styleguide/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 the primary development language for applications on OS X and on iOS.
>
> Apple has already written a very good, and widely accepted, [Cocoa Coding
> Guidelines](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html)
> 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 that should be used for iOS and
> OS X code. 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](https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html).


## 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.

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.

```objectivec 
// 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;

/**
 * Designated initializer.
 *
 * @param bar A string that represents a thing that does a thing.
 */
- (instancetype)initWithBar:(Bar *)bar;

/**
 * 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.

```objectivec 
// 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
```

## 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.

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:

```objectivec 
// 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 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.

```objectivec 
// GOOD:

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

When the second or later parameter name is longer than the first, indent the
second and later lines by at least four spaces, maintaining colon alignment:

```objectivec 
// GOOD:

- (void)short:(GTMFoo *)theFoo
          longKeyword:(NSRect)theRect
    evenLongerKeyword:(float)theInterval
                error:(NSError **)theError {
  ...
}
```

### Conditionals 

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

```objectivec 
// 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.

```objectivec 
// GOOD:

if (hasSillyName) LaughOutLoud();

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

```objectivec 
// 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.

```objectivec 
// GOOD:

if (hasBaz) {
  foo();
} else {
  bar();
}
```

```objectivec 
// 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.

```objectivec 
// 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.

```objectivec 
// GOOD:

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

Factors in an expression may omit spaces.

```objectivec 
// 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:

```objectivec 
// GOOD:

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

or have one argument per line, with colons aligned:

```objectivec 
// GOOD:

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

Don't use any of these styles:

```objectivec 
// 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:

```objectivec 
// 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.

```objectivec 
// 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.

```objectivec 
// 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.

```objectivec 
// 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.

## Naming 

Names should be as descriptive as possible, within reason. Follow standard
[Objective-C naming
rules](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html).

Avoid non-standard abbreviations. 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:

```objectivec 
// GOOD:

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

```objectivec 
// 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](https://en.wikipedia.org/wiki/Initialism)
within 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.

### 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

### 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 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 `NSString+GTMParsing.h`, and the category itself
would be named `GTMNSStringParsingAdditions`. The file name and the category may
not match, as 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's global namespace.

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

```objectivec 
// GOOD:

// Using a category to extend a Foundation class.
@interface NSString (GTMNSStringParsingAdditions)
- (NSString *)gtm_parsedString;
@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.

```objectivec 
// GOOD:

+ (NSURL *)URLWithString;
```

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.

```objectivec 
// 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.
```

A method that returns an object should have a name beginning with a noun
identifying the object returned:

```objectivec 
// GOOD:

- (Sandwich *)sandwich;      // GOOD.
```

```objectivec 
// AVOID:

- (Sandwich *)makeSandwich;  // 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:

```objectivec 
// GOOD:

- (id)delegate;     // GOOD.
```

```objectivec 
// 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.

```objectivec 
// GOOD:

@property(nonatomic, getter=isGlorious) BOOL glorious;
- (BOOL)isGlorious;

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

```objectivec 
// AVOID:

BOOL isGood = object.isGlorious;    // AVOID.
```

```objectivec 
// GOOD:

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

```objectivec 
// AVOID:

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

See [Apple's Guide to Naming
Methods](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingMethods.html#//apple_ref/doc/uid/20001282-BCIGIJJF)
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 

Regular functions have mixed case.

Ordinarily, functions should start with a capital letter and have a capital
letter for each new word (a.k.a. "[Camel
Case](https://en.wikipedia.org/wiki/Camel_case)" or "Pascal case").

```objectivec 
// GOOD:

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

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

```objectivec 
// GOOD:

extern NSTimeZone *GTMGetDefaultTimeZone();
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.

```objectivec 
// 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.

Constants should have an appropriate prefix.

```objectivec 
// GOOD:

extern NSString *const GTLServiceErrorDomain;

typedef NS_ENUM(NSInteger, GTLServiceError) {
  GTLServiceErrorQueryResultMissing = -3000,
  GTLServiceErrorWaitTimedOut       = -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:

```objectivec 
// GOOD:

typedef NS_ENUM(NSInteger, DisplayTinge) {
  DisplayTingeGreen = 1,
  DisplayTingeBlue = 2,
};
```

Constants may use a lowercase k prefix when appropriate:

```objectivec 
// GOOD:

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

## Types and Declarations 

### Local Variables 

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

```objectivec 
// 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:

```objectivec 
// AVOID:

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

Under Automatic Reference Counting, pointers to Objective-C objects are by
default initialized to `nil`, so explicit initialization to `nil` is not
required.

### 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.

```objectivec 
// GOOD:

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

```objectivec 
// 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 

Due to sizes that differ in 32- and 64-bit builds, avoid types long, NSInteger,
NSUInteger, and CGFloat except when matching system interfaces.

Types long, NSInteger, NSUInteger, and CGFloat vary in size between 32- and
64-bit builds. Use of these types is appropriate when handling values exposed by
system interfaces, but they should be avoided for most other computations.

```objectivec 
// GOOD:

int32_t scalar1 = proto.intValue;

int64_t scalar2 = proto.longValue;

NSUInteger numberOfObjects = array.count;

CGFloat offset = view.bounds.origin.x;
```

```objectivec 
// 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`.

## 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.

```objectivec 
// 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
```

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.

### Implementation Comments 

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

```objectivec 
// 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.

```objectivec 
// 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`:

```objectivec 
// GOOD:

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

or when quoting something which already contains quotes

```objectivec 
// GOOD:

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

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

```objectivec 
// GOOD:

// This class serves as a delegate to GTMDepthCharge.
```

Doxygen formatting is also suitable for identifying symbols.

```objectivec 
// 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:

```objectivec 
// 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
  MyObjcParent * __weak _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.

```objectivec 
// 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.
```

```objectivec 
// 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.

```objectivec 
// 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 }))`
and variable-length arrays.

`__attribute__` is an approved exception, as it is used in Objective-C API
specifications.

The binary form of the conditional operator, `A ?: B`, is an approved exception.

## Cocoa and Objective-C Features 

### Identify Designated Initializer 

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
override a single initializer (of potentially several) to guarantee the
initializer of their subclass 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. Identify the designated initializer using comments or the
`NS_DESIGNATED_INITIALIZER` macro. If you use `NS_DESIGNATED_INITIALIZER`, mark
unsupported initializers with `NS_UNAVAILABLE`.

### Override Designated Initializer 

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

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

### 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,
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](https://developer.apple.com/library/mac/documentation/General/Conceptual/CocoaEncyclopedia/ObjectAllocation/ObjectAllocation.html)
`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`.

```objectivec 
// GOOD:

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

### 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, 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.

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


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

### 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.

A blank line may separate logically distinct groups of included headers.

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

```objectivec 
// GOOD:

#import "ProjectX/BazViewController.h"

#import <Foundation/Foundation.h>

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

#include "base/basictypes.h"
#include "base/integral_types.h"
#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.

```objectivec 
// GOOD:

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

```objectivec 
// AVOID:

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

### 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 on
self.

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

```objectivec 
// GOOD:

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

```objectivec 
// AVOID:

- (instancetype)init {
  self = [super init];
  if (self) {
    self.bar = 23;  // AVOID.
  }
  return self;
}
```

```objectivec 
// GOOD:

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

```objectivec 
// AVOID:

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

### Setters copy NSStrings 

Setters taking an `NSString` should always copy the string it accepts. This is
often also appropriate for collections like `NSArray` and `NSDictionary`.

Never just retain the string, as it may be a `NSMutableString`. This avoids the
caller changing it under you without your knowledge.

Code receiving and holding collection objects should also consider that the
passed collection may be mutable, and thus the collection could be more safely
held as a copy or mutable copy of the original.

```objectivec 
// GOOD:

@property(nonatomic, copy) NSString *name;

- (void)setZigfoos:(NSArray<Zigfoo *> *)zigfoos {
  // Ensure that we're holding an immutable collection.
  _zigfoos = [zigfoos copy];
}
```

### 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.

```objectivec 
// 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.

```objectivec 
// 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
heterogenous using id.

```objectivec 
// 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](https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Exceptions/Exceptions.html).

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. Sending a message to `nil` [reliably
returns](http://www.sealiesoftware.com/blog/archive/2012/2/29/objc_explain_return_value_of_message_to_nil.html)
`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` in OS X and in 32-bit iOS builds 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` that could, depending on
the value of the last byte of the integer value, still result in a `NO` value.
When converting a general integral value to a `BOOL` use ternary 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
ternary operator.

```objectivec 
// AVOID:

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

```objectivec 
// GOOD:

- (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, but the first point above
demonstrates that return values may not always be what you expect.

```objectivec 
// AVOID:

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

```objectivec 
// GOOD:

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

### Interfaces Without Instance Variables 

Omit the empty set of braces on interfaces that do not declare any instance
variables.

```objectivec 
// GOOD:

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

```objectivec 
// AVOID:

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

```objectivec++ 
// 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.

## 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*`.