Objective-C is the main programming language used by Apple for the OS X and iOS operating systems and their respective frameworks, Cocoa and Cocoa Touch. It is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.
Programming with Objective-C. Apple PDF book
Programming with Objective-C for iOS
Programming with Objective-C for Mac OSX
iOS For High School Students: Getting Started
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Originally contributed by Eugene Yagrushkin, and updated by 17 contributor(s).
© 2018 Eugene Yagrushkin, Yannick Loriot, Levi Bostian, Clayton Walker, Fernando Valverde
Google Objective-C Style Guide
Objective-C is a dynamic, object-oriented extension of C. It’s designed to be easy to use and read, while enabling sophisticated object-oriented design. It is the primary development language for applications on OS X and on iOS.
Apple has already written a very good, and widely accepted, Cocoa Coding Guidelines for Objective-C. Please read it in addition to this guide.
The purpose of this document is to describe the Objective-C (and Objective-C++) coding guidelines and practices that should be used for iOS and OS X code. These guidelines have evolved and been proven over time on other projects and teams. Open-source projects developed by Google conform to the requirements in this guide.
Note that this guide is not an Objective-C tutorial. We assume that the reader is familiar with the language. If you are new to Objective-C or need a refresher, please read Programming with Objective-C.
Optimize for the reader, not the writer
Codebases often have extended lifetimes and more time is spent reading the code than writing it. We explicitly choose to optimize for the experience of our average software engineer reading, maintaining, and debugging code in our codebase rather than the ease of writing said code. For example, when something surprising or unusual is happening in a snippet of code, leaving textual hints for the reader is valuable.
When the style guide allows multiple options it is preferable to pick one option over mixed usage of multiple options. Using one style consistently throughout a codebase lets engineers focus on other (more important) issues. Consistency also enables better automation because consistent code allows more efficient development and operation of tools that format or refactor code. In many cases, rules that are attributed to “Be Consistent” boil down to “Just pick one and stop worrying about it”; the potential value of allowing flexibility on these points is outweighed by the cost of having people argue over them.
Be consistent with Apple SDKs
Consistency with the way Apple SDKs use Objective-C has value for the same reasons as consistency within our code base. If an Objective-C feature solves a problem that’s an argument for using it. However, sometimes language features and idioms are flawed, or were just designed with assumptions that are not universal. In those cases it is appropriate to constrain or ban language features or idioms.
Style rules should pull their weight
The benefit of a style rule must be large enough to justify asking engineers to remember it. The benefit is measured relative to the codebase we would get without the rule, so a rule against a very harmful practice may still have a small benefit if people are unlikely to do it anyway. This principle mostly explains the rules we don’t have, rather than the rules we do: for example, goto contravenes many of the following principles, but is not discussed due to its extreme rarity.
They say an example is worth a thousand words, so let’s start off with an example that should give you a feel for the style, spacing, naming, and so on.
Here is an example header file, demonstrating the correct commenting and spacing for an declaration.
An example source file, demonstrating the correct commenting and spacing for the of an interface.
Spacing and Formatting
Spaces vs. Tabs
Use only spaces, and indent 2 spaces at a time. We use spaces for indentation. Do not use tabs in your code.
You should set your editor to emit spaces when you hit the tab key, and to trim trailing spaces on lines.
The maximum line length for Objective-C files is 100 columns.
You can make violations easier to spot by enabling Preferences > Text Editing > Page guide at column: 100 in Xcode.
Method Declarations and Definitions
One space should be used between the or and the return type, and no spacing in the parameter list except between parameters.
Methods should look like this:
The spacing before the asterisk is optional. When adding new code, be consistent with the surrounding file’s style.
If you have too many parameters to fit on one line, giving each its own line is preferred. If multiple lines are used, align each using the colon before the parameter.
When the second or later parameter name is longer than the first, indent the second and later lines by at least four spaces, maintaining colon alignment:
Include a space after , , , and , and around comparison operators.
Braces may be omitted when a loop body or conditional statement fits on a single line.
If an clause has an clause, both clauses should use braces.
Intentional fall-through to the next case should be documented with a comment unless the case has no intervening code before the next case.
Use a space around binary operators and assignments. Omit a space for a unary operator. Do not add spaces inside parentheses.
Factors in an expression may omit spaces.
Method invocations should be formatted much like method declarations.
When there’s a choice of formatting styles, follow the convention already used in a given source file. Invocations should have all arguments on one line:
or have one argument per line, with colons aligned:
Don’t use any of these styles:
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:
Invocations containing multiple inlined blocks may have their parameter names left-aligned at a four space indent.
Function calls should include as many parameters as fit on each line, except where shorter lines are needed for clarity or documentation of the parameters.
Continuation lines for function parameters may be indented to align with the opening parenthesis, or may have a four-space indent.
Use local variables with descriptive names to shorten function calls and reduce nesting of calls.
Format exceptions with and labels on the same line as the preceding . Add a space between the label and the opening brace (), as well as between the and the caught object declaration. If you must use Objective-C exceptions, format them as follows. However, see Avoid Throwing Exceptions for reasons why you should not be using exceptions.
Prefer small and focused functions.
Long functions and methods are occasionally appropriate, so no hard limit is placed on function length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.
Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.
When updating legacy code, consider also breaking long functions into smaller and more manageable pieces.
Use vertical whitespace sparingly.
To allow more code to be easily viewed on a screen, avoid putting blank lines just inside the braces of functions.
Limit blank lines to one or two between functions and between logical groups of code.
Names should be as descriptive as possible, within reason. Follow standard Objective-C naming rules.
Avoid non-standard abbreviations. Don’t worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. For example:
Any class, category, method, function, or variable name should use all capitals for acronyms and initialisms within the name. This follows Apple’s standard of using all capitals within a name for acronyms such as URL, ID, TIFF, and EXIF.
Names of C functions and typedefs should be capitalized and use camel case as appropriate for the surrounding code.
File names should reflect the name of the class implementation that they contain—including case.
Follow the convention that your project uses. File extensions should be as follows:
|.h||C/C++/Objective-C header file|
|.m||Objective-C implementation file|
|.mm||Objective-C++ implementation file|
|.cc||Pure C++ implementation file|
|.c||C implementation file|
Files containing code that may be shared across projects or used in a large project should have a clearly unique name, typically including the project or class prefix.
File names for categories should include the name of the class being extended, like GTMNSString+Utils.h or NSTextView+GTMAutocomplete.h
Class names (along with category and protocol names) should start as uppercase and use mixed case to delimit words.
When designing code to be shared across multiple applications, prefixes are acceptable and recommended (e.g. GTMSendMessage). Prefixes are also recommended for classes of large applications that depend on external libraries.
Category names should start with a 3 character prefix identifying the category as part of a project or open for general use.
The category name should incorporate the name of the class it’s extending. For example, if we want to create a category on for parsing, we would put the category in a file named , and the category itself would be named . The file name and the category may not match, as this file could have many separate categories related to parsing. Methods in that category should share the prefix () in order to prevent collisions in Objective-C’s global namespace.
There should be a single space between the class name and the opening parenthesis of the category.
Objective-C Method Names
Method and parameter names typically start as lowercase and then use mixed case.
Proper capitalization should be respected, including at the beginning of names.
The method name should read like a sentence if possible, meaning you should choose parameter names that flow with the method name. Objective-C method names tend to be very long, but this has the benefit that a block of code can almost read like prose, thus rendering many implementation comments unnecessary.
Use prepositions and conjunctions like “with”, “from”, and “to” in the second and later parameter names only where necessary to clarify the meaning or behavior of the method.
A method that returns an object should have a name beginning with a noun identifying the object returned:
An accessor method should be named the same as the object it’s getting, but it should not be prefixed with the word . For example:
Accessors that return the value of boolean adjectives have method names beginning with , but property names for those methods omit the .
Dot notation is used only with property names, not with method names.
See Apple’s Guide to Naming Methods for more details on Objective-C naming.
These guidelines are for Objective-C methods only. C++ method names continue to follow the rules set in the C++ style guide.
Regular functions have mixed case.
Ordinarily, functions should start with a capital letter and have a capital letter for each new word (a.k.a. “Camel Case” or “Pascal case”).
Because Objective-C does not provide namespacing, non-static functions should have a prefix that minimizes the chance of a name collision.
Variable names typically start with a lowercase and use mixed case to delimit words.
Instance variables have leading underscores. File scope or global variables have a prefix . For example: , , .
Common Variable Names
Readers should be able to infer the variable type from the name, but do not use Hungarian notation for syntactic attributes, such as the static type of a variable (int or pointer).
File scope or global variables (as opposed to constants) declared outside the scope of a method or function should be rare, and should have the prefix g.
Instance variable names are mixed case and should be prefixed with an underscore, like .
NOTE: Google’s previous convention for Objective-C ivars was a trailing underscore. Existing projects may opt to continue using trailing underscores in new code in order to maintain consistency within the project codebase. Consistency of prefix or suffix underscores should be maintained within each class.
Constant symbols (const global and static variables and constants created with #define) should use mixed case to delimit words.
Global and file scope constants should have an appropriate prefix.
Because Objective-C does not provide namespacing, constants with external linkage should have a prefix that minimizes the chance of a name collision, typically like or .
For interoperability with Swift code, enumerated values should have names that extend the typedef name:
Constants may use a lowercase k prefix when appropriate:
Types and Declarations
Declare variables in the narrowest practical scopes, and close to their use. Initialize variables in their declarations.
Occasionally, efficiency will make it more appropriate to declare a variable outside the scope of its use. This example declares meters separate from initialization, and needlessly sends the lastKnownLocation message each time through the loop:
Under Automatic Reference Counting, pointers to Objective-C objects are by default initialized to , so explicit initialization to is not required.
Avoid unsigned integers except when matching types used by system interfaces.
Subtle errors crop up when doing math or counting down to zero using unsigned integers. Rely only on signed integers in math expressions except when matching NSUInteger in system interfaces.
Unsigned integers may be used for flags and bitmasks, though often NS_OPTIONS or NS_ENUM will be more appropriate.
Types with Inconsistent Sizes
Due to sizes that differ in 32- and 64-bit builds, avoid types long, NSInteger, NSUInteger, and CGFloat except when matching system interfaces.
Types long, NSInteger, NSUInteger, and CGFloat vary in size between 32- and 64-bit builds. Use of these types is appropriate when handling values exposed by system interfaces, but they should be avoided for most other computations.
File and buffer sizes often exceed 32-bit limits, so they should be declared using , not with , , or .
Comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names and then trying to explain them through comments.
Pay attention to punctuation, spelling, and grammar; it is easier to read well-written comments than badly written ones.
Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but use a consistent style. When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous—the next one may be you!
A file may optionally start with a description of its contents. Every file may contain the following items, in order:
- License boilerplate if necessary. Choose the appropriate boilerplate for the license used by the project.
- A basic description of the contents of the file if necessary.
If you make significant changes to a file with an author line, consider deleting the author line since revision history already provides a more detailed and accurate record of authorship.
Every non-trivial interface, public and private, should have an accompanying comment describing its purpose and how it fits into the larger picture.
Comments should be used to document classes, properties, ivars, functions, categories, protocol declarations, and enums.
Doxygen-style comments are encouraged for interfaces as they are parsed by Xcode to display formatted documentation. There is a wide variety of Doxygen commands; use them consistently within a project.
If you have already described an interface in detail in the comments at the top of your file, feel free to simply state, “See comment at top of file for a complete description”, but be sure to have some sort of comment.
Additionally, each method should have a comment explaining its function, arguments, return value, thread or queue assumptions, and any side effects. Documentation comments should be in the header for public methods, or immediately preceding the method for non-trivial private methods.
Use descriptive form (“Opens the file”) rather than imperative form (“Open the file”) for method and function comments. The comment describes the function; it does not tell the function what to do.
Document the thread usage assumptions the class, properties, or methods make, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.
Any sentinel values for properties and ivars, such as or , should be documented in comments.
Declaration comments explain how a method or function is used. Comments explaining how a method or function is implemented should be with the implementation rather than with the declaration.
Provide comments explaining tricky, subtle, or complicated sections of code.
When useful, also provide comments about implementation approaches that were considered or abandoned.
End-of-line comments should be separated from the code by at least 2 spaces. If you have several comments on subsequent lines, it can often be more readable to line them up.
Where needed to avoid ambiguity, use backticks or vertical bars to quote variable names and symbols in comments in preference to using quotation marks or naming the symbols inline.
In Doxygen-style comments, prefer demarcating symbols with a monospace text command, such as .
Demarcation helps provide clarity when a symbol is a common word that might make the sentence read like it was poorly constructed. A common example is the symbol :
or when quoting something which already contains quotes
Backticks or vertical bars are not needed when a symbol is self-apparent.
Doxygen formatting is also suitable for identifying symbols.
For objects not managed by ARC, make the pointer ownership model as explicit as possible when it falls outside the most common Objective-C usage idioms.
Manual Reference Counting
Instance variables for NSObject-derived objects are presumed to be retained; if they are not retained, they should be either commented as weak or declared with the lifetime qualifier.
An exception is in Mac software for instance variables labeled as , which are presumed to not be retained.
Where instance variables are pointers to Core Foundation, C++, and other non-Objective-C objects, they should always be declared with strong and weak comments to indicate which pointers are and are not retained. Core Foundation and other non-Objective-C object pointers require explicit memory management, even when building for automatic reference counting.
Examples of strong and weak declarations:
Automatic Reference Counting
Object ownership and lifetime are explicit when using ARC, so no additional comments are required for automatically retained objects.
C Language Features
Avoid macros, especially where variables, enums, XCode snippets, or C functions may be used instead.
Macros make the code you see different from the code the compiler sees. Modern C renders traditional uses of macros for constants and utility functions unnecessary. Macros should only be used when there is no other solution available.
Where a macro is needed, use a unique name to avoid the risk of a symbol collision in the compilation unit. If practical, keep the scope limited by the macro after its use.
Macro names should use —all uppercase letters with underscores between words. Function-like macros may use C function naming practices. Do not define macros that appear to be C or Objective-C keywords.
Avoid macros that expand to unbalanced C or Objective-C constructs. Avoid macros that introduce scope, or may obscure the capturing of values in blocks.
Avoid macros that generate class, property, or method definitions in headers to be used as public API. These only make the code hard to understand, and the language already has better ways of doing this.
Avoid macros that generate method implementations, or that generate declarations of variables that are later used outside of the macro. Macros shouldn’t make code hard to understand by hiding where and how a variable is declared.
Examples of acceptable macro use include assertion and debug logging macros that are conditionally compiled based on build settings—often, these are not compiled into release builds.
Nonstandard extensions to C/Objective-C may not be used unless otherwise specified.
Compilers support various extensions that are not part of standard C. Examples include compound statement expressions (e.g. and variable-length arrays.
is an approved exception, as it is used in Objective-C API specifications.
The binary form of the conditional operator, , is an approved exception.
Cocoa and Objective-C Features
Identify Designated Initializer
Clearly identify your designated initializer.
It is important for those who might be subclassing your class that the designated initializer be clearly identified. That way, they only need to override a single initializer (of potentially several) to guarantee the initializer of their subclass is called. It also helps those debugging your class in the future understand the flow of initialization code if they need to step through it. Identify the designated initializer using comments or the macro. If you use , mark unsupported initializers with .
Override Designated Initializer
When writing a subclass that requires an method, make sure you override the designated initializer of the superclass.
If you fail to override the designated initializer of the superclass, your initializer may not be called in all cases, leading to subtle and very difficult to find bugs.
Overridden NSObject Method Placement
Put overridden methods of NSObject at the top of an .
This commonly applies to (but is not limited to) the , , and methods. The methods should be grouped together, followed by other typical methods such as , , and .
Convenience class factory methods for creating instances may precede the methods.
Don’t initialize instance variables to or in the method; doing so is redundant.
All instance variables for a newly allocated object are initialized to (except for isa), so don’t clutter up the init method by re-initializing variables to or .
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 or .
Do not invoke the class method , nor override it in a subclass. Instead, use and methods to instantiate retained objects.
Modern Objective-C code explicitly calls and an method to create and retain an object. As the class method is rarely used, it makes reviewing code for correct memory management more difficult.
Keep the Public API Simple
Keep your class simple; avoid “kitchen-sink” APIs. If a method doesn’t need to be public, keep it out of the public interface.
Unlike C++, Objective-C doesn’t differentiate between public and private methods; any message may be sent to an object. As a result, avoid placing methods in the public API unless they are actually expected to be used by a consumer of the class. This helps reduce the likelihood they’ll be called when you’re not expecting it. This includes methods that are being overridden from the parent class.
Since internal methods are not really private, it’s easy to accidentally override a superclass’s “private” method, thus making a very difficult bug to squash. In general, private methods should have a fairly unique name that will prevent subclasses from unintentionally overriding them.
#import and #include
Objective-C and Objective-C++ headers, and C/C++ headers.
Choose between and based on the language of the header that you are including.
When including a header that uses Objective-C or Objective-C++, use . When including a standard C or C++ header, use . The header should provide its own guard.
Order of Includes
The standard order for header inclusion is the related header, operating system headers, language library headers, and finally groups of headers for other dependencies.
The related header precedes others to ensure it has no hidden dependencies. For implementation files the related header is the header file. For test files the related header is the header containing the tested interface.
A blank line may separate logically distinct groups of included headers.
Import headers using their path relative to the project’s source directory.
Import umbrella headers for system frameworks and system libraries rather than include individual files.
While it may seem tempting to include individual system headers from a framework such as Cocoa or Foundation, in fact it’s less work on the compiler if you include the top-level root framework. The root framework is generally pre-compiled and can be loaded much more quickly. In addition, remember to use or rather than for Objective-C frameworks.
Avoid Messaging the Current Object Within Initializers and
Code in initializers and should avoid invoking instance methods.
Superclass initialization completes before subclass initialization. Until all classes have had a chance to initialize their instance state any method invocation on self may lead to a subclass operating on uninitialized instance state.
A similar issue exists for , where a method invocation may cause a class to operate on state that has been deallocated.
One case where this is less obvious is property accessors. These can be overridden just like any other selector. Whenever practical, directly assign to and release ivars in initializers and , rather than rely on accessors.
Beware of factoring common initialization code into helper methods:
- Methods can be overridden in subclasses, either deliberately, or accidentally due to naming collisions.
- When editing a helper method, it may not be obvious that the code is being run from an initializer.
Setters copy NSStrings
Setters taking an should always copy the string it accepts. This is often also appropriate for collections like and .
Never just retain the string, as it may be a . This avoids the caller changing it under you without your knowledge.
Code receiving and holding collection objects should also consider that the passed collection may be mutable, and thus the collection could be more safely held as a copy or mutable copy of the original.
Use Lightweight Generics to Document Contained Types
All projects compiling on Xcode 7 or newer versions should make use of the Objective-C lightweight generics notation to type contained objects.
Every , , or reference should be declared using lightweight generics for improved type safety and to explicitly document usage.
If the fully-annotated types become complex, consider using a typedef to preserve readability.
Use the most descriptive common superclass or protocol available. In the most generic case when nothing else is known, declare the collection to be explicitly heterogenous using id.
Avoid Throwing Exceptions
Don’t Objective-C exceptions, but you should be prepared to catch them from third-party or OS calls.
This follows the recommendation to use error objects for error delivery in Apple’s Introduction to Exception Programming Topics for Cocoa.
We do compile with (mainly so we get ), but we don’t . Use of , , and 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.
Use checks for logic flow only.
Use pointer checks for logic flow of the application, not for preventing crashes when sending messages. Sending a message to reliably returns as a pointer, zero as an integer or floating-point value, structs initialized to , and values equal to .
Note that this applies to as a message target, not as a parameter value. Individual methods may or may not safely handle parameter values.
Note too that this is distinct from checking C/C++ pointers and block pointers against , which the runtime does not handle and will cause your application to crash. You still need to make sure you do not dereference a pointer.
Be careful when converting general integral values to . Avoid comparing directly with .
in OS X and in 32-bit iOS builds is defined as a signed , so it may have values other than () and (). Do not cast or convert general integral values directly to .
Common mistakes include casting or converting an array’s size, a pointer value, or the result of a bitwise logic operation to a that could, depending on the value of the last byte of the integer value, still result in a value. When converting a general integral value to a use ternary operators to return a or value.
You can safely interchange and convert , and (see C++ Std 4.7.4, 4.12 and C99 Std 184.108.40.206). Use in Objective-C method signatures.
Using logical operators (, and ) with is also valid and will return values that can be safely converted to without the need for a ternary operator.
Also, don’t directly compare variables directly with . Not only is it harder to read for those well-versed in C, but the first point above demonstrates that return values may not always be what you expect.
Interfaces Without Instance Variables
Omit the empty set of braces on interfaces that do not declare any instance variables.
Delegates, target objects, and block pointers should not be retained when doing so would create a retain cycle.
To avoid causing a retain cycle, a delegate or target pointer should be released as soon as it is clear there will no longer be a need to message the object.
If there is no clear time at which the delegate or target pointer is no longer needed, the pointer should only be retained weakly.
Block pointers cannot be retained weakly. To avoid causing retain cycles in the client code, block pointers should be used for callbacks only where they can be explicitly released after they have been called or once they are no longer needed. Otherwise, callbacks should be done via weak delegate or target pointers.
Style Matches the Language
Within an Objective-C++ source file, follow the style for the language of the function or method you’re implementing. In order to minimize clashes between the differing naming styles when mixing Cocoa/Objective-C and C++, follow the style of the method being implemented.
For code in an block, use the Objective-C naming rules. For code in a method of a C++ class, use the C++ naming rules.
For code in an Objective-C++ file outside of a class implementation, be consistent within the file.
Projects may opt to use an 80 column line length limit for consistency with Google’s C++ style guide.
Objective-C Style Exceptions
Indicating style exceptions
Lines of code that are not expected to adhere to these style recommendations require at the end of the line or at the end of the previous line. Sometimes it is required that parts of Objective-C code must ignore these style recommendations (for example code may be machine generated or code constructs are such that its not possible to style correctly).
A comment on that line or on the previous line can be used to indicate to the reader that code is intentionally ignoring style guidelines. In addition these annotations can also be picked up by automated tools such as linters and handle code correctly. Note that there is a single space between and .