are considered best practice.

// GOOD:

/** An example error domain. */
GTM_EXTERN NSString *GTMExampleErrorDomain;

/** Gets the default time zone. */
GTM_EXTERN NSTimeZone *GTMGetDefaultTimeZone(void);

/** An example delegate. */
@protocol GTMExampleDelegate <NSObject>
@end

/** An example class. */
@interface GTMExample : NSObject
@end

Class Names

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

Classes and protocols in code shared across multiple applications must have an appropriate prefix (e.g. GTMSendMessage). Prefixes are recommended, but not required, for other classes and protocols.

Category Naming

Category names should start with an appropriate prefix identifying the category as part of a project or open for general use.

Category source file names should begin with the class being extended followed by a plus sign and the name of the category, e.g., NSString+GTMParsing.h. Methods in a category should be prefixed with a lowercase version of the prefix used for the category name followed by an underscore (e.g., gtm_myCategoryMethodOnAString:) in order to prevent collisions in Objective-C's global namespace.

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

// GOOD:

// UIViewController+GTMCrashReporting.h

/** A category that adds metadata to include in crash reports to UIViewController. */
@interface UIViewController (GTMCrashReporting)

/** A unique identifier to represent the view controller in crash reports. */
@property(nonatomic, setter=gtm_setUniqueIdentifier:) int gtm_uniqueIdentifier;

/** Returns an encoded representation of the view controller's current state. */
- (nullable NSData *)gtm_encodedState;

@end

If a class is not shared with other projects, categories extending it may omit name prefixes and method name prefixes.

// GOOD:

/** This category extends a class that is not shared with other projects. */
@interface XYZDataObject (Storage)
- (NSString *)storageIdentifier;
@end

Objective-C Method Names

Method and parameter names typically start as lowercase and then use mixed case.

Proper capitalization should be respected, including at the beginning of names.

// GOOD:

+ (NSURL *)URLWithString:(NSString *)URLString;

The method name should read like a sentence if possible, meaning you should choose parameter names that flow with the method name. 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 implementation comments unnecessary.

Use prepositions and conjunctions like "with", "from", and "to" in the second and later parameter names only where necessary to clarify the meaning or behavior of the method.

// GOOD:

- (void)addTarget:(id)target action:(SEL)action;                          // GOOD; no conjunction needed
- (CGPoint)convertPoint:(CGPoint)point fromView:(UIView *)view;           // GOOD; conjunction clarifies parameter
- (void)replaceCharactersInRange:(NSRange)aRange
            withAttributedString:(NSAttributedString *)attributedString;  // GOOD.

If the method returns an attribute of the receiver, name the method after the attribute.

// GOOD:

/** Returns this instance's sandwich. */
- (Sandwich *)sandwich;      // GOOD.

- (CGFloat)height;           // GOOD.

// GOOD; Returned value is not an attribute.
- (UIBackgroundTaskIdentifier)beginBackgroundTask;

// AVOID:

- (CGFloat)calculateHeight;  // AVOID.
- (id)theDelegate;           // AVOID.

An accessor method should be named the same as the object it's getting, but it should not be prefixed with the word get. For example:

// GOOD:

- (id)delegate;     // GOOD.

// AVOID:

- (id)getDelegate;  // AVOID.

Accessors that return the value of boolean adjectives have method names beginning with is, but property names for those methods omit the is.

Dot notation is used only with property names, not with method names.

// GOOD:

@property(nonatomic, getter=isGlorious) BOOL glorious;
// The method for the getter of the property above is:
// - (BOOL)isGlorious;

BOOL isGood = object.glorious;      // GOOD.
BOOL isGood = [object isGlorious];  // GOOD.

// AVOID:

BOOL isGood = object.isGlorious;    // AVOID.

// GOOD:

NSArray<Frog *> *frogs = [NSArray<Frog *> arrayWithObject:frog];
NSEnumerator *enumerator = [frogs reverseObjectEnumerator];  // GOOD.

// AVOID:

NSEnumerator *enumerator = frogs.reverseObjectEnumerator;    // AVOID.

See Apple's Guide to Naming Methods for more details on Objective-C naming.

These guidelines are for Objective-C methods only. C++ method names continue to follow the rules set in the C++ style guide.

Function Names

Function names should start with a capital letter and have a capital letter for each new word (a.k.a. "camel case" or "Pascal case").

// GOOD:

static void AddTableEntry(NSString *tableEntry);
static BOOL DeleteFile(const char *filename);

Because Objective-C does not provide namespacing, non-static functions should have a prefix that minimizes the chance of a name collision.

// GOOD:

GTM_EXTERN NSTimeZone *GTMGetDefaultTimeZone(void);
GTM_EXTERN NSString *GTMGetURLScheme(NSURL *URL);

Variable Names

Variable names typically start with a lowercase and use mixed case to delimit words.

Instance variables have leading underscores. File scope or global variables have a prefix g. For example: myLocalVariable, _myInstanceVariable, gMyGlobalVariable.

Common Variable Names

Readers should be able to infer the variable type from the name, but do not use Hungarian notation for syntactic attributes, such as the static type of a variable (int or pointer).

File scope or global variables (as opposed to constants) declared outside the scope of a method or function should be rare, and should have the prefix g.

// GOOD:

static int gGlobalCounter;

Instance Variables

Instance variable names are mixed case and should be prefixed with an underscore, like _usernameTextField.

NOTE: Google's previous convention for Objective-C ivars was a trailing underscore. Existing projects may opt to continue using trailing underscores in new code in order to maintain consistency within the project codebase. Consistency of prefix or suffix underscores should be maintained within each class.

Constants

Constant symbols (const global and static variables and constants created with #define) should use mixed case to delimit words.

Global and file scope constants should have an appropriate prefix.

// GOOD:

/** The domain for GTL service errors. */
GTL_EXTERN NSString *const GTLServiceErrorDomain;

/** An enumeration of GTL service error codes. */
typedef NS_ENUM(int32_t, GTLServiceError) {
  /** An error code indicating that a query result was missing. */
  GTLServiceErrorQueryResultMissing = -3000,
  /** An error code indicating that the query timed out. */
  GTLServiceErrorQueryTimedOut      = -3001,
};

Because Objective-C does not provide namespacing, constants with external linkage should have a prefix that minimizes the chance of a name collision, typically like ClassNameConstantName or ClassNameEnumName.

For interoperability with Swift code, enumerated values should have names that extend the typedef name:

// GOOD:

/** An enumeration of supported display tinges. */
typedef NS_ENUM(int32_t, DisplayTinge) {
  DisplayTingeGreen = 1,
  DisplayTingeBlue = 2,
};

A lowercase k can be used as a standalone prefix for constants of static storage duration declared within implementation files:

// GOOD:

static const int kFileCount = 12;
static NSString *const kUserKey = @"kUserKey";

NOTE: Previous convention was for public constant names to begin with a lowercase k followed by a project-specific prefix. This practice is no longer recommended.

Types and Declarations

Method Declarations

As shown in the example, the recommended order for declarations in an @interface declaration are: properties, class methods, initializers, and then finally instance methods. The class methods section should begin with any convenience constructors.

Local Variables

Declare variables in the narrowest practical scopes, and close to their use. Initialize variables in their declarations.

// GOOD:

CLLocation *location = [self lastKnownLocation];
for (int meters = 1; meters < 10; meters++) {
  reportFrogsWithinRadius(location, meters);
}

Occasionally, efficiency will make it more appropriate to declare a variable outside the scope of its use. This example declares meters separate from initialization, and needlessly sends the lastKnownLocation message each time through the loop:

// AVOID:

int meters;                                         // AVOID.
for (meters = 1; meters < 10; meters++) {
  CLLocation *location = [self lastKnownLocation];  // AVOID.
  reportFrogsWithinRadius(location, meters);
}

Under Automatic Reference Counting, strong and weak pointers to Objective-C objects are automatically initialized to nil, so explicit initialization to nil is not required for those common cases. However, automatic initialization does not occur for many Objective-C pointer types, including object pointers declared with the __unsafe_unretained ownership qualifier and CoreFoundation object pointer types. When in doubt, prefer to initialize all Objective-C local variables.

Static Variables

When file scope variable/constant declarations in an implementation file do not need to be referenced outside that file, declare them static (or in an anonymous namespace in Objective-C++). Do not declare file scope variables or constants with static storage duration (or in anonymous namespaces in Objective-C++) in .h files.

// GOOD:

// file: Foo.m
static const int FOORequestLimit = 5;

// AVOID:

// file: Foo.h
static const int FOORequestLimit = 5;  // AVOID.

Unsigned Integers

Avoid unsigned integers except when matching types used by system interfaces.

Subtle errors crop up when doing math or counting down to zero using unsigned integers. Rely only on signed integers in math expressions except when matching NSUInteger in system interfaces.

// GOOD:

NSUInteger numberOfObjects = array.count;
for (NSInteger counter = numberOfObjects - 1; counter >= 0; --counter)

// AVOID:

for (NSUInteger counter = numberOfObjects - 1; counter >= 0; --counter)  // AVOID.

Unsigned integers may be used for flags and bitmasks, though often NS_OPTIONS or NS_ENUM will be more appropriate.

Types with Inconsistent Sizes

Be aware that types long, NSInteger, NSUInteger and CGFloat have sizes that differ in 32- and 64-bit builds. Their use is appropriate when matching system interfaces but should be avoided when dealing with APIs that require exact sizing, e.g., proto APIs.

// GOOD:

int32_t scalar1 = proto.intValue;

int64_t scalar2 = proto.longValue;

NSUInteger numberOfObjects = array.count;

CGFloat offset = view.bounds.origin.x;

// AVOID:

NSInteger scalar2 = proto.longValue;  // AVOID.

File and buffer sizes often exceed 32-bit limits, so they should be declared using int64_t, not with long, NSInteger, or NSUInteger.

Floating Point Constants

When defining CGFloat constants, please keep in mind the following.

Previously for projects targeting 32-bit platforms, using float literals (numbers with the f suffix) could be necessary to avoid type-conversion warnings.

Since all Google iOS projects are now targeting only 64-bit runtime, CGFloat constants may omit the suffix (use double values). However, teams may choose to continue using float numbers for legacy code consistency, until they eventually migrate to double values everywhere. Avoid a mixture of float and double values in the same code.

// GOOD:

// Good since CGFloat is double
static const CGFloat kHorizontalMargin = 8.0;
static const CGFloat kVerticalMargin = 12.0;

// This is OK as long as all values for CGFloat constants in your project are float
static const CGFloat kHorizontalMargin = 8.0f;
static const CGFloat kVerticalMargin = 12.0f;

// AVOID:

// Avoid a mixture of float and double constants
static const CGFloat kHorizontalMargin = 8.0f;
static const CGFloat kVerticalMargin = 12.0;

Comments

Comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are 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.

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but use a consistent style.

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!

File Comments

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

Every file may contain the following items, in order

• License boilerplate if necessary. Choose the appropriate boilerplate for the license used by the project.
• 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 non-trivial interface, public and private, should have an accompanying comment describing its purpose and how it fits into the larger picture.

Comments should be used to document classes, properties, ivars, functions, categories, protocol declarations, and enums.

// GOOD:

/**
 * A delegate for NSApplication to handle notifications about app
 * launch and shutdown. Owned by the main app controller.
 */
@interface MyAppDelegate : NSObject {
  /**
   * The background task in progress, if any. This is initialized
   * to the value UIBackgroundTaskInvalid.
   */
  UIBackgroundTaskIdentifier _backgroundTaskID;
}

/** The factory that creates and manages fetchers for the app. */
@property(nonatomic) GTMSessionFetcherService *fetcherService;

@end

Doxygen-style comments are encouraged for interfaces as they are parsed by Xcode to display formatted documentation. There is a wide variety of Doxygen commands; use them consistently within a project.

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 should have a comment explaining its function, arguments, return value, thread or queue assumptions, and any side effects. Documentation comments should be in the header for public methods, or immediately preceding the method for non-trivial private methods.

Use descriptive form ("Opens the file") rather than imperative form ("Open the file") for method and function comments. The comment describes the function; it does not tell the function what to do.

Document the thread usage assumptions the class, properties, or methods make, 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.

Any sentinel values for properties and ivars, such as NULL or -1, should be documented in comments.

Declaration comments explain how a method or function is used. Comments explaining how a method or function is implemented should be with the implementation rather than with the declaration.

Declaration comments may be omitted on test case classes and test methods if comments would communicate no additional information beyond the method's name. Utility methods in tests or test-specific classes (such as helpers) should be commented.

Implementation Comments

Provide comments explaining tricky, subtle, or complicated sections of code.

// GOOD:

// Set the property to nil before invoking the completion handler to
// avoid the risk of reentrancy leading to the callback being
// invoked again.
CompletionHandler handler = self.completionHandler;
self.completionHandler = nil;
handler();

When useful, also provide comments about implementation approaches that were considered or abandoned.

End-of-line comments should be separated from the code by at least 2 spaces. If you have several comments on subsequent lines, it can often be more readable to line them up.

// GOOD:

[self doSomethingWithALongName];  // Two spaces before the comment.
[self doSomethingShort];          // More spacing to align the comment.

Disambiguating Symbols

Where needed to avoid ambiguity, use backticks or vertical bars to quote variable names and symbols in comments in preference to using quotation marks or naming the symbols inline.

In Doxygen-style comments, prefer demarcating symbols with a monospace text command, such as @c.

Demarcation helps provide clarity when a symbol is a common word that might make the sentence read like it was poorly constructed. A common example is the symbol count:

// GOOD:

// Sometimes `count` will be less than zero.

or when quoting something which already contains quotes

// GOOD:

// Remember to call `StringWithoutSpaces("foo bar baz")`

Backticks or vertical bars are not needed when a symbol is self-apparent.

// GOOD:

// This class serves as a delegate to GTMDepthCharge.

Doxygen formatting is also suitable for identifying symbols.

// GOOD:

/** @param maximum The highest value for @c count. */

Object Ownership

For objects not managed by ARC, 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 for NSObject-derived objects are presumed to be retained; if they are not retained, they should be either commented as weak or declared with the __weak lifetime qualifier.

An exception is in Mac software for instance variables labeled as @IBOutlets, 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.

Examples of strong and weak declarations:

// GOOD:

@interface MyDelegate : NSObject

@property(nonatomic) NSString *doohickey;
@property(nonatomic, weak) NSString *parent;

@end


@implementation MyDelegate {
  IBOutlet NSButton *_okButton;  // Normal NSControl; implicitly weak on Mac only

  AnObjcObject *_doohickey;  // My doohickey
  __weak MyObjcParent *_parent;  // To send messages back (owns this instance)

  // non-NSObject pointers...
  CWackyCPPClass *_wacky;  // Strong, some cross-platform object
  CFDictionaryRef *_dict;  // Strong
}
@end

Automatic Reference Counting

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

C Language Features

Macros

Avoid macros, especially where const variables, enums, Xcode snippets, or C functions may be used instead.

Macros make the code you see different from the code the compiler sees. Modern C renders traditional uses of macros for constants and utility functions unnecessary. Macros should only be used when there is no other solution available.

Where a macro is needed, use a unique name to avoid the risk of a symbol collision in the compilation unit. If practical, keep the scope limited by #undefining the macro after its use.

Macro names should use SHOUTY_SNAKE_CASE—all uppercase letters with underscores between words. Function-like macros may use C function naming practices. Do not define macros that appear to be C or Objective-C keywords.

// GOOD:

#define GTM_EXPERIMENTAL_BUILD ...      // GOOD

// Assert unless X > Y
#define GTM_ASSERT_GT(X, Y) ...         // GOOD, macro style.

// Assert unless X > Y
#define GTMAssertGreaterThan(X, Y) ...  // GOOD, function style.

// AVOID:

#define kIsExperimentalBuild ...        // AVOID

#define unless(X) if(!(X))              // AVOID

Avoid macros that expand to unbalanced C or Objective-C constructs. Avoid macros that introduce scope, or may obscure the capturing of values in blocks.

Avoid macros that generate class, property, or method definitions in headers to be used as public API. These only make the code hard to understand, and the language already has better ways of doing this.

Avoid macros that generate method implementations, or that generate declarations of variables that are later used outside of the macro. Macros shouldn't make code hard to understand by hiding where and how a variable is declared.

// AVOID:

#define ARRAY_ADDER(CLASS) \
  -(void)add ## CLASS ## :(CLASS *)obj toArray:(NSMutableArray *)array

ARRAY_ADDER(NSString) {
  if (array.count > 5) {              // AVOID -- where is 'array' defined?
    ...
  }
}

Examples of acceptable macro use include assertion and debug logging macros that are conditionally compiled based on build settings—often, these are not compiled into release builds.

Nonstandard Extensions

Nonstandard extensions to C/Objective-C may not be used unless otherwise specified.

Compilers support various extensions that are not part of standard C. Examples include compound statement expressions (e.g. foo = ({ int x; Bar(&x); x })).

The __typeof__ Keyword

The __typeof__ keyword is allowed in cases where the type doesn't aid in clarity for the reader. The __typeof__ keyword is encouraged over other similar keywords (e.g., the typeof keyword) as it is supported in all language variants.

// GOOD:

  __weak __typeof__(self) weakSelf = self;

// AVOID:

  __typeof__(data) copiedData = [data copy];  // AVOID.
  __weak typeof(self) weakSelf = self;        // AVOID.

The __auto_type Keyword and Type Deduction

Type deduction using the __auto_type keyword is allowed only for local variables of block and function pointer types. Avoid type deduction if a typedef already exists for the block or pointer type.

// GOOD:

__auto_type block = ^(NSString *arg1, int arg2) { ... };
__auto_type functionPointer = &MyFunction;

typedef void(^SignInCallback)(Identity *, NSError *);
SignInCallback signInCallback = ^(Identity *identity, NSError *error) { ... };

// AVOID:

__auto_type button = [self createButtonForInfo:info];
__auto_type viewController = [[MyCustomViewControllerClass alloc] initWith...];

typedef void(^SignInCallback)(Identity *, NSError *);
__auto_type signInCallback = ^(Identity *identity, NSError *error) { ... };

Approved Nonstandard Extensions

• The __attribute__ keyword is approved as it is used in Apple API declarations.
• The binary form of the conditional operator, A ?: B, is approved.

Cocoa and Objective-C Features

Identify Designated Initializers

Clearly identify your designated initializer(s).

It is important for subclassing that a class clearly identify its designated initializers. This allows a subclass to override a subset of initializers to initialize subclass state or invoke a new designated initializer provided by the subclass. Clearly identified designated initializers also make tracing through and debugging initialization code easier.

Prefer identifying designated initializers by annotating them with designated initializer attributes, e.g., NS_DESIGNATED_INITIALIZER. Declare designated initializers in comments when designated initializer attributes are not available. Prefer a single designated initializer unless there is a compelling reason or requirement for multiple designated initializers.

Support initializers inherited from superclasses by overriding superclass designated initializers to ensure that all inherited initializers are directed through subclass designated initializers. When there is a compelling reason or requirement that an inherited initializer should not be supported, the initializer may be annotated with availability attributes (e.g., NS_UNAVAILABLE) to discourage usage; however, note that availability attributes alone do not completely protect against invalid initialization.

Override Designated Initializers

When writing a subclass that requires a new designated initializer, make sure you override any designated initializers of the superclass.

When declaring designated initializers on a class, remember that any initializers that were considered designated initializers on the superclass become convenience initializers of the subclass unless declared otherwise. Failure to override superclass designated initializers can result in bugs due to invalid initialization using superclass initializers. To avoid invalid initialization, ensure convenience initializers call through to a designated initializer.

Overridden NSObject Method Placement

Put overridden methods of NSObject at the top of an @implementation.

This commonly applies to (but is not limited to) the init..., copyWithZone:, and dealloc methods. The init... methods should be grouped together, including those init... methods that are not NSObject overrides, followed by other typical NSObject methods such as description, isEqual:, and hash.

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

Initialization

Don't initialize instance variables to 0 or nil in the init method; doing so is redundant.

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

Instance Variables In Headers Should Be @protected or @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 @protected or @private.

// GOOD:

@interface MyClass : NSObject {
 @protected
  id _myInstanceVariable;
}
@end

Do Not Use +new

Do not invoke the NSObject class method new, nor override it in a subclass. +new is rarely used and contrasts greatly with initializer usage. Instead, use +alloc and -init methods to instantiate retained objects.

Keep the Public API Simple

Keep your class simple; avoid "kitchen-sink" APIs. If a method doesn't need to be public, keep it out of the public interface.

Unlike C++, Objective-C doesn't differentiate between public and private methods; any message may be sent to an object. 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.

Since internal methods are not really private, it's easy to 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.

#import and #include

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

C/C++ headers include other C/C++ headers using #include. Using #import on C/C++ headers prevents future inclusions using #include and could result in unintended compilation behavior.

C/C++ headers should provide their own #define guard.

Order of Includes

The standard order for header inclusion is the related header, operating system headers, language library headers, and finally groups of headers for other dependencies.

The related header precedes others to ensure it has no hidden dependencies. For implementation files the related header is the header file. For test files the related header is the header containing the tested interface.

Separate each non-empty group of includes with one blank line. Within each group the includes should be ordered alphabetically.

Import headers using their path relative to the project's source directory.

// GOOD:

#import "ProjectX/BazViewController.h"

#import <Foundation/Foundation.h>

#include <unistd.h>
#include <vector>

#include "base/basictypes.h"
#include "base/integral_types.h"
#import "base/mac/FOOComplexNumberSupport"
#include "util/math/mathutil.h"

#import "ProjectX/BazModel.h"
#import "Shared/Util/Foo.h"

Use Umbrella Headers for System Frameworks

Import umbrella headers for system frameworks and system libraries rather than include 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 or #import rather than #include for Objective-C frameworks.

// GOOD:

@import UIKit;     // GOOD.
#import <Foundation/Foundation.h>     // GOOD.

// AVOID:

#import <Foundation/NSArray.h>        // AVOID.
#import <Foundation/NSString.h>
...

Avoid Messaging the Current Object Within Initializers and -dealloc

Code in initializers and -dealloc should avoid invoking instance methods when possible.

Superclass initialization completes before subclass initialization. Until all classes have had a chance to initialize their instance state any method invocation on self may lead to a subclass operating on uninitialized instance state.

A similar issue exists for -dealloc, where a method invocation may cause a class to operate on state that has been deallocated.

One case where this is less obvious is property accessors. These can be overridden just like any other selector. Whenever practical, directly assign to and release ivars in initializers and -dealloc, rather than rely on accessors.

// GOOD:

- (instancetype)init {
  self = [super init];
  if (self) {
    _bar = 23;  // GOOD.
  }
  return self;
}

Beware of factoring common initialization code into helper methods:

• Methods can be overridden in subclasses, either deliberately, or accidentally due to naming collisions.
• When editing a helper method, it may not be obvious that the code is being run from an initializer.

// AVOID:

- (instancetype)init {
  self = [super init];
  if (self) {
    self.bar = 23;  // AVOID.
    [self sharedMethod];  // AVOID. Fragile to subclassing or future extension.
  }
  return self;
}

// GOOD:

- (void)dealloc {
  [_notifier removeObserver:self];  // GOOD.
}

// AVOID:

- (void)dealloc {
  [self removeNotifications];  // AVOID.
}

There are common cases where a class may need to use properties and methods provided by a superclass during initialization. This commonly occurs for classes derived from UIKit and AppKit base classes, among other base classes. Use your judgement and knowledge of common practice when deciding whether to make an exception to this rule.

Avoid redundant property access

Code should avoid redundant property access. Prefer to assign a property value to a local variable when the property value is not expected to change and needs to be used multiple times.

// GOOD:

UIView *view = self.view;
UIScrollView *scrollView = self.scrollView;
[scrollView.leadingAnchor constraintEqualToAnchor:view.leadingAnchor].active = YES;
[scrollView.trailingAnchor constraintEqualToAnchor:view.trailingAnchor].active = YES;

// AVOID:

[self.scrollView.loadingAnchor constraintEqualToAnchor:self.view.loadingAnchor].active = YES;
[self.scrollView.trailingAnchor constraintEqualToAnchor:self.view.trailingAnchor].active = YES;

When repeatedly referencing chained property invocations, prefer to capture the repeated expression in a local variable:

// AVOID:

foo.bar.baz.field1 = 10;
foo.bar.baz.field2 = @"Hello";
foo.bar.baz.field3 = 2.71828183;

// GOOD:

Baz *baz = foo.bar.baz;
baz.field1 = 10;
baz.field2 = @"Hello";
baz.field3 = 2.71828183;

Redundantly accessing the same properties results in multiple message dispatches to fetch the same value, and under ARC requires retains and releases of any returned objects; the compiler cannot optimize away these extra operations, leading to slower execution and substantial increases in binary size.

Mutables, Copies and Ownership

For Foundation and other hierarchies containing both immutable and mutable subclasses a mutable subclass may be substituted for an immutable so long as the immutable's contract is honored.

The most common example of this sort of substitution are ownership transfers, particularly for return values. In these cases an additional copy is not necessary and returning the mutable subclass is more efficient. Callers are expected to treat return values as their declared type, and thus the return value will be treated as an immutable going forward.

// GOOD:

- (NSArray *)listOfThings {
  NSMutableArray *generatedList = [NSMutableArray array];
  for (NSInteger i = 0; i < _someLimit; i++) {
    [generatedList addObject:[self thingForIndex:i]];
  }
  // Copy not necessary, ownership of generatedList is transferred.
  return generatedList;
}

This rule also applies to classes where only a mutable variant exists so long as the ownership transfer is clear. Protos are a common example.

// GOOD:

- (SomeProtoMessage *)someMessageForValue:(BOOL)value {
  SomeProtoMessage *message = [SomeProtoMessage message];
  message.someValue = value;
  return message;
}

It is not necessary to create a local immutable copy of a mutable type to match the method signature of a method being called so long as the mutable argument will not change for the duration of the method call. Called methods are expected to treat arguments as the declared type, and take defensive copies (referred to by Apple as "snapshots") if they intend to retain those arguments beyond the duration of the call.

// AVOID:

NSMutableArray *updatedThings = [NSMutableArray array];
[updatedThings addObject:newThing];
[_otherManager updateWithCurrentThings:[updatedThings copy]];  // AVOID

Copy Potentially Mutable Objects

Code receiving and retaining collections or other types with mutable variants should consider that the passed object may be mutable, and thus an immutable or mutable copy should be retained instead of the original object. In particular, initializers and setters should copy instead of retaining objects whose types have mutable variants.

Synthesized accessors should use the copy keyword to ensure the generated code matches these expectations.

NOTE: The copy property keyword only affects the synthesized setter and has no effect on getters. Since property keywords have no effect on direct ivar access custom accessors must implement the same copy semantics.

// GOOD:

@property(nonatomic, copy) NSString *name;
@property(nonatomic, copy) NSSet<FilterThing *> *filters;

- (instancetype)initWithName:(NSString *)name
                     filters:(NSSet<FilterThing *> *)filters {
  self = [super init];
  if (self) {
    _name = [name copy];
    _filters = [filters copy];
  }
  return self;
}

- (void)setFilters:(NSSet<FilterThing *> *)filters {
  // Ensure that we retain an immutable collection.
  _filters = [filters copy];
}

Similarly, getters must return types that match the contract expectations of the immutable types they return.

// GOOD:


@implementation Foo {
  NSMutableArray<ContentThing *> *_currentContent;
}

- (NSArray<ContentThing *> *)currentContent {
  return [_currentContent copy];
}

All Objective-C protos are mutable and typically should be copied rather than retained except in clear cases of ownership transfer.

// GOOD:

- (void)setFooMessage:(FooMessage *)fooMessage {
  // Copy proto to ensure no other retainer can mutate our value.
  _fooMessage = [fooMessage copy];
}

- (FooMessage *)fooMessage {
  // Copy proto to return so that caller cannot mutate our value.
  return [_fooMessage copy];
}

Asynchronous code should copy potentially mutable objects prior to dispatch. Objects captured by blocks are retained but not copied.

// GOOD:

- (void)doSomethingWithThings:(NSArray<Thing *> *)things {
  NSArray<Thing *> *thingsToWorkOn = [things copy];
  dispatch_async(_workQueue, ^{
    for (id<Thing> thing in thingsToWorkOn) {
      ...
    }
  });
}

NOTE: It is unnecessary to copy objects that do not have mutable variants, e.g. NSURL, NSNumber, NSDate, UIColor, etc.

Use Lightweight Generics to Document Contained Types

All projects compiling on Xcode 7 or newer versions should make use of the Objective-C lightweight generics notation to type contained objects.

Every NSArray, NSDictionary, or NSSet reference should be declared using lightweight generics for improved type safety and to explicitly document usage.

// GOOD:

@property(nonatomic, copy) NSArray<Location *> *locations;
@property(nonatomic, copy, readonly) NSSet<NSString *> *identifiers;

NSMutableArray<MyLocation *> *mutableLocations = [otherObject.locations mutableCopy];

If the fully-annotated types become complex, consider using a typedef to preserve readability.

// GOOD:

typedef NSSet<NSDictionary<NSString *, NSDate *> *> TimeZoneMappingSet;
TimeZoneMappingSet *timeZoneMappings = [TimeZoneMappingSet setWithObjects:...];

Use the most descriptive common superclass or protocol available. In the most generic case when nothing else is known, declare the collection to be explicitly heterogeneous using id.

// GOOD:

@property(nonatomic, copy) NSArray<id> *unknowns;

Avoid Throwing Exceptions

Don't @throw Objective-C exceptions, but you should be prepared to catch them from third-party or OS calls.

This follows the recommendation to use error objects for error delivery in Apple's Introduction to Exception Programming Topics for Cocoa.

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

Avoid nil pointer checks that exist only to prevent sending messages to nil. 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}.

// AVOID:

if (dataSource) {  // AVOID.
  [dataSource moveItemAtIndex:1 toIndex:0];
}

// GOOD:

[dataSource moveItemAtIndex:1 toIndex:0];  // GOOD.

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.

Nullability

Interfaces can be decorated with nullability annotations to describe how the interface should be used and how it behaves. Use of nullability regions (e.g., NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END) and explicit nullability annotations are both accepted. Prefer using the _Nullable and _Nonnull keywords over the __nullable and __nonnull keywords. For Objective-C methods and properties prefer using the context-sensitive, non-underscored keywords, e.g., nonnull and nullable.

// GOOD:

/** A class representing an owned book. */
@interface GTMBook : NSObject

/** The title of the book. */
@property(nonatomic, readonly, copy, nonnull) NSString *title;

/** The author of the book, if one exists. */
@property(nonatomic, readonly, copy, nullable) NSString *author;

/** The owner of the book. Setting nil resets to the default owner. */
@property(nonatomic, copy, null_resettable) NSString *owner;

/** Initializes a book with a title and an optional author. */
- (nonnull instancetype)initWithTitle:(nonnull NSString *)title
                               author:(nullable NSString *)author
    NS_DESIGNATED_INITIALIZER;

/** Returns nil because a book is expected to have a title. */
- (nullable instancetype)init;

@end

/** Loads books from the file specified by the given path. */
NSArray<GTMBook *> *_Nullable GTMLoadBooksFromFile(NSString *_Nonnull path);

// AVOID:

NSArray<GTMBook *> *__nullable GTMLoadBooksFromTitle(NSString *__nonnull path);

Do not assume that a pointer is not null based on a nonnull qualifier, because the compiler only checks a subset of such cases, and does not guarantee that the pointer is not null. Avoid intentionally violating nullability semantics of function, method, and property declarations.

BOOL Pitfalls

BOOL Expressions and Conversions

Be careful when converting general integral values to BOOL. Avoid comparing directly with YES or comparing multiple BOOL values with comparison operators.

BOOL on some Apple platforms (notably Intel macOS, watchOS, and 32-bit iOS) is defined as a signed char, so it may 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. These operations can depend on the value of the last byte of the integer value and result in an unexpected NO value. Operations with NS_OPTIONS values and flag masking are especially common errors.

When converting a general integral value to a BOOL, use conditional 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). 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 conditional operator.

// AVOID:

- (BOOL)isBold {
  return [self fontTraits] & NSFontBoldTrait;  // AVOID.
}
- (BOOL)isValid {
  return [self stringValue];  // AVOID.
}
- (BOOL)isLongEnough {
  return (BOOL)([self stringValue].count);  // AVOID.
}

// GOOD:

- (BOOL)isBold {
  return ([self fontTraits] & NSFontBoldTrait) ? YES : NO;
}
- (BOOL)isValid {
  return [self stringValue] != nil;
}
- (BOOL)isLongEnough {
  return [self stringValue].count > 0;
}
- (BOOL)isEnabled {
  return [self isValid] && [self isBold];
}

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

// AVOID:

BOOL great = [foo isGreat];
if (great == YES) {  // AVOID.
  // ...be great!
}

// GOOD:

BOOL great = [foo isGreat];
if (great) {         // GOOD.
  // ...be great!
}

Don't directly compare BOOL values using comparison operators. BOOL values that are true may not be equal. Use logical operators in place of bitwise comparisons of BOOL values.

// AVOID:

if (oldBOOLValue != newBOOLValue) {  // AVOID.
  // ... code that should only run when the value changes.
}

// GOOD:

if ((!oldBoolValue && newBoolValue) || (oldBoolValue && !newBoolValue)) {  // GOOD.
  // ... code that should only run when the value changes.
}

// GOOD, the results of logical operators on BOOLs are safe to compare.
if (!oldBoolValue != !newBoolValue) {
  // ... code that should only run when the value changes.
}

BOOL Literals

The BOOL NSNumber literals are @YES and @NO which are equivalent to [NSNumber numberWithBool:...].

Avoid using boxed expressions to create BOOL values, including simple expressions like @(YES). Boxed expressions suffer from [the same pitfalls as other BOOL expressions] (#BOOL_Expressions_Conversions) as boxing general integral values can produce true or false NSNumbers that are not equal to @YES and @NO.

When converting a general integral value to a BOOL literal, use conditional operators to convert to @YES or @NO. Do not embed a conditional operator inside a boxed expression as this is equivalent to boxing general integral values even when the result of the operation is a BOOL.

// AVOID:

[_boolArray addValue:@(YES)];  // AVOID boxing even in simple cases.
NSNumber *isBold = @(self.fontTraits & NSFontBoldTrait);  // AVOID.
NSNumber *hasContent = @([self stringValue].length);  // AVOID.
NSNumber *isValid = @([self stringValue]);  // AVOID.
NSNumber *isStringNotNil = @([self stringValue] ? YES : NO);  // AVOID.

// GOOD:

[_boolArray addValue:@YES];  // GOOD.
NSNumber *isBold = self.fontTraits & NSFontBoldTrait ? @YES : @NO;  // GOOD.
NSNumber *hasContent = [self stringValue].length ? @YES : @NO;  // GOOD.
NSNumber *isValid = [self stringValue] ? @YES : @NO;  // GOOD.
NSNumber *isStringNotNil = [self stringValue] ? @YES : @NO;  // GOOD.

Containers Without Instance Variables

Omit the empty set of braces on interfaces, class extensions, and implementations without any instance variable declarations.

// GOOD:

@interface MyClass : NSObject
// Does a lot of stuff.
- (void)fooBarBam;
@end

@interface MyClass ()
- (void)classExtensionMethod;
@end

@implementation MyClass
// Actual implementation.
@end

// AVOID:

@interface MyClass : NSObject {
}
// Does a lot of stuff.
- (void)fooBarBam;
@end

@interface MyClass () {
}
- (void)classExtensionMethod;
@end

@implementation MyClass {
}
// Actual implementation.
@end

Cocoa Patterns

Delegate Pattern

Delegates, target objects, and block pointers should not be retained when doing so would create a retain cycle.

To avoid causing a retain cycle, a delegate or target pointer should be released as soon as it is clear there will no longer be a need to message the object.

If there is no clear time at which the delegate or target pointer is no longer needed, the pointer should only be retained weakly.

Block pointers cannot be retained weakly. To avoid causing retain cycles in the client code, block pointers should be used for callbacks only where they can be explicitly released after they have been called or once they are no longer needed. Otherwise, callbacks should be done via weak delegate or target pointers.

Objective-C++

Style Matches the Language

Within an Objective-C++ source file, follow the style for the language of the function or 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.

For code in an @implementation block, use the Objective-C naming rules. For code in a method of a C++ class, use the C++ naming rules.

For code in an Objective-C++ file outside of a class implementation, be consistent within the file.

// GOOD:

// 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];
}

Projects may opt to use an 80 column line length limit for consistency with Google's C++ style guide.

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, and to trim trailing spaces on lines.

Line Length

The maximum line length for Objective-C files is 100 columns.

Method Declarations and Definitions

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

Methods should look like this:

// GOOD:

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

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

If a method declaration does not fit on a single line, put each parameter on its own line. All lines except the first should be indented at least four spaces. Colons before parameters should be aligned on all lines. If the colon before the parameter on the first line of a method declaration is positioned such that colon alignment would cause indentation on a subsequent line to be less than four spaces, then colon alignment is only required for all lines except the first. If a parameter declared after the : in a method declaration or definition would cause the line limit to be exceeded, wrap the content to the next line indented by at least four spaces.

// GOOD:

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

- (void)shortKeyword:(GTMFoo *)theFoo
            longerKeyword:(NSRect)theRect
    someEvenLongerKeyword:(float)theInterval
                    error:(NSError **)theError {
  ...
}

- (id<UIAdaptivePresentationControllerDelegate>)
    adaptivePresentationControllerDelegateForViewController:(UIViewController *)viewController;

- (void)presentWithAdaptivePresentationControllerDelegate:
    (id<UIAdaptivePresentationControllerDelegate>)delegate;

- (void)updateContentHeaderViewForExpansionToContentOffset:(CGPoint)contentOffset
                                            withController:
                                                (GTMCollectionExpansionController *)controller;

Function Declarations and Definitions

Prefer putting the return type on the same line as the function name and append all parameters on the same line if they will fit. Wrap parameter lists which do not fit on a single line as you would wrap arguments in a function call.

// GOOD:

NSString *GTMVersionString(int majorVersion, int minorVersion) {
  ...
}

void GTMSerializeDictionaryToFileOnDispatchQueue(
    NSDictionary<NSString *, NSString *> *dictionary,
    NSString *filename,
    dispatch_queue_t queue) {
  ...
}

Function declarations and definitions should also satisfy the following conditions:

• The opening parenthesis must always be on the same line as the function name.
• If you cannot fit the return type and the function name on a single line, break between them and do not indent the function name.
• There should never be a space before the opening parenthesis.
• There should never be a space between function parentheses and parameters.
• The open curly brace is always on the end of the last line of the function declaration, not the start of the next line.
• The close curly brace is either on the last line by itself or on the same line as the open curly brace.
• There should be a space between the close parenthesis and the open curly brace.
• All parameters should be aligned if possible.
• Function scopes should be indented 2 spaces.
• Wrapped parameters should have a 4 space indent.

Conditionals

Include a space after if, while, for, and switch, and around comparison operators.

// GOOD:

for (int i = 0; i < 5; ++i) {
}

while (test) {};

Braces may be omitted when a loop body or conditional statement fits on a single line.

// GOOD:

if (hasSillyName) LaughOutLoud();

for (int i = 0; i < 10; i++) {
  BlowTheHorn();
}

// AVOID:

if (hasSillyName)
  LaughOutLoud();               // AVOID.

for (int i = 0; i < 10; i++)
  BlowTheHorn();                // AVOID.

If an if clause has an else clause, both clauses should use braces.

// GOOD:

if (hasBaz) {
  foo();
} else {  // The else goes on the same line as the closing brace.
  bar();
}

// AVOID:

if (hasBaz) foo();
else bar();        // AVOID.

if (hasBaz) {
  foo();
} else bar();      // AVOID.

Intentional fall-through to the next case should be documented with a comment unless the case has no intervening code before the next case.

// GOOD:

switch (i) {
  case 1:
    ...
    break;
  case 2:
    j++;
    // Falls through.
  case 3: {
    int k;
    ...
    break;
  }
  case 4:
  case 5:
  case 6: break;
}

Expressions

Use a space around binary operators and assignments. Omit a space for a unary operator. Do not add spaces inside parentheses.

// GOOD:

x = 0;
v = w * x + y / z;
v = -y * (x + z);

Factors in an expression may omit spaces.

// GOOD:

v = w*x + y/z;

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:

// GOOD:

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

or have one argument per line, with colons aligned:

// GOOD:

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

Don't use any of these styles:

// AVOID:

[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:

// GOOD:

[myObj short:arg1
          longKeyword:arg2
    evenLongerKeyword:arg3
                error:arg4];

Invocations containing multiple inlined blocks may have their parameter names left-aligned at a four space indent.

Function Calls

Function calls should include as many parameters as fit on each line, except where shorter lines are needed for clarity or documentation of the parameters.

Continuation lines for function parameters may be indented to align with the opening parenthesis, or may have a four-space indent.

// GOOD:

CFArrayRef array = CFArrayCreate(kCFAllocatorDefault, objects, numberOfObjects,
                                 &kCFTypeArrayCallBacks);

NSString *string = NSLocalizedStringWithDefaultValue(@"FEET", @"DistanceTable",
    resourceBundle,  @"%@ feet", @"Distance for multiple feet");

UpdateTally(scores[x] * y + bases[x],  // Score heuristic.
            x, y, z);

TransformImage(image,
               x1, x2, x3,
               y1, y2, y3,
               z1, z2, z3);

Use local variables with descriptive names to shorten function calls and reduce nesting of calls.

// GOOD:

double scoreHeuristic = scores[x] * y + bases[x];
UpdateTally(scoreHeuristic, x, y, z);

Exceptions

Format exceptions with @catch and @finally labels on the same line as the preceding }. Add a space between the @ label and the opening brace ({), as well as between the @catch and the caught object declaration. If you must use Objective-C exceptions, format them as follows. However, see Avoid Throwing Exceptions for reasons why you should not be using exceptions.

// GOOD:

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

Function Length

Prefer small and focused functions.

Long functions and methods are occasionally appropriate, so no hard limit is placed on function length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.

Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.

When updating legacy code, consider also breaking long functions into smaller and more manageable pieces.

Vertical Whitespace

Use vertical whitespace sparingly.

To allow more code to be easily viewed on a screen, avoid putting blank lines just inside the braces of functions.

Limit blank lines to one or two between functions and between logical groups of code.

Objective-C Style Exceptions

Indicating style exceptions

Lines of code that are not expected to adhere to these style recommendations require // NOLINT at the end of the line or // NOLINTNEXTLINE at the end of the previous line. Sometimes it is required that parts of Objective-C code must ignore these style recommendations (for example code may be machine generated or code constructs are such that its not possible to style correctly).

A // NOLINT comment on that line or // NOLINTNEXTLINE on the previous line can be used to indicate to the reader that code is intentionally ignoring style guidelines. In addition these annotations can also be picked up by automated tools such as linters and handle code correctly. Note that there is a single space between // and NOLINT*.