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styleguide/objcguide.xml
mmentovai cd4ce0fff0 Update C++ style guide to 3.188: 
- Revise "Smart Pointers" section.
 - Clarify that it's OK to have spaces after '#' in a preprocessor directive,
   even though '#' itself must not be indented.
 - Revise "TODO Comments" section.
 - Fix wording.
 - Explicitly recommend foo.h be the first file #included by foo_test.cc.

Update Objective-C style guide to 2.24:
 - Clarify the spacing of @property declarations.
 - Add "Overridden NSObject Method Placement" section.
 - Explicitly permit blank lines around @interface, @implementation, and @end.

Update JavaScript style guide to 2.20:
 - Provide additional guidance with respect to the compiler.
 - Add {function(new:Type)} as a type syntax for constructors of Type.
 - Revise "Method and Function Comments" section.
 - Harmonize text and example in the "Passing Anonymous Functions" section.
 - Explicitly state that @param and @return types must be enclosed in braces.
 - Add documentation on the UNKNOWN type.
 - Replace a CODE_SNIPPET with BAD_CODE_SNIPPET in the "Internet Explorer's
   Conditional Comments" section.
 - Remove a redundant "Expand for more information" in the "Naming" section
   and fully spell out "information" in the "Code Formatting" section.
 - Provide a positive example in the "Multiline string literals" section.
 - Provide guidance for indentation within nested functions.

Update Python style guide to 2.20:
 - Clarify shebang rule.
2011-03-29 20:30:47 +00:00

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<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="styleguide.xsl"?>
<GUIDE title="Google Objective-C Style Guide">
<p align="right">
Revision 2.24
</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="http://developer.apple.com/documentation/Cocoa/Conceptual/CodingGuidelines/index.html">
Apple's Cocoa Coding Guidelines
</a>
</li>
<li>
<div>
<a href="http://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="http://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="http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/index.html">
The Objective-C Programming Language
</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>
// GTMFoo.h
// FooProject
//
// Created by Greg Miller on 6/13/08.
// Copyright 2008 Google, Inc. All rights reserved.
//
#import &lt;Foundation/Foundation.h&gt;
// A sample class demonstrating good Objective-C style. All interfaces,
// categories, and protocols (read: all top-level declarations in a header)
// MUST be commented. Comments must also be adjacent to the object they're
// documenting.
//
// (no blank line between this comment and the interface)
@interface GTMFoo : NSObject {
@private
NSString *foo_;
NSString *bar_;
}
// Returns an autoreleased instance of GMFoo. See -initWithString: for details
// about the argument.
+ (id)fooWithString:(NSString *)string;
// Designated initializer. |string| will be copied and assigned to |foo_|.
- (id)initWithString:(NSString *)string;
// Gets and sets the string for |foo_|.
- (NSString *)foo;
- (void)setFoo:(NSString *)newFoo;
// Does some work on |blah| and returns YES if the work was completed
// successfuly, and NO otherwise.
- (BOOL)doWorkWithString:(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>
//
// GTMFoo.m
// FooProject
//
// Created by Greg Miller on 6/13/08.
// Copyright 2008 Google, Inc. All rights reserved.
//
#import "GTMFoo.h"
@implementation GTMFoo
+ (id)fooWithString:(NSString *)string {
return [[[self alloc] initWithString:string] autorelease];
}
// Must always override super's designated initializer.
- (id)init {
return [self initWithString:nil];
}
- (id)initWithString:(NSString *)string {
if ((self = [super init])) {
foo_ = [string copy];
bar_ = [[NSString alloc] initWithFormat:@"hi %d", 3];
}
return self;
}
- (void)dealloc {
[foo_ release];
[bar_ release];
[super dealloc];
}
- (NSString *)foo {
return foo_;
}
- (void)setFoo:(NSString *)newFoo {
[foo_ autorelease];
foo_ = [newFoo copy];
}
- (BOOL)doWorkWithString:(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, as most do, any 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>
Each line of text in your code should be at most 80 characters long.
</SUMMARY>
<BODY>
<p>
Even though Objective-C tends to be a more verbose language than C++,
to aid in the interoperability with that guide, we have decided
to keep the limit at 80 columns as well. It's easier to live with
than you might expect.
</p>
<p>
We recognize that this rule is controversial, but so much existing
code already adheres to it, and we feel that consistency is
important.
</p>
<p>
You can make violations easier to spot in Xcode by going to <i>Xcode
&gt; Preferences &gt; Text Editing &gt; Show page guide</i>.
</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. You can do this by making keywords
line up vertically, not aligning colons:
</p>
<CODE_SNIPPET>
- (void)short:(GTMFoo *)theFoo
longKeyword:(NSRect)theRect
evenLongerKeyword:(float)theInterval {
...
}
</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 keyword lengths make
it impossible to align colons and still have four leading
spaces, indent later lines by four spaces and align keywords after the
first one, instead of aligning the colons.
</p>
<CODE_SNIPPET>
[myObj short:arg1
longKeyword:arg2
evenLongerKeyword:arg3];
</CODE_SNIPPET>
</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
delcarations. 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>
</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="http://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="http://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 stringWithInt:instanceVar_];
NSLog(@"%@", tempString);
}
@end
// The platform-specific implementation of the C++ class, using
// C++ naming.
int CrossPlatformAPI::DoSomethingPlatformSpecific() {
NSString* temp_string = [NSString stringWithInt:an_instance_var_];
NSLog(@"%@", temp_string);
return [temp_string intValue];
}
</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>
In <em>application-level</em> code, prefixes on class names should
generally be avoided. Having every single class with same prefix
impairs readability for no benefit. When designing code to be shared
across multiple applications, prefixes are acceptable and recommended
(e.g. <code>GTMSendMessage</code>).
</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>
</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="http://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. Class member variables have trailing underscores. For example:
<var>myLocalVariable</var>, <var>myInstanceVariable_</var>. Members
used for KVO/KVC bindings may begin with a leading underscore
<i>iff</i> use of Objective-C 2.0's <code>@property</code> isn't
allowed.
</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 suffixed with a
trailing underscore, e.g. <var>usernameTextField_</var>. However,
we permit an exception when binding to a member variable using
KVO/KVC and Objective-C 2.0 cannot be used (due to OS release
constraints). In this case, it is acceptable to prefix the variable
with an underscore, per Apple's accepted practices for key/value
naming. If Objective-C 2.0 can be used, <code>@property</code> and
<code>@synthesize</code> provide a solution that conforms to the
naming guidelines.
</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, i.e. <var>kInvalidHandle</var>,
<var>kWritePerm</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 &#8212; 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>
Start each file with a copyright notice, followed by a
description of the contents of the file.
</SUMMARY>
<BODY>
<SUBSECTION title="Legal Notice and Author Line">
<p>
Every file should contain the following items, in order:
<ul>
<li>a copyright statement (for example,
<code>Copyright 2008 Google Inc.</code>)</li>
<li>a license boilerplate. Choose the appropriate boilerplate
for the license used by the project (for example,
Apache 2.0, BSD, LGPL, GPL)</li>
</ul>
</p>
<p>
If you make significant changes to a file that someone else
originally wrote, add yourself to the author line. This can
be very helpful when another
contributor
has questions about the file and needs to know whom to contact
about it.
</p>
</SUBSECTION>
</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>
<p>
Instance variable pointers to objects derived from NSObject are
presumed to be retained, and should be documented as <b>weak</b> if
they are not retained by the class. However, instance variables which
are labeled as IBOutlets are presumed to not be retained by the class,
and should be documented as <b>strong</b> if the class does retain
them.
</p>
<p>
Where instance variables are pointers to CoreFoundation, C++, and
other non-Objective-C objects, they should always be documented in
comments as strong or weak. Be mindful that support for automatic C++
objects encapsulated in Objective-C objects is disabled by default, as
described <a href="http://chanson.livejournal.com/154253.html">here</a>.
</p>
<p>
Examples of strong and weak documentation:
<CODE_SNIPPET>
@interface MyDelegate : NSObject {
@private
IBOutlet NSButton* okButton_; // normal NSControl
IBOutlet NSMenu* myContextMenu_; // manually-loaded menu (strong)
AnObjcObject* doohickey_; // my doohickey
MyController* controller_; // so we can send msgs back (weak, owns me)
// non-NSObject pointers...
CWackyCPPClass* wacky_; // some cross-platform object (strong)
CFDictionaryRef* dict_; // (strong)
}
@end
</CODE_SNIPPET>
<dl>
<dt>strong</dt><dd>The object will be <code>retain</code>'d by this class</dd>
<dt>weak</dt><dd>The object will be <b>not</b> be <code>retain</code>'d by this class
(e.g. a delegate).</dd>
</dl>
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<CATEGORY title="Cocoa and Objective-C Features">
<STYLEPOINT title="Member Variables Should Be @private">
<SUMMARY>
Member variables should be declared <code>@private</code>.
</SUMMARY>
<BODY>
<CODE_SNIPPET>
@interface MyClass : NSObject {
@private
id myInstanceVariable_;
}
// public accessors, setter takes ownership
- (id)myInstanceVariable;
- (void)setMyInstanceVariable:(id)theVar;
@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>
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
the <code>copyWithZone:</code> method, and finally the
<code>dealloc</code> method.
</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 &#8212; 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>
// GTMFoo.m
#import "GTMFoo.h"
@interface GTMFoo (PrivateDelegateHandling)
- (NSString *)doSomethingWithDelegate; // Declare private method
@end
@implementation GTMFoo(PrivateDelegateHandling)
...
- (NSString *)doSomethingWithDelegate {
// Implement this method
}
...
@end
</CODE_SNIPPET>
<p>
Before Objective-C 2.0, if you declare a method in the private
<code>@interface</code>, but forget to implement it in the main
<code>@implementation</code>, the compiler will <i>not</i> object.
(This is because you don't implement these private methods in a
separate category.) The solution is to put the functions within
an <code>@implementation</code> that specifies the category.
</p>
<p>
If you are using Objective-C 2.0, you should instead declare your
private category using a <a href="http://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="dealloc should process 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>
<p>
Do not use the <code>NS_DURING</code>, <code>NS_HANDLER</code>,
<code>NS_ENDHANDLER</code>, <code>NS_VALUERETURN</code> and
<code>NS_VOIDRETURN</code> macros unless you are writing code that
needs to run on Mac OS X 10.2 or before.
</p>
<p>
Also be aware when writing Objective-C++ code that stack based objects
are <b>not</b> cleaned up when you throw an Objective-C exception.
Example:
</p>
<CODE_SNIPPET>
class exceptiontest {
public:
exceptiontest() { NSLog(@"Created"); }
~exceptiontest() { NSLog(@"Destroyed"); }
};
void foo() {
exceptiontest a;
NSException *exception = [NSException exceptionWithName:@"foo"
reason:@"bar"
userInfo:nil];
@throw exception;
}
int main(int argc, char *argv[]) {
GMAutoreleasePool pool;
@try {
foo();
}
@catch(NSException *ex) {
NSLog(@"exception raised");
}
return 0;
}
</CODE_SNIPPET>
<p>
will give you:
</p>
<CODE_SNIPPET>
2006-09-28 12:34:29.244 exceptiontest[23661] Created
2006-09-28 12:34:29.244 exceptiontest[23661] exception raised
</CODE_SNIPPET>
<p>
Note that the destructor for <i>a</i> never got called. This is a
major concern for stack based smartptrs such as
<code>shared_ptr</code> and <code>linked_ptr</code>, as well as any
STL objects that you may want to use. Therefore it pains us to say
that if you must use exceptions in your Objective-C++ code, use C++
exceptions whenever possible. You should never re-throw an Objective-C
exception, nor are stack based C++ objects (such as
<code>std::string</code>, <code>std::vector</code> etc.) allowed in
the body of any <code>@try</code>, <code>@catch</code>, or
<code>@finally</code> blocks.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="nil Checks">
<SUMMARY>
Use <code>nil</code> checks for logic flow only.
</SUMMARY>
<BODY>
<p>
Use <code>nil</code> checks for logic flow of the application, not for
crash prevention. Sending a message to a <code>nil</code> object is
handled by the Objective-C runtime. If the method has no return
result, you're good to go. However if there is one, there may be
differences based on runtime architecture, return size, and OS X
version (see <a href="http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/Articles/chapter_2_section_3.html#//apple_ref/doc/uid/TP30001163-CH11-SW7">Apple's
documentation</a> for specifics).
</p>
<p>
Note that this is very different from checking C/C++ 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 an unsigned 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>
Properties in general are allowed 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
trailing _ requirement. The property's name should be the same as its
associated instance variable without the trailing _. The optional
space between the <code>@property</code> and the opening parenthesis
should be omitted, as seen in the examples.
</p>
<p>
Use the @synthesize directive to rename the property correctly.
</p>
<CODE_SNIPPET>
@interface MyClass : NSObject {
@private
NSString *name_;
}
@property(copy, nonatomic) NSString *name;
@end
@implementation MyClass
@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 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_;
- (id)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="Never synthesize CFType properties">
<p>
CFTypes should always have the <code>@dynamic</code> implementation
directive.
</p>
<p>
Since CFTypes can't have the <code>retain</code> property
attribute, the developer must handle retaining and releasing the
value themselves. In the rare case that you do actually want
assignment it is better to make that completely clear by actually
implementing the setter and getter and commenting why that is the
case.
</p>
</SUBSECTION>
<SUBSECTION title="List out all implementation directives">
<p>
Use implementation directives for all properties even if they are
<code>@dynamic</code> by default.
</p>
<p>
Even though <code>@dynamic</code> is default, explicitly list it out
with all of the other property implementation directives making it
clear how every property in a class is handled at a single glance.
</p>
<BAD_CODE_SNIPPET>
@interface MyClass : NSObject
@property(readonly) NSString *name;
@end
@implementation MyClass
.
.
.
- (NSString*)name {
return @"foo";
}
@end
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
@interface MyClass : NSObject
@property(readonly) NSString *name;
@end
@implementation MyClass
@dynamic name;
.
.
.
- (NSString*)name {
return @"foo";
}
@end
</CODE_SNIPPET>
</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>
</CATEGORY>
<CATEGORY title="Cocoa Patterns">
<STYLEPOINT title="Delegate Pattern">
<SUMMARY>
Delegate objects should not be retained.
</SUMMARY>
<BODY>
<p>
A class that implements the delegate pattern should:
<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 <b>not</b> be retained.
</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.
(Exception: when using Objective-C 1.0, <code>@optional</code> isn't
available, so use a category to define an "informal protocol".)
</li>
</ul>
</p>
</BODY>
</STYLEPOINT>
</CATEGORY>
<HR/>
<p align="right">
Revision 2.24
</p>
<address>
Mike Pinkerton <br/>
Greg Miller <br/>
Dave MacLachlan <br/>
</address>
</GUIDE>
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