Objective-C Program Structure

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Before we study basic building blocks of the Objective-C programming language, let us look a bare minimum Objective-C program structure so that we can take it as a reference in upcoming chapters.

Objective-C Hello World Example

A Objective-C program basically consists of the following parts −

  • Preprocessor Commands
  • Interface
  • Implementation
  • Method
  • Variables
  • Statements & Expressions
  • Comments

Let us look at a simple code that would print the words "Hello World" −

Live Demo 
#import <Foundation/Foundation.h>

@interface SampleClass:NSObject
- (void)sampleMethod;
@end

@implementation SampleClass

- (void)sampleMethod {
   NSLog(@"Hello, World! \n");
}

@end

int main() {
   /* my first program in Objective-C */
   SampleClass *sampleClass = [[SampleClass alloc]init];
   [sampleClass sampleMethod];
   return 0;
}

Let us look various parts of the above program −

  • The first line of the program #import <Foundation/Foundation.h> is a preprocessor command, which tells a Objective-C compiler to include Foundation.h file before going to actual compilation.

  • The next line @interface SampleClass:NSObject shows how to create an interface. It inherits NSObject, which is the base class of all objects.

  • The next line - (void)sampleMethod; shows how to declare a method.

  • The next line @end marks the end of an interface.

  • The next line @implementation SampleClass shows how to implement the interface SampleClass.

  • The next line - (void)sampleMethod{} shows the implementation of the sampleMethod.

  • The next line @end marks the end of an implementation.

  • The next line int main() is the main function where program execution begins.

  • The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program.

  • The next line NSLog(...) is another function available in Objective-C which causes the message "Hello, World!" to be displayed on the screen.

  • The next line return 0; terminates main()function and returns the value 0.

Compile & Execute Objective-C Program

Now when we compile and run the program, we will get the following result.

2017-10-06 07:48:32.020 demo[65832] Hello, World!
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