Objective-C creating objects, methods and variables

In this tutorial we will be creating a console application to try every part of our code out. Please make sure that the console is not set to its default language, which is C and is instead set to foundation.

The first thing we need to do is create a no project with Xcode. I will be naming mine “test” but you really don’t need to follow along with that much detail since it is not needed at this stage with the level of coding we will be doing.

The next thing we will need to do is open up our “test.m” file through Xcode. This file contains the default main method but we are going to be creating objects and variables.

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#import <Foundation/Foundation.h> int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; // insert code here... NSLog(@"Hello, World!"); [pool drain]; return 0; }

Now you are going to need to create an interface and an implementation of the class made in the interface. Luckily for us Xcode has an auto complete feature built in that look something like this.

Until we start modifying the information in the interface and implementation you can just leave it with its default values that are given when it auto completes.

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#import <Foundation/Foundation.h> @interface <#class#> : <#superclass#> { <#ivars#> } <#methods#> @end @implementation <#class#> <#methods#> @end int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; // insert code here... NSLog(@"Hello, World!"); [pool drain]; return 0; }

The next thing we will be doing is creating the actual object from our interface. This requires us to create an object name, which in this case will be a “Person”, and it also requires a super class, which we will send to “NSObject.”

1 2	@interface Person : NSObject {

Note: It is standard and common practice to associate objects as follows: “PersonObjectSomething.” You will notice that when naming all of the words are capitalized. This is common practice when naming classes. When naming methods it is standard to leave that first letter as a lower case like so “thisTestMethod.”

The next things that are required are instance variables. These are variables/information that is associated with the object. For example if we created an object that was a cup then we would have instance variables for weight, liquid capacity, circumference of base, etc.

For our person object we will be creating the instance variables for age and height. Normally I would like to do a name but I will be leaving the string library for another tutorial.

When declaring instance variables, or any variables, you will need to create the data type first then give the variable a name. A simple example of this would be an integer data type named x as you commonly see in math. You can also assign the variable a value when it is first declared.

1 2	int x; int y = 32;

Once we have declared our instance variables for person you should have something that looks like this.

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#import <Foundation/Foundation.h> @interface Person : NSObject { int height; // in CM int age; } @end

Now that we have created our Person interface’ variables we will need to create methods. If you have never worked with methods you may know them by another name, which is a function. If you still haven’t worked with them then you should know that a method is essentially a small sub program running in a larger one. Methods naturally return a value as well however, you can change the method type to void and not have to worry about returning a value.

There are two kinds of basic methods used in programming languages. The first one is called an accessor method and the second one is called a mutator method. The accessor method simply accesses a variable from the object it is derived from and returns said variable. And the mutator changes the value of the variable in some way. For this program we will be using these two types along with another void type that will print information on the screen.

In order to create a method, we must first link to it in the interface section of the code so our object can use it with the variables it contains. For this program we will need to set the height and age, access the height and age, and print the information on the screen. Add this section of code to your interface.

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-(void)setHeight: (int) x; -(void)setAge: (int) x; -(int)getHeight; -(int)getAge; -(void)printInfo;

You should now have an interface that looks like this.

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#import <Foundation/Foundation.h> @interface Person : NSObject { int height; // in CM int age; } -(void)setHeight: (int) x; -(void)setAge: (int) x; -(int)getHeight; -(int)getAge; -(void)printInfo; @end

The next thing we will need to do is create the body of our object. We can do this by filling out the implementation section of our code. As you can see in your Xcode’ code view we have a place for the class to be written and a place for the methods. We are going to change the class to “Person” and start writing in our accessor and mutator methods. When done we will have the following code.

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@implementation Person -(void)setHeight: (int) x{ height = x; } -(void)setAge: (int) x{ age = x; } -(int)getHeight{ return height; } -(int)getAge{ return age; } -(void)printInfo{ NSLog(@"height: %i age: %i",height, age); } @end

Now that we have created our methods and instance variables we will go ahead and create this object and put it into memory. The following code is what we currently have for a main method.

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int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; NSLog(@"Hello, World!"); [pool drain]; return 0; }

When creating objects there are two main terms you will need to familiarize yourself with. The first term is “init” Which of course stands for initialize. This is when you tell the machine that you are ready to use the object. The next term you should be familiar with is “alloc,” it is short for allocate. This is the process of freeing up memory in your ram for the storage of the object.

The code to initialize and allocate an object is written as follows.

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Person *p1; p1 = [Person alloc]; p1 = [p1 init];

In this process we first named the object p1 and assigned it enough space to hold the object “Person” in memory. The next line of code simply initializes the object so it is ready for our use.

When you are done using your object and you would like to delete it you must include an object release.

1	[p1 release];

In between creating the object in memory and removing it from memory we are going to add some code to set the variables, access the variables and display what the object holds on screen.

In order to display an object’s methods you will need to follow the form of writing the code as follows.

1	[p1 setAge:18];

You can see that the object is placed within square brackets and the first statement is the object name itself. The second statement accepts the method that you would like to call on then a colon and some parameters if you made a method that accepts arguments.

When using a method that returns a value it is possible to load the data type returned into another variable as long as it is of the same data type. We use an example of this with the following lines. You can also use a variable as an argument when setting an objects variable.

1 2	int g = [p1 getHeight]; [p1 setAge: g];

The entire program looks as follows:

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#import <Foundation/Foundation.h> @interface Person : NSObject { int height; // in CM int age; } -(void)setHeight: (int) x; -(void)setAge: (int) x; -(int)getHeight; -(int)getAge; -(void)printInfo; @end @implementation Person -(void)setHeight: (int) x{ height = x; } -(void)setAge: (int) x{ age = x; } -(int)getHeight{ return height; } -(int)getAge{ return age; } -(void)printInfo{ NSLog(@"height: %i age: %i",height, age); } @end int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; Person *p1; p1 = [Person alloc]; p1 = [p1 init]; [p1 setAge:18]; [p1 setHeight:188]; [p1 printInfo]; int g = [p1 getHeight]; [p1 setAge: g]; [p1 printInfo]; [p1 release]; // insert code here... NSLog(@"Hello, World!"); [pool drain]; return 0; }

If you have any questions or concerns please feel free to comment below.