Home > Articles > Programming > Mac and iOS Programming

Using Object-Oriented Features in Objective-C

📄 Contents

  1. Communicating to Methods with Messages
  2. Allocating and Initializing Objects
  3. Summary
  4. Q&A
  5. Workshop
  • Print
  • + Share This
Learn about the messaging structure that is at the heart of Objective-C. It is not just another way of talking about the calling of methods that other languages use; it is a different way of constructing software.
This chapter is from the book

Communicating to Methods with Messages

Perhaps the biggest difference between Objective-C and languages such as C++ is its messaging syntax as well as the way people talk about it. Objective-C has classes just as other object-oriented languages do, and those classes can have methods within them. You communicate with those methods with messages. A message is enclosed within square brackets, and it consists of the name of the object to which it is being sent followed by the message itself.

The implementation files that you create carry the .m suffix because originally they were referred to as message files that contained the code for the messages defined in the header (.h) files. (This is possibly an apocryphal tale, but the importance of messaging in Objective-C is indisputed.)

Looking at a Simple Message

Here is a simple message that is sent to an object called myObject, which is assumed to be of type NSObject—the object that is the root class of most class hierarchies in Objective-C.

[myObject init];

This message calls the init method on myObject. (That is the point made previously about the different way many people talk about Objective-C methods: They often refer to calling a message “on” an object.)

Methods can return a value. If a method returns a value, you can set it to a local variable as in the following:

myVariable = [myObject init];

Declaring a Method

When you declare a simple method, you use an Objective-C variation on C function syntax. NSObject, the root class of almost all of the Objective-C objects you use, does declare an init method.

The following is the declaration that supports the messages shown in the previous section:

– (id)init

As you might surmise, the init method shown here returns a result of type id. (You find out more about id shortly.)

The minus sign at the start of the method is an important part of the declaration: It is the method type. It indicates that this is a method that is defined for instances of a class. Any instance of the class in which this declaration is used can invoke this method. To put it another way, you (or an instance of a class) can send the init method to any instance of this class). Because this is the NSObject superclass of every other object, that means you can send the init message to any instance of any class.

  • There is more on allocating and initializing objects later in this hour.

Using Class Methods

The minus sign at the start of the method shown in the previous section indicates that it is an instance method. There is another type of method in Objective-C: a class method. It is indicated by a plus sign.

A message to an instance method can be sent to any instance of that class subject to constraints for that specific class. Whereas you call an instance method on an instance of a class, you call a class method on the class itself. No instance is involved.

Class methods are used most frequently as factory methods. Perhaps the most common class method is alloc. For NSObject, its declaration is

+ (id)alloc;

Whereas you send init to an instance, as in this case:

[myObject init];

alloc allocates an instance of a class as in

[MyClass alloc];

This returns an instance of MyClass. As you can see in the declaration, this result is of type id. It is time to explore that type.

Working with id—Strongly and Weakly Typed Variables

Objective-C supports strongly and weakly typed variables. When you reference a variable using a strong type, you specify the type of the variable. The actual variable must be of that type or a subclass of that type; if it is a subclass, it is, by definition, the type of all of its superclasses.

In Cocoa, you see that declaring to a variable as

NSArray *myArray

means that you could be referring to an object of type NSMutableArray, which is a subclass. You can write the same code to work with elements of the array no matter what its actual type is. If necessary, you might have to coerce a specific instance to the subclass that you want (if you know that is what it is).

id is the ultimate weakly typed variable; it could be any class. That is why it is used as the return type from alloc. alloc is a class method on NSObject so if you call it on an NSArray, you get an instance of NSArray returned through id.

Nesting Messages

Messages can be nested within one another. You could write the following:

myObject = [MyClass alloc];
myObject = [myObject init];

This would use the class method of MyClass to allocate a new instance of MyClass, which you immediately assign to myObject.

You can nest them together as follows:

myObject = [[MyClass alloc] init];

The rules for nesting square brackets are the same as for nesting parentheses.

Looking at Method Signatures and Parameters

alloc and init are very good basic examples because they have no parameters. Most methods in any language do have parameters. For example, you can write an area function that takes two parameters (height and width) and returns the product as its return value.

Other languages generally specify a name and a type for each parameter, and so does Objective-C. However, it adds another dimension: It labels each parameter.

This naming means that the code is more readable, but you do have to understand what is going on when there is more than one parameter. When there is no parameter, the message is simply the receiver and the name of the method:

[myObject init];

If there is a parameter, it follows the method name. In the message, a colon precedes the parameter itself. For example, in NSSet, you can initialize a set with an NSArray using code like this:

mySet =[NSSet alloc];
[mySet initWithArray: myArray;

The declaration needs to specify not only the parameter name, which is used in the code of the method, but also its type:

– (id)initWithArray:(NSArray *)array;

The second and subsequent parameters are also labeled. The difference is that the first parameter is labeled in effect by the name of the method. If you add more parameters, their names and types are needed; they are preceded by a keyword (which, in the case of the first parameter is the method name). Here is another NSSet method. It initializes an NSSet to the elements of another NSSet (the first parameter). The second parameter specifies whether the elements of the first set are to be copied or not.

Here is a typical invocation:

[mySet: initWithArray: myArray copyItems:YES];

Here is the declaration:

– (id)initWithSet:(NSSet *)set copyItems:(BOOL)flag
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020