objcguide.xml

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<?xml-stylesheet type="text/xsl" href="styleguide.xsl"?>
<GUIDE title="Google Objective-C Style Guide">

<p align="right">

Revision 2.59
</p>


<div align="right">
  <address>
    Mike Pinkerton<br/>
    Greg Miller <br/>
    Dave MacLachlan
  </address>
</div>

<OVERVIEW>

<CATEGORY title="Important Note">
  <STYLEPOINT title="Displaying Hidden Details in this Guide">
     <SUMMARY>
       This style guide contains many details that are initially
       hidden from view.  They are marked by the triangle icon, which you
       see here on your left.  Click it now.
       You should see "Hooray" appear below.
     </SUMMARY>
     <BODY>
       <p>
        Hooray!  Now you know you can expand points to get more
        details.  Alternatively, there's an "expand all" at the
        top of this document.
       </p>
     </BODY>
  </STYLEPOINT>
</CATEGORY>

<CATEGORY title="Background">

  <p>
    Objective-C is a very dynamic, object-oriented extension of C.  It's
    designed to be easy to use and read, while enabling sophisticated
    object-oriented design. It is the primary development language for new
    applications on Mac OS X and the iPhone.
  </p>

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

  <p>
    Apple has already written a very good, and widely accepted, coding guide
    for Objective-C. Google has also written a similar guide for C++. This
    Objective-C guide aims to be a very natural combination of Apple's and
    Google's general recommendations. So, before reading this guide, please make
    sure you've read:
    <ul>
      <li>
        <a href="https://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/index.html">
          Apple's Cocoa Coding Guidelines
        </a>
      </li>
      <li>
        
        <div>
          <a href="https://google-styleguide.googlecode.com/svn/trunk/cppguide.xml">
            Google's Open Source C++ Style Guide
          </a>
        </div>
      </li>
    </ul>
  </p>

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

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

  <p>
    Google has already released open-source code that conforms to these
    guidelines as part of the
    <a href="https://code.google.com/p/google-toolbox-for-mac/">
      Google Toolbox for Mac project
    </a>
    (abbreviated GTM throughout this document).
    Code meant to be shared across different projects is a good candidate to
    be included in this repository.
  </p>

  
  <p>
    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
    <a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html">
      Programming with Objective-C
    </a>.
  </p>
</CATEGORY>
</OVERVIEW>

<CATEGORY title="Example">

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

  <p>
    An example header file, demonstrating the correct commenting and spacing
    for an <code>@interface</code> declaration
  </p>

<CODE_SNIPPET>
    #import &lt;Foundation/Foundation.h&gt;

    // A sample class demonstrating good Objective-C style. All interfaces,
    // categories, and protocols (read: all non-trivial top-level declarations
    // in a header) MUST be commented. Comments must also be adjacent to the
    // object they're documenting.
    //
    // (no blank line between this comment and the interface)
    @interface Foo : NSObject

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

    // Designated initializer. |bar| is a thing that represents a thing that
    // does a thing.
    - (instancetype)initWithBar:(NSString *)bar;

    // Gets and sets |_bar|.
    - (NSString *)bar;
    - (void)setBar:(NSString *)bar;

    // Does some work with |blah| and returns YES if the work was completed
    // successfully, and NO otherwise.
    - (BOOL)doWorkWithBlah:(NSString *)blah;

    @end
  </CODE_SNIPPET>

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

  <CODE_SNIPPET>
    #import "Foo.h"


    @implementation Foo {
      NSString *_bar;
      NSString *_foo;
    }

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

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

    - (instancetype)initWithBar:(NSString *)bar {
      if ((self = [super init])) {
        _bar = [bar copy];
        _bam = [[NSString alloc] initWithFormat:@"hi %d", 3];
      }
      return self;
    }

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

    - (NSString *)bar {
      return _bar;
    }

    - (void)setBar:(NSString *)bar {
      [_bar autorelease];
      _bar = [bar copy];
    }

    - (BOOL)doWorkWithBlah:(NSString *)blah {
      // ...
      return NO;
    }

    @end
  </CODE_SNIPPET>

  <p>
    Blank lines before and after <code>@interface</code>,
    <code>@implementation</code>, and <code>@end</code> are optional. If your
    <code>@interface</code> declares instance variables, a blank
    line should come after the closing brace (<code>}</code>).
  <p>
  </p>
    Unless an interface or implementation is very short, such as when declaring
    a handful of private methods or a bridge class, adding blank lines usually
    helps readability.
  </p>

</CATEGORY>

<CATEGORY title="Spacing And Formatting">

  <STYLEPOINT title="Spaces vs. Tabs">
    <SUMMARY>
      Use only spaces, and indent 2 spaces at a time.
    </SUMMARY>
    <BODY>
      <p>
        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.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Line Length">
    <SUMMARY>
      The maximum line length for Objective-C and Objective-C++ files is 100
      columns. Projects may opt to use an 80 column limit for consistency with
      the C++ style guide.
    </SUMMARY>
    <BODY>
      <p>
        You can make violations easier to spot by enabling <i>Preferences &gt;
        Text Editing &gt; Page guide at column: 100</i> in Xcode.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Method Declarations and Definitions">
    <SUMMARY>
      One space should be used between the <code>-</code> or <code>+</code>
      and the return type, and no spacing in the parameter list except between
      parameters.
    </SUMMARY>
    <BODY>
      <p>
        Methods should look like this:
      </p>
      <CODE_SNIPPET>
        - (void)doSomethingWithString:(NSString *)theString {
          ...
        }
      </CODE_SNIPPET>
      <p>
        The spacing before the asterisk is optional. When adding new code,
        be consistent with the surrounding file's style.
      </p>
      <p>
        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.
      </p>
      <CODE_SNIPPET>
        - (void)doSomethingWith:(GTMFoo *)theFoo
                           rect:(NSRect)theRect
                       interval:(float)theInterval {
          ...
        }
      </CODE_SNIPPET>
      <p>
        When the first keyword is shorter than the others, indent the later
        lines by at least four spaces, maintaining colon alignment:
      </p>
      <CODE_SNIPPET>
        - (void)short:(GTMFoo *)theFoo
                  longKeyword:(NSRect)theRect
            evenLongerKeyword:(float)theInterval
                        error:(NSError **)theError {
          ...
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Method Invocations">
    <SUMMARY>
      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.
    </SUMMARY>
    <BODY>
      <p>
        Invocations should have all arguments on one line:
      </p>
      <CODE_SNIPPET>
        [myObject doFooWith:arg1 name:arg2 error:arg3];
      </CODE_SNIPPET>
      <p>
        or have one argument per line, with colons aligned:
      </p>
      <CODE_SNIPPET>
        [myObject doFooWith:arg1
                       name:arg2
                      error:arg3];
      </CODE_SNIPPET>
      <p>
        Don't use any of these styles:
      </p>
      <BAD_CODE_SNIPPET>
        [myObject doFooWith:arg1 name:arg2  // some lines with &gt;1 arg
                      error:arg3];

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

        [myObject doFooWith:arg1
                  name:arg2  // aligning keywords instead of colons
                  error:arg3];
      </BAD_CODE_SNIPPET>

      <p>
        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:
      </p>
      <CODE_SNIPPET>
        [myObj short:arg1
                  longKeyword:arg2
            evenLongerKeyword:arg3
                        error:arg4];
      </CODE_SNIPPET>
      <p>
        Invocations containing inlined <a href="#Blocks">blocks</a> may have
        their segments left-aligned at a four space indent.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="@public and @private">
    <SUMMARY>
      The <code>@public</code> and <code>@private</code> access modifiers
      should be indented by 1 space.
    </SUMMARY>
    <BODY>
      <p>
        This is similar to <code>public</code>, <code>private</code>, and
        <code>protected</code> in C++.
      </p>
      <CODE_SNIPPET>
        @interface MyClass : NSObject {
         @public
          ...
         @private
          ...
        }
        @end
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Exceptions">
    <SUMMARY>
      Format exceptions with each <code>@</code> label on its own line and a
      space between the <code>@</code> label and the opening brace
      (<code>{</code>), as well as between the <code>@catch</code> and the
      caught object declaration.
    </SUMMARY>
    <BODY>
      <p>
        If you must use Obj-C exceptions, format them as follows.  However, see
        <a href="#Avoid_Throwing_Exceptions">Avoid Throwing Exceptions</a> for
        reasons why you <b>should not</b> be using exceptions.
      </p>
      <CODE_SNIPPET>
        @try {
          foo();
        }
        @catch (NSException *ex) {
          bar(ex);
        }
        @finally {
          baz();
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Protocols">
    <SUMMARY>
      There should not be a space between the type identifier and the name
      of the protocol encased in angle brackets.
    </SUMMARY>
    <BODY>
      <p>
        This applies to class declarations, instance variables, and method
        declarations. For example:
      </p>
      <CODE_SNIPPET>
        @interface MyProtocoledClass : NSObject&lt;NSWindowDelegate&gt; {
         @private
          id&lt;MyFancyDelegate&gt; _delegate;
        }
        - (void)setDelegate:(id&lt;MyFancyDelegate&gt;)aDelegate;
        @end
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Blocks">
    <SUMMARY>
      Code inside blocks should be indented four spaces.
    </SUMMARY>
    <BODY>
      <p>
        There are several appropriate style rules, depending on how long the
        block is:
      </p>
      <ul>
        <li>If the block can fit on one line, no wrapping is necessary.</li>
        <li>
          If it has to wrap, the closing brace should line up with the first
          character of the line on which the block is declared.
        </li>
        <li>Code within the block should be indented four spaces.</li>
        <li>
          If the block is large, e.g. more than 20 lines, it is recommended to
          move it out-of-line into a local variable.
        </li>
        <li>
          If the block takes no parameters, there are no spaces between the
          characters <code>^{</code>. If the block takes parameters, there is no
          space between the <code>^(</code> characters, but there is one space
          between the <code>) {</code> characters.
        </li>
        <li>
          Invocations containing inlined blocks may have their segments
          left-aligned at a four-space indent. This helps when invocations
          contain multiple inlined blocks.
        </li>
        <li>
          Two space indents inside blocks are also allowed, but should only
          be used when it's consistent with the rest of the project's code.
        </li>
      </ul>
      <CODE_SNIPPET>
        // The entire block fits on one line.
        [operation setCompletionBlock:^{ [self onOperationDone]; }];

        // The block can be put on a new line, indented four spaces, with the
        // closing brace aligned with the first character of the line on which
        // block was declared.
        [operation setCompletionBlock:^{
            [self.delegate newDataAvailable];
        }];

        // Using a block with a C API follows the same alignment and spacing
        // rules as with Objective-C.
        dispatch_async(_fileIOQueue, ^{
            NSString* path = [self sessionFilePath];
            if (path) {
              // ...
            }
        });

        // An example where the parameter wraps and the block declaration fits
        // on the same line. Note the spacing of |^(SessionWindow *window) {|
        // compared to |^{| above.
        [[SessionService sharedService]
            loadWindowWithCompletionBlock:^(SessionWindow *window) {
                if (window) {
                  [self windowDidLoad:window];
                } else {
                  [self errorLoadingWindow];
                }
            }];

        // An example where the parameter wraps and the block declaration does
        // not fit on the same line as the name.
        [[SessionService sharedService]
            loadWindowWithCompletionBlock:
                ^(SessionWindow *window) {
                    if (window) {
                      [self windowDidLoad:window];
                    } else {
                      [self errorLoadingWindow];
                    }
                }];

        // Large blocks can be declared out-of-line.
        void (^largeBlock)(void) = ^{
            // ...
        };
        [_operationQueue addOperationWithBlock:largeBlock];

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

  <STYLEPOINT title="Container Literals">
    <SUMMARY>
      For projects using Xcode 4.4 or later and clang, the use of container
      (array and dictionary) literals is encouraged. If split across multiple
      lines, the contents should be indented two spaces.
    </SUMMARY>
    <BODY>
      <p>
        If the collection fits on one line, put a single space after the opening
        and before the closing brackets.
      </p>
      <CODE_SNIPPET>
        NSArray* array = @[ [foo description], @"Another String", [bar description] ];

        NSDictionary* dict = @{ NSForegroundColorAttributeName : [NSColor redColor] };
      </CODE_SNIPPET>
      <p>
        Not:
      </p>
      <BAD_CODE_SNIPPET>
        NSArray* array = @[[foo description], [bar description]];

        NSDictionary* dict = @{NSForegroundColorAttributeName: [NSColor redColor]};
      </BAD_CODE_SNIPPET>

      <p>
        If the collection spans more than a single line, place the opening
        bracket on the same line as the declaration, indent the body by two
        spaces, and place the closing bracket on a new line that is indented to
        the same level as the opening bracket.
      </p>
      <CODE_SNIPPET>
        NSArray* array = @[
          @"This",
          @"is",
          @"an",
          @"array"
        ];

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

      <p>
        For dictionary literals, there should be one space before the colon and
        at least one space after it (to optionally align the values).
      </p>
      <CODE_SNIPPET>
        NSDictionary* option1 = @{
          NSFontAttributeName : [NSFont fontWithName:@"Helvetica-Bold" size:12],
          NSForegroundColorAttributeName : fontColor
        };

        NSDictionary* option2 = @{
          NSFontAttributeName :            [NSFont fontWithName:@"Arial" size:12],
          NSForegroundColorAttributeName : fontColor
        };
      </CODE_SNIPPET>
      <p>
        The following are all incorrect:
      </p>
      <BAD_CODE_SNIPPET>
        // There should be a space before the colon.
        NSDictionary* wrong = @{
          AKey:       @"b",
          BLongerKey: @"c",
        };

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

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

<CATEGORY title="Naming">

  <p>
    Naming rules are very important in maintainable code. Objective-C method
    names tend to be very long, but this has the benefit that a block of code
    can almost read like prose, thus rendering many comments unnecessary. </p>
    <p> When writing pure Objective-C code, we mostly follow standard <a href="https://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/CodingGuidelines.html">Objective-C
    naming rules</a>. These naming guidelines may differ
    significantly from those outlined in the C++ style guide. For example,
    Google's C++ style guide recommends the use of underscores between words
    in variable names, whereas this guide recommends the use of intercaps,
    which is standard in the Objective-C community.
  </p>
  <p>
    Any class, category, method, or variable name may use all capitals for
    <a href="https://en.wikipedia.org/wiki/Initialism">initialisms</a>
    within the name. This follows Apple's standard of using all capitals
    within a name for initialisms such as URL, TIFF, and EXIF.
  </p>
  <p>
    When writing Objective-C++, however, things are not so cut and dry. Many
    projects need to implement cross-platform C++ APIs with some Objective-C
    or Cocoa, or bridge between a C++ back-end and a native Cocoa front-end.
    This leads to situations where the two guides are directly at odds.
  </p>
  <p>
    Our solution is that the style follows that of the method/function being
    implemented. If you're in an <code>@implementation</code> block, use the
    Objective-C naming rules. If you're implementing a method for a C++
    <code>class</code>, use the C++ naming rules. This avoids the situation
    where instance variable and local variable naming rules are mixed within a
    single function, which would be a serious detriment to readability.
  </p>

  <STYLEPOINT title="File Names">
    <SUMMARY>
      File names should reflect the name of the class implementation that
      they contain—including case. Follow the convention that your
      
      project
      uses.
    </SUMMARY>
    <BODY>
      <p>
        File extensions should be as follows:
      </p>
      <table>
        <tr>
          <td><code>.h</code></td>
          <td>C/C++/Objective-C header file</td>
        </tr>
        <tr>
          <td><code>.m</code></td>
          <td>Objective-C implementation file</td>
        </tr>
        <tr>
          <td><code>.mm</code></td>
          <td>Objective-C++ implementation file</td>
        </tr>
        <tr>
          <td><code>.cc</code></td>
          <td>Pure C++ implementation file</td>
        </tr>
        <tr>
          <td><code>.c</code></td>
          <td>C implementation file</td>
        </tr>
      </table>
      <p>
        File names for categories should include the name of the class being
        extended, e.g. <code>GTMNSString+Utils.h</code> or
        <code>GTMNSTextView+Autocomplete.h</code>
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Objective-C++">
    <SUMMARY>
      Within a source file, Objective-C++ follows the style of the
      function/method you're implementing.
    </SUMMARY>
    <BODY>
      <p>
        In order to minimize clashes between the differing naming styles when
        mixing Cocoa/Objective-C and C++, follow the style of the method being
        implemented. If you're in an <code>@implementation</code> block, use
        the Objective-C naming rules. If you're implementing a method for a
        C++ <code>class</code>, use the C++ naming rules.
      </p>
      <CODE_SNIPPET>
        // 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-&gt;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];
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Class Names">
    <SUMMARY>
      Class names (along with category and protocol names) should start as
      uppercase and use mixed case to delimit words.
    </SUMMARY>
    <BODY>
      <p>
         When designing code to be shared across multiple applications,
         prefixes are acceptable and recommended (e.g. <code>GTMSendMessage</code>).
         Prefixes are also recommended for classes of large applications that
         depend on external libraries.
      </p>
      
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Category Names">
    <SUMMARY>
      Category names should start with a 2 or 3 character prefix
      identifying the category as part of a project or open for general
      use. The category name should incorporate the name of the class it's
      extending.
    </SUMMARY>
    <BODY>
      <p>
        For example, if we want to create a category on <code>NSString</code>
        for parsing, we would put the category in a file named
        <code>GTMNSString+Parsing.h</code>, and the category itself would be
        named <code>GTMStringParsingAdditions</code> (yes, we know the file
        name and the category name do not match, but this file could have many
        separate categories related to parsing). Methods in that category
        should share the prefix (<code>gtm_myCategoryMethodOnAString:</code>)
        in order to prevent collisions in Objective-C which only has a single
        namespace. If the code isn't meant to be shared and/or doesn't run in
        a different address-space, the method naming isn't quite as
        important.
      </p>
      <p>
        There should be a single space between the class name and the opening
        parenthesis of the category.
      </p>
      <CODE_SNIPPET>
        // Extending a framework class:
        @interface NSString (GTMStringParsingAdditions)
        - (NSString *)gtm_foobarString;
        @end

        // Making your methods and properties private:
        @interface FoobarViewController ()
        @property(nonatomic, retain) NSView *dongleView;
        - (void)performLayout;
        @end
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Objective-C Method Names">
    <SUMMARY>
      Method names should start as lowercase and then use mixed case.
      Each named parameter should also start as lowercase.
    </SUMMARY>
    <BODY>
      <p>
        The method name should read like a sentence if possible, meaning you
        should choose parameter names that flow with the method name. (e.g.
        <code>convertPoint:fromRect:</code> or
        <code>replaceCharactersInRange:withString:</code>). See <a href="https://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingMethods.html#//apple_ref/doc/uid/20001282-BCIGIJJF">Apple's
        Guide to Naming Methods</a> for more details.
      </p>
      <p>
        Accessor methods should be named the same as the variable they're
        "getting", but they should <em>not</em> be prefixed with the word
        "get". For example:
        <BAD_CODE_SNIPPET>
          - (id)getDelegate;  // AVOID
        </BAD_CODE_SNIPPET>
        <CODE_SNIPPET>
          - (id)delegate;    // GOOD
        </CODE_SNIPPET>
      </p>
      <p>
        This is for Objective-C methods only. C++ method names and functions
        continue to follow the rules set in the C++ style guide.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Variable Names">
    <SUMMARY>
      Variables names start with a lowercase and use mixed case to delimit
      words. Instance variables have leading underscores. For example:
      <var>myLocalVariable</var>, <var>_myInstanceVariable</var>.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Common Variable Names">
        <p>
          Do <em>not</em> use Hungarian notation for syntactic attributes,
          such as the static type of a variable (int or pointer). Give as
          descriptive a name as possible, within reason. Don't worry about
          saving horizontal space as it is far more important to make your
          code immediately understandable by a new reader. For example:
        </p>
        <BAD_CODE_SNIPPET>
          int w;
          int nerr;
          int nCompConns;
          tix = [[NSMutableArray alloc] init];
          obj = [someObject object];
          p = [network port];
        </BAD_CODE_SNIPPET>
        <CODE_SNIPPET>
          int numErrors;
          int numCompletedConnections;
          tickets = [[NSMutableArray alloc] init];
          userInfo = [someObject object];
          port = [network port];
        </CODE_SNIPPET>
      </SUBSECTION>

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

      <SUBSECTION title="Constants">
        <p>
          Constant names (#defines, enums, const local variables, etc.) should
          start with a lowercase <var>k</var> and then use mixed case to
          delimit words.  For example:
        </p>
        <CODE_SNIPPET>
          const int kNumberOfFiles = 12;
          NSString *const kUserKey = @"kUserKey";
          enum DisplayTinge {
            kDisplayTingeGreen = 1,
            kDisplayTingeBlue = 2
          };
        </CODE_SNIPPET>
        <p>
          Because Objective-C does not provide namespacing, constants with global
          scope should have an appropriate prefix to minimize the chance of name
          collision, typically like <var>kClassNameFoo</var>.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

</CATEGORY>

<CATEGORY title="Comments">

  <p>
    Though a pain to write, they are absolutely vital to keeping our code
    readable. The following rules describe what you should comment and where.
    But remember: while comments are very important, the best code is
    self-documenting. Giving sensible names to types and variables is much
    better than using obscure names and then trying to explain them through
    comments.
  </p>
  <p>
    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!
  </p>
  <p>
    Remember that all of the rules and conventions listed in the C++ Style
    Guide are in effect here, with a few additional points, below.
  </p>

  <STYLEPOINT title="File Comments">
    <SUMMARY>
      A file may optionally start with a description of its contents.
    </SUMMARY>
    <BODY>
      
      <p>
        Every file should contain the following items, in order:
        <ul>
          <li>license boilerplate if neccessary. Choose the appropriate
              boilerplate for the license used by the project (e.g.
              Apache 2.0, BSD, LGPL, GPL).</li>
          <li>a basic description of the contents of the file if necessary.</li>
        </ul>
      </p>
      <p>
        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.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Declaration Comments">
    <SUMMARY>
      Every interface, category, and protocol declaration should have an
      accompanying comment describing its purpose and how it fits into the
      larger picture.
    </SUMMARY>
    <BODY>
      <CODE_SNIPPET>
        // A delegate for NSApplication to handle notifications about app
        // launch and shutdown. Owned by the main app controller.
        @interface MyAppDelegate : NSObject {
          ...
        }
        @end
      </CODE_SNIPPET>
      <p>
        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.
      </p>
      <p>
        Additionally, each method in the public interface should have a
        comment explaining its function, arguments, return value, and any
        side effects.
      </p>
      <p>
        Document the synchronization assumptions the class makes, if
        any. If an instance of the class can be accessed by multiple
        threads, take extra care to document the rules and invariants
        surrounding multithreaded use.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Implementation Comments">
    <SUMMARY>
      Use vertical bars to quote variable names and symbols in comments rather
      than quotes or naming the symbol inline.
    </SUMMARY>
    <BODY>
      <p>
        This helps eliminate ambiguity, especially when the symbol is a common
        word that might make the sentence read like it was poorly constructed.
        E.g. for a symbol "count":
      </p>
      <CODE_SNIPPET>
        // Sometimes we need |count| to be less than zero.
      </CODE_SNIPPET>
      <p>
        or when quoting something which already contains quotes
      </p>
      <CODE_SNIPPET>
        // Remember to call |StringWithoutSpaces("foo bar baz")|
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Object Ownership">
    <SUMMARY>
      Make the pointer ownership model as explicit as possible when it falls
      outside the most common Objective-C usage idioms.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Manual Reference Counting">
        <p>
          Instance variables which are pointers to objects derived from NSObject
          are presumed to be retained, and should be either commented as weak or
          declared with the <b>__weak</b> lifetime qualifier when applicable.
          Similarly, declared properties must specify an <b>assign</b> property
          attribute if they are not retained by the class.  An exception is
          instance variables labeled as IBOutlets in desktop Mac software,
          which are presumed to not be retained.
        </p>
        <p>
          Where instance variables are pointers to Core Foundation, C++, and
          other non-Objective-C objects, they should always be declared with
          <code>strong</code> and <code>weak</code> comments to indicate which
          pointers are and are not retained. Core Foundation and other
          non-Objective-C object pointers require explicit memory management,
          even when building for automatic reference counting or garbage
          collection.
        </p>
        <p>
          Examples of strong and weak declarations:
          <CODE_SNIPPET>
            @interface MyDelegate : NSObject {
             @private
              IBOutlet NSButton *_okButton;  // Normal NSControl; implicitly weak on Mac only

              AnObjcObject* _doohickey;  // My doohickey
              __weak MyObjcParent *_parent;  // So we can send msgs back (owns me)

              // non-NSObject pointers...
              CWackyCPPClass *_wacky;  // Strong, some cross-platform object
              CFDictionaryRef *_dict;  // Strong
            }
            @property(strong, nonatomic) NSString *doohickey;
            @property(weak, nonatomic) NSString *parent;
            @end
          </CODE_SNIPPET>
        </p>
      </SUBSECTION>
      <SUBSECTION title="Automatic Reference Counting">
        <p>
          Object ownership and lifetime are explicit when using ARC, so no
          additional comments are required.
        </p>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

</CATEGORY>

<CATEGORY title="Cocoa and Objective-C Features">

  <STYLEPOINT title="Instance Variables In Headers Should Be @private">
    <SUMMARY>
      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 <code>@private</code>.
    </SUMMARY>
    <BODY>
      <CODE_SNIPPET>
        @interface MyClass : NSObject {
         @private
          id _myInstanceVariable;
        }
        @end
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Identify Designated Initializer">
    <SUMMARY>
      Comment and clearly identify your designated initializer.
    </SUMMARY>
    <BODY>
      <p>
        It is important for those who might be subclassing your class that the
        designated initializer be clearly identified. That way, they only need
        to subclass a single initializer (of potentially several) to guarantee
        their subclass' initializer is called. It also helps those debugging
        your class in the future understand the flow of initialization code if
        they need to step through it.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Override Designated Initializer">
    <SUMMARY>
      When writing a subclass that requires an <code>init...</code> method,
      make <i>sure</i> you override the superclass' designated initializer.
    </SUMMARY>
    <BODY>
      <p>
        If you fail to override the superclass' designated initializer, your
        initializer may not be called in all cases, leading to subtle and
        very difficult to find bugs.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Overridden NSObject Method Placement">
    <SUMMARY>
      It is strongly recommended and typical practice to place overridden
      methods of <code>NSObject</code> at the top of an
      <code>@implementation</code>.
    </SUMMARY>
    <BODY>
      <p>
        This commonly applies (but is not limited) to the <code>init...</code>,
        <code>copyWithZone:</code>, and <code>dealloc</code> methods.
        <code>init...</code> methods should be grouped together, followed by
        other <code>NSObject</code> methods.
      </p>
      <p>
        Convenience class methods for creating instances may precede the
        <code>NSObject</code> methods.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Initialization">
    <SUMMARY>
      Don't initialize variables to <code>0</code> or <code>nil</code> in the
      init method; it's redundant.
    </SUMMARY>
    <BODY>
      <p>
        All memory for a newly allocated object is initialized to 0 (except
        for <var>isa</var>), so don't clutter up the <code>init</code> method
        by re-initializing variables to 0 or <code>nil</code>.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Avoid +new">
    <SUMMARY>
      Do not invoke the <code>NSObject</code> class method <code>new</code>,
      nor override it in a subclass. Instead, use <code>alloc</code> and
      <code>init</code> methods to instantiate retained objects.
    </SUMMARY>
    <BODY>
      <p>
        Modern Objective-C code explicitly calls <code>alloc</code> and an
        <code>init</code> method to create and retain an object.  As the
        <code>new</code> class method is rarely used, it makes reviewing code
        for correct memory management more difficult.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Keep the Public API Simple">
    <SUMMARY>
      Keep your class simple; avoid "kitchen-sink" APIs. If a method doesn't
      need to be public, don't make it so. Use a private category to prevent
      cluttering the public header.
    </SUMMARY>
    <BODY>
      <p>
        Unlike C++, Objective-C doesn't have a way to differentiate between
        public and private methods—everything is public. As a result,
        avoid placing methods in the public API unless they are actually
        expected to be used by a consumer of the class. This helps reduce the
        likelihood they'll be called when you're not expecting it. This includes
        methods that are being overridden from the parent class. For internal
        implementation methods, use a category defined in the implementation
        file as opposed to adding them to the public header.
      </p>
      <CODE_SNIPPET>
        #import "GTMFoo.h"

        @interface GTMFoo (PrivateDelegateHandling)
        - (NSString *)doSomethingWithDelegate;  // Declare private method
        @end

        @implementation GTMFoo (PrivateDelegateHandling)
        ...
        - (NSString *)doSomethingWithDelegate {
          // Implement this method
        }
        ...
        @end
      </CODE_SNIPPET>
      <p>
        If you are using Objective-C 2.0, you should instead declare your
        private category using a <a href="https://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/chapter_4_section_5.html#">class
        extension</a>, for example:
      </p>
      <CODE_SNIPPET>
      @interface GMFoo () { ... }
      </CODE_SNIPPET>
      <p>
        which will guarantee that the declared methods are implemented in the
        <code>@implementation</code> section by issuing a compiler warning if
        they are not.
      </p>
      <p>
        Again, "private" methods are not really private. You could
        accidentally override a superclass's "private" method, thus making a
        very difficult bug to squash. In general, private methods should have
        a fairly unique name that will prevent subclasses from unintentionally
        overriding them.
      </p>
      <p>
        Finally, Objective-C categories are a great way to segment a large
        <code>@implementation</code> section into more understandable chunks
        and to add new, application-specific functionality to the most
        appropriate class. For example, instead of adding "middle truncation"
        code to a random object in your app, make a new category on
        <code>NSString</code>).
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="#import and #include">
    <SUMMARY>
      <code>#import</code> Objective-C/Objective-C++ headers, and
      <code>#include</code> C/C++ headers.
    </SUMMARY>
    <BODY>
      <p>
        Choose between <code>#import</code> and <code>#include</code> based
        on the language of the header that you are including.
      </p>
      <ul>
        <li>When including a header that uses Objective-C or Objective-C++,
            use <code>#import</code>.</li>
        <li>When including a standard C or C++ header, use
            <code>#include</code>.  The header should provide its own <a href="cppguide.xml?showone=The__define_Guard#The__define_Guard">#define
            guard</a>.</li>
      </ul>
      <p>
        Some Objective-C headers lack <code>#define</code> guards, and expect
        to be included only by <code>#import</code>.  As Objective-C headers
        may only be included in Objective-C source files and other Objective-C
        headers, using <code>#import</code> across the board is appropriate.
      </p>
      <p>
        Standard C and C++ headers without any Objective-C in them can expect
        to be included by ordinary C and C++ files.  Since there is no
        <code>#import</code> in standard C or C++, such files will be
        included by <code>#include</code> in those cases.  Using
        <code>#include</code> for them in Objective-C source files as well
        means that these headers will always be included with the same
        semantics.
      </p>
      <p>
        This rule helps avoid inadvertent errors in cross-platform
        projects.  A Mac developer introducing a new C or C++ header might
        forget to add <code>#define</code> guards, which would not cause
        problems on the Mac if the new header were included with
        <code>#import</code>, but would break builds on other platforms
        where <code>#include</code> is used.  Being consistent by using
        <code>#include</code> on all platforms means that compilation is
        more likely to succeed everywhere or fail everywhere, and avoids
        the frustration of files working only on some platforms.
      </p>
      <CODE_SNIPPET>
        #import &lt;Cocoa/Cocoa.h&gt;
        #include &lt;CoreFoundation/CoreFoundation.h&gt;
        #import "GTMFoo.h"
        #include "base/basictypes.h"
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Use Root Frameworks">
    <SUMMARY>
      Include root frameworks over individual files.
    </SUMMARY>
    <BODY>
      <p>
        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 <code>#import</code> rather than
        <code>#include</code> for Objective-C frameworks.
      </p>
      <CODE_SNIPPET>
        #import &lt;Foundation/Foundation.h&gt;     // good
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        #import &lt;Foundation/NSArray.h&gt;        // avoid
        #import &lt;Foundation/NSString.h&gt;
        ...
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Prefer To autorelease At Time of Creation">
    <SUMMARY>
      When creating new temporary objects, <code>autorelease</code> them on
      the same line as you create them rather than a separate
      <code>release</code> later in the same method.
    </SUMMARY>
    <BODY>
      <p>
        While ever so slightly slower, this prevents someone from accidentally
        removing the <code>release</code> or inserting a <code>return</code>
        before it and introducing a memory leak. E.g.:
      </p>
      <BAD_CODE_SNIPPET>
        // AVOID (unless you have a compelling performance reason)
        MyController* controller = [[MyController alloc] init];
        // ... code here that might return ...
        [controller release];
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        // BETTER
        MyController* controller = [[[MyController alloc] init] autorelease];
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Autorelease Then Retain">
    <SUMMARY>
      Assignment of objects follows the <code>autorelease</code> then
      <code>retain</code> pattern.
    </SUMMARY>
    <BODY>
      <p>
        When assigning a new object to a variable, one must first release the
        old object to avoid a memory leak. There are several "correct" ways to
        handle this. We've chosen the "autorelease then retain" approach
        because it's less prone to error. Be aware in tight loops it can fill
        up the autorelease pool, and may be slightly less efficient, but we
        feel the tradeoffs are acceptable.
      </p>
      <CODE_SNIPPET>
        - (void)setFoo:(GMFoo *)aFoo {
          [_foo autorelease];  // Won't dealloc if |_foo| == |aFoo|
          _foo = [aFoo retain];
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Avoid Accessors During init and dealloc">
    <SUMMARY>
      Instance subclasses may be in an inconsistent state during
      <code>init</code> and <code>dealloc</code> method execution, so code in
      those methods should avoid invoking accessors.
    </SUMMARY>
    <BODY>
      <p>
        Subclasses have not yet been initialized or have already deallocated
        when <code>init</code> and <code>dealloc</code> methods execute, making
        accessor methods potentially unreliable. Whenever practical, directly
        assign to and release ivars in those methods rather than rely on
        accessors.
      </p>
      <CODE_SNIPPET>
        - (instancetype)init {
          self = [super init];
          if (self) {
            _bar = [[NSMutableString alloc] init];  // good
          }
          return self;
        }

        - (void)dealloc {
          [_bar release];                           // good
          [super dealloc];
        }
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        - (instancetype)init {
          self = [super init];
          if (self) {
            self.bar = [NSMutableString string];  // avoid
          }
          return self;
        }

        - (void)dealloc {
          self.bar = nil;                         // avoid
          [super dealloc];
        }
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Dealloc Instance Variables in Declaration                      Order">
    <SUMMARY>
      <code>dealloc</code> should process instance variables in the same order
      the <code>@interface</code> declares them, so it is easier for a reviewer
      to verify.
    </SUMMARY>
    <BODY>
      <p>
        A code reviewer checking a new or revised <code>dealloc</code>
        implementation needs to make sure that every retained instance
        variable gets released.
      </p>
      <p>
        To simplify reviewing <code>dealloc</code>, order the code so that
        the retained instance variables get released in the same order that
        they are declared in the <code>@interface</code>. If
        <code>dealloc</code> invokes other methods that release instance
        variables, add comments describing what instance variables those
        methods handle.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Setters copy NSStrings">
    <SUMMARY>
      Setters taking an <code>NSString</code>, should always <code>copy</code>
      the string it accepts.
    </SUMMARY>
    <BODY>
      <p>
        Never just <code>retain</code> the string. This avoids the caller
        changing it under you without your knowledge. Don't assume that
        because you're accepting an <code>NSString</code> that it's not
        actually an <code>NSMutableString</code>.
      </p>
      <CODE_SNIPPET>
        - (void)setFoo:(NSString *)aFoo {
          [_foo autorelease];
          _foo = [aFoo copy];
        }
      </CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Avoid Throwing Exceptions">
    <SUMMARY>
      Don't <code>@throw</code> Objective-C exceptions, but you should be
      prepared to catch them from third-party or OS calls.
    </SUMMARY>
    <BODY>
      <p>
        We do compile with <code>-fobjc-exceptions</code> (mainly so we get
        <code>@synchronized</code>), but we don't <code>@throw</code>. Use of
        <code>@try</code>, <code>@catch</code>, and <code>@finally</code> 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.
      </p>
    </BODY>
  </STYLEPOINT>


  <STYLEPOINT title="nil Checks">
    <SUMMARY>
      Use <code>nil</code> checks for logic flow only.
    </SUMMARY>
    <BODY>
      <p>
        Use <code>nil</code> pointer checks for logic flow of the application,
        not for preventing crashes when sending messages. With current compilers
        (<a href="http://www.sealiesoftware.com/blog/archive/2012/2/29/objc_explain_return_value_of_message_to_nil.html">
        as of LLVM 3.0/Xcode 4.2</a>), sending a message to <code>nil</code>
        reliably returns nil as a pointer, zero as an integer or floating-point
        value, structs initialized to 0, and <code>_Complex</code> values equal
        to {0, 0}.
      </p>
      <p>
        Note that this applies to <code>nil</code> as a message target, not as
        a parameter value. Individual methods may or may not safely handle
        <code>nil</code> parameter values.
      </p>
      <p>
        Note too that this is distinct from checking C/C++ pointers and block
        pointers against <code>NULL</code>, which the runtime does not handle
        and will cause your application to crash. You still need to make sure
        you do not dereference a <code>NULL</code> pointer.
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="BOOL Pitfalls">
    <SUMMARY>
      Be careful when converting general integral values to <code>BOOL</code>.
      Avoid comparing directly with <code>YES</code>.
    </SUMMARY>
    <BODY>
      <p>
        <code>BOOL</code> is defined as a signed char in Objective-C which means
        that it can have values other than <code>YES</code> (1) and
        <code>NO</code> (0). Do not cast or convert general integral values
        directly to <code>BOOL</code>. Common mistakes include casting or
        converting an array's size, a pointer value, or the result of a bitwise
        logic operation to a <code>BOOL</code> which, depending on the value of
        the last byte of the integral result, could still result in a
        <code>NO</code> value. When converting a general integral value to a
        <code>BOOL</code> use ternery operators to return a <code>YES</code> or
        <code>NO</code> value.
      </p>
      <p>
        You can safely interchange and convert <code>BOOL</code>,
        <code>_Bool</code> and <code>bool</code> (see C++ Std 4.7.4, 4.12 and
        C99 Std 6.3.1.2). You cannot safely interchange <code>BOOL</code> and
        <code>Boolean</code> so treat <code>Booleans</code> as a general
        integral value as discussed above. Only use <code>BOOL</code> in
        Objective C method signatures.
      </p>
      <p>
        Using logical operators (<code>&amp;&amp;</code>, <code>||</code> and
        <code>!</code>) with <code>BOOL</code> is also valid and will return
        values that can be safely converted to <code>BOOL</code> without the
        need for a ternery operator.
      </p>
      <BAD_CODE_SNIPPET>
        - (BOOL)isBold {
          return [self fontTraits] &amp; NSFontBoldTrait;
        }
        - (BOOL)isValid {
          return [self stringValue];
        }
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        - (BOOL)isBold {
          return ([self fontTraits] &amp; NSFontBoldTrait) ? YES : NO;
        }
        - (BOOL)isValid {
          return [self stringValue] != nil;
        }
        - (BOOL)isEnabled {
          return [self isValid] &amp;&amp; [self isBold];
        }
      </CODE_SNIPPET>
      <p>
        Also, don't directly compare <code>BOOL</code> variables directly
        with <code>YES</code>. Not only is it harder to read for those
        well-versed in C, the first point above demonstrates that return
        values may not always be what you expect.
      </p>
      <BAD_CODE_SNIPPET>
        BOOL great = [foo isGreat];
        if (great == YES)
          // ...be great!
      </BAD_CODE_SNIPPET>
      <CODE_SNIPPET>
        BOOL great = [foo isGreat];
        if (great)
          // ...be great!
      </CODE_SNIPPET>

    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Properties">
    <SUMMARY>
      Use of the @property directive is preferred, with the following caveat:
      properties are an Objective-C 2.0 feature which will limit your code to
      running on the iPhone and Mac OS X 10.5 (Leopard) and higher. Dot notation
      is allowed only for access to a declared <code>@property</code>.
    </SUMMARY>
    <BODY>
      <SUBSECTION title="Naming">
        <p>
          A property's associated instance variable's name must conform to the
          leading _ requirement. The property's name should be the same as its
          associated instance variable without the leading _. The optional space
          between the <code>@property</code> and the opening parenthesis
          should be omitted, as seen in the examples.
        </p>

        <CODE_SNIPPET>
          @interface MyClass : NSObject
          @property(copy, nonatomic) NSString *name;
          @end

          @implementation MyClass
          // No code required for auto-synthesis, else use:
          //   @synthesize name = _name;
          @end
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="Location">
        <p>
          A property's declaration must come immediately after the instance
          variable block of a class interface. A property's definition (if
          not using automatic synthesis) must come immediately after the
          <code>@implementation</code> block in a class definition. They are
          indented at the same level as the <code>@interface</code> or
          <code>@implementation</code> statements that they are enclosed in.
        </p>
        <CODE_SNIPPET>
          @interface MyClass : NSObject {
           @private
            NSString *_name;
          }
          @property(copy, nonatomic) NSString *name;
          @end

          @implementation MyClass
          @synthesize name = _name;

          - (instancetype)init {
            ...
          }
          @end
        </CODE_SNIPPET>
      </SUBSECTION>
      <SUBSECTION title="Use Copy Attribute For Strings">
        <p>
          NSString properties should always be declared with the
          <code>copy</code> attribute.
        </p>
        <p>
          This logically follows from the requirement that setters for
          NSStrings always must use <code>copy</code> instead of
          <code>retain</code>.
        </p>
      </SUBSECTION>
      <SUBSECTION title="Atomicity">
        <p>
          Be aware of the overhead of properties. By default, all synthesized
          setters and getters are atomic. This gives each set and get calls a
          substantial amount of synchronization overhead. Declare your
          properties <code>nonatomic</code> unless you require atomicity.
        </p>
      </SUBSECTION>
      <SUBSECTION title="Dot notation">
        <p>
          Dot notation is idiomatic style for Objective-C 2.0. It may be used
          when doing simple operations to get and set a <code>@property</code>
          of an object, but should not be used to invoke other object behavior.
        </p>
        <CODE_SNIPPET>
          NSString *oldName = myObject.name;
          myObject.name = @"Alice";
        </CODE_SNIPPET>
        <BAD_CODE_SNIPPET>
          NSArray *array = [[NSArray arrayWithObject:@"hello"] retain];

          NSUInteger numberOfItems = array.count;  // not a property
          array.release;                           // not a property
        </BAD_CODE_SNIPPET>
      </SUBSECTION>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Interfaces Without Instance Variables">
    <SUMMARY>
      Omit the empty set of braces on interfaces that do not declare any
      instance variables.
    </SUMMARY>
    <BODY>
      <CODE_SNIPPET>
        @interface MyClass : NSObject
        // Does a lot of stuff
        - (void)fooBarBam;
        @end
      </CODE_SNIPPET>
      <BAD_CODE_SNIPPET>
        @interface MyClass : NSObject {
        }
        // Does a lot of stuff
        - (void)fooBarBam;
        @end
      </BAD_CODE_SNIPPET>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Automatically Synthesized Instance Variables">
    <SUMMARY>
      <p>
        Use of automatically synthesized instance variables is preferred.  Code
        that must support earlier versions of the compiler toolchain (Xcode 4.3
        or earlier or when compiling with GCC) or is using properties inherited
        from a protocol should prefer the @synthesize directive.
      </p>
    </SUMMARY>
    <BODY>
      <CODE_SNIPPET>
        // Header file
        @protocol Thingy
        @property(nonatomic, copy) NSString *widgetName;
        @end

        @interface Foo : NSObject&lt;Thingy&gt;
        // A guy walks into a bar.
        @property(nonatomic, copy) NSString *bar;
        @end

        // Implementation file
        @interface Foo ()
        @property(nonatomic, retain) NSArray *baz;
        @end

        @implementation Foo
        @synthesize widgetName = _widgetName;
        @end
      </CODE_SNIPPET>

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

  <STYLEPOINT title="Automatic Reference Counting (ARC)">
    <SUMMARY>
      <p>
        For projects that use Xcode 4.2 or later and will run only on 64-bit
        Mac OS X 10.7 and iOS 5.0 and later, ARC is preferred. Use manual
        reference counting when supporting earlier environments where zeroing
        weak pointers are not available.
      </p>
      <p>
        Classes that require ARC should include a preprocessor directive to
        prevent compilation using manual reference counting.
      </p>
      <p>
        Ownership qualifiers like <code>__unsafe_unretained</code> and
        <code>__weak</code> should precede variable names. Specifying
        <code>__strong</code> for variables is not required since it is
        the default. Properties, on the other hand, should always specify the
        <code>strong</code> keyword rather than relying on the compiler default.
      </p>
      <p>
        Files that are compiled using ARC need to have preprocessor directives
        to prevent compilation without ARC. See the code snippet below for
        details.
      </p>
    </SUMMARY>
    <BODY>
      <p>
        Example of an implementation file enforcing ARC style. Note that
        declaring instance variables in the @implementation is permitted when
        using ARC.
        <CODE_SNIPPET>
          #if !defined(__has_feature) || !__has_feature(objc_arc)
          #error "This file requires ARC support."
          #endif

          #import "Foo.h"

          @implementation Foo {
            Bar* __weak _bar;
            Baz* __unsafe_unretained _baz;
          }
          // ...
          @end
        </CODE_SNIPPET>
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="NSNumber Literals">
    <SUMMARY>
      <p>
        For projects that use Xcode 4.4 or later with clang, the use of
        <a href="http://clang.llvm.org/docs/ObjectiveCLiterals.html">NSNumber literals</a>
        is allowed. Note however that this will limit the portability of your
        code to other toolchains.
      </p>
      </SUMMARY>
    <BODY>
      <p>
        NSNumber literals are used just like Objective C string literals.
        Boxing is used when necessary. Code using NSNumber literals can be
        deployed on any iOS/MacOS system.
        <CODE_SNIPPET>
          NSNumber *fortyTwo = @42;
          NSNumber *piOverTwo = @(M_PI / 2);
          enum {
            kMyEnum = 2;
          };
          NSNumber *myEnum = @(kMyEnum);
        </CODE_SNIPPET>
      </p>
    </BODY>
  </STYLEPOINT>
</CATEGORY>


<CATEGORY title="Cocoa Patterns">

  <STYLEPOINT title="Delegate Pattern">
    <SUMMARY>
      Delegate objects should not be retained when doing so would create
      a retain cycle.
    </SUMMARY>
    <BODY>
      <p>
        A class that implements the delegate pattern should typically:
        <ol>
          <li>
            Have an instance variable named <var>_delegate</var> to reference
            the delegate.
          </li>
          <li>
            Thus, the accessor methods should be named <code>delegate</code>
            and <code>setDelegate:</code>.
          </li>
          <li>
            The <var>_delegate</var> object should be weak if the class
            is typically retained by its delegate, such that a strong delegate
            would create a retain cycle.
          </li>
        </ol>
      </p>
    </BODY>
  </STYLEPOINT>

  <STYLEPOINT title="Model/View/Controller">
    <SUMMARY>
      Separate the model from the view. Separate the controller from the
      view and the model. Use <code>@protocol</code>s for callback APIs.
    </SUMMARY>
    <BODY>
      <p>
        <ul>
          <li>
            Separate model from view: don't build assumptions about the
            presentation into the model or data source. Keep the interface
            between the data source and the presentation abstract. Don't give
            the model knowledge of its view. (A good rule of thumb is to ask
            yourself if it's possible to have multiple presentations, with
            different states, on a single instance of your data source.)
          </li>
          <li>
            Separate controller from view and model: don't put all of the
            "business logic" into view-related classes; this makes the code
            very unusable. Make controller classes to host this code, but
            ensure that the controller classes don't make too many assumptions
            about the presentation.
          </li>
          <li>
            Define callback APIs with <code>@protocol</code>, using
            <code>@optional</code> if not all the methods are required.
          </li>
        </ul>
      </p>
    </BODY>
  </STYLEPOINT>

</CATEGORY>

<CATEGORY title="Historical Notes">

  <STYLEPOINT title="Trailing vs Leading Underscores">
    <SUMMARY>
      Trailing underscores were once preferred for instance variable names.
    </SUMMARY>
    <BODY>
      <p>
        Our style guide used to have a rule saying that instance variables
        should be named with a trailing underscore, similar to the naming of
        member variables in C++.  This was changed to leading underscores to
        be consistent with the broader Objective-C community, to better follow
        Apple's official guidelines, and to allow for use of new compiler
        features like automatic instance variable synthesis. New projects are
        strongly encouraged to use leading underscores. Existing projects may
        continue to use trailing underscores in new code to maintain
        consistency with the rest of their codebase.
      </p>
    </BODY>
  </STYLEPOINT>

</CATEGORY>

<HR/>

<p align="right">
Revision 2.59
</p>


<address>
Mike Pinkerton <br/>
Greg Miller <br/>
Dave MacLachlan <br/>
</address>
</GUIDE>