Here a simple example of how inheritance works in objective-c (based on this older example).

1. Define the parent class.

Rectangle.h

#import <Foundation/NSObject.h>

@interface Rectangle: NSObject 

@property (nonatomic) int width;
@property (nonatomic) int height;

-(Rectangle*) initWithWidth: (int) w height: (int) h;
-(void) setWidth: (int) w height: (int) h;
-(void) print;

@end

Rectangle.m

#import "Rectangle.h"
#import <stdio.h>

@implementation Rectangle

@synthesize width = _width;
@synthesize height = _height;

-(Rectangle*) initWithWidth: (int) w height: (int) h {
    self = [super init];
    
    if ( self ) {
        [self setWidth: w height: h];
    }
    
    return self;
}

-(void) setWidth: (int) w height: (int) h {
    self.width = w;
    self.height = h;
}

-(void) print {
    printf( "width = %i, height = %i", self.width, self.height );
}

@end

2. Inherit from the parent.

Here we inherit our behavior from the parent with this line:

@interface Square: Rectangle


<strong>Square.h</strong>


#import "Rectangle.h"

@interface Square: Rectangle
-(Square*) initWithSize: (int) s;
-(void) setSize: (int) s;
-(int) size;
@end

And add our own customer behaviour:

Square.m

#import "Square.h"

@implementation Square
-(Square*) initWithSize: (int) s {
    self = [super init];
    
    if ( self ) {
        [self setSize: s];
    }
    
    return self;
}

-(void) setSize: (int) s {
    self.width = s;
    self.height = s;
}

-(int) size {
    return self.width;
}

@end

You can see here we inherit the Rectangle’s setWidth setHeight and print methods while providing our own setSize implementation.

3. Run it.

SpikeInheritanceTests.m

#import "SpikeInheritanceTests.h"
#import "Square.h"
#import "Rectangle.h"
#import <stdio.h>

@implementation SpikeInheritanceTests

- (void)testExample
{
    Rectangle *rec = [[Rectangle alloc] initWithWidth: 10 height: 20];
    Square *sq = [[Square alloc] initWithSize: 15];
    
    // print em
    printf( "Rectangle: " );
    [rec print];
    printf( "\n" );
    
    printf( "Square: " );
    [sq print];
    printf( "\n" );
    
    // update square
    [sq setWidth: 20];
    printf( "Square after change: " );
    [sq print];
    printf( "\n" );
}

@end

If all goes well output should look something like this: