iOS app development for C/C++/C# Developers : Defining default constructor in Objective-C

In this post I am going to share how we do whats called "constructors" in C++ or C# and for my C-only fellows, object initializers or <insert some term here>.

As I mentioned in my previous blog post that Objective-C has no such thing as constructors. We use the native non-instance method or what we call "static" method to allocate a new instance. That method is called alloc.

 MyClass* myObj = [MyClass alloc]

The above code just creates an object in a fashion similar to having no constructors defined in our C++ or C# class.

But consider the following code in C++.

 class Circle
  {
    private int m_radius;
    public Circle()
     {
       m_radius = 10; // C++ folks can also use the "initializer list" for this assignment
     }
  }

If we want to implement the same logic in Objective-C, we are going to have to define something what most Obj-C developers call "initializers"

Initializers are nothing but methods that may take arguments and meant to be called right after allocation of a new object. So we could do the following in Objective-C. Lets do a "default constructor" in this post. I will make another post for parameter-ized constructor


Circle.h file

 #import <foundation/foundation.h>

@interface Circle : NSObject {
  NSInteger m_radius;
}

 - (id) init;
@end

Circle.m file

 @implementation Circle
  - (id) init
  {
    self = [super init]; // calling base class's init in this case NSObject's init

    if (self) { // checking to see if an object has been returned by NSObject's init
            self.m_radius = 10; // set the default value to 10
      }
   return self; // In order for it to act like a constructor, it should return the object
  }
 @end

And this is how we create an object using the initializer defined above

 Circle* circleObj = [[Circle alloc] init];

By the way, 'super' is a Java term for base class too. Somebody gets a title of 'copy-cat' here.

That's all folks for this post. Please let me know your comments in the comment box below if you have any.

A few Objective-C kick start tips for C++ Developers

Here are a few things I as a C++ developer had to learn the hard way.

Basic Language Constructs

+ sign for static methods - sign for methods that require objects

For instance in C++ we do the following

public:
static void SomeMethod();

The objective-C equivalent would be

+(void) SomeMethod;

Similarly for non-static methods just change the + sign to -. They are called Instance Methods.

-(void) SomeNonStaticMethod;

You may also have noticed that the return type is enclosed in () parenthesis which was weird to me.

No private methods

Thats right, no method is private. You can have member variables or attributes private by using @private but methods cannot be private.

Method calling

In C++ we have, when we have to call a method; we do

myObj.MethodName();

In Objective-C, its totally changed. 

[myObj MethodName];

Tada! No Constructor

Yes, there is no such thing as constructor. All you do is call a native/built-in method alloc to create an object with default properties.

[MyClass alloc]; //Here is the object being created

But Objective-C developers have come up with something called initializers, so immediately after the object is created, an init method is called. Keep in mind that init is something developer has to create as a method that does some initialization work.

[ [MyClass alloc] init];

Weird asterisk * that reminds you of pointer will get on your nerve

Yes, that asterisk is for pointers. Every object reference is stored in pointers. 

So in C++ we create objects by simply writing the following code

MyClass MyObj = MyClass();

In Objective-C, you will have to write the following code

MyClass *MyObj = [ [ MyClass alloc ] init];

Project files

Objective-C has similar to .h/.cpp structure for class declaration and definition but its most often called .h/.m or .h/.mm (for Objective-C++)

Keyword interface is not what you think

interface keyword in Objective-C is used for class declarations.

C++ code

class MyClass

{

 public:

MyClass();

};

Objective-C will require you to write the following code for the above

@interface MyClass {

// non-static variables go here

}

- (MyClass*) init;

@end

Class definition or .m file

.m file or .cpp file counterpart in Objective-C would look something like this

@implementation MyClass { // Yes 'class' keyword is never seen

-(MyClass*) init  {

 if (self) {

// initialize self here

 }

return self;

}

@end

Thats all folks! There is more information on the topic which I intend to write in future blog posts. If you are a C++ developer and thinking about taking a nosedive into Objective-C, above mentioned things should be enough for you to brace for.

I would love to hear your comments. They help me understand how useful my postings are.