Question

I am new to programming.I have seen this code.returning a derived class object to the base class.
So that the base class can then point to the derived class methods.
Here a static function in class B is returning its object to the base
class.

base-derivedclass.m

#import <Foundation/Foundation.h>
#import "B.h"
int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    [B p]; 
    [pool drain];
    return 0;
}

A.h

#import <Foundation/Foundation.h>


@interface A : NSObject {

}
@end

A.m

#import "A.h"

@implementation A
 @end

B.h

#import <Foundation/Foundation.h>
#import "A.h"

@interface B : A {

}
+(A*)p;
-(void)display;
@end

B.m

#import "B.h"


@implementation B
+(A*)p
{
    NSLog(@"returning derived class object to the base class!!");
    return [B new];
}
-(void)display
{
    NSLog(@"Hello");
}


@end

Answer 1

p is a class method. In Obj-C you denote class method by using + in method declaration and - to denote instance method. You can call the class method by using this:

// [ClassName methodName];
[B p];

Or you can change p to instance method by this:

- (A *)p;
// and call
// [instanceName methodName];
[dep p];

You can check Objective-C: A Primer to get started with these.