Question

I noticed whilst playing around today that both the objects below when sent “init” print the same message “_init: TireSnow” to the console. Can anyone shed any light on why this is happening?

// INTERFACE
@interface TireBasic : NSObject {
}
@end

@interface TireSnow : TireBasic {
}
@end
// IMPLEMENT
@implementation TireBasic
- (id) init {
    self = [super init];
    if(self) {
        NSLog(@"TB_init: %@", NSStringFromClass([self class]));
    }
    return self;
}
- (NSString *) description {
return @"This is a BASIC TIRE.";
}
@end

@implementation TireSnow
- (id) init {
    self = [super init];
    if(self) {
        NSLog(@"TS_init: %@", NSStringFromClass([self class]));
    }
    return self;
}
- (NSString *) description {
return @"This is a SNOW TIRE.";
}

@end

EDIT2 – Added all of main()

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

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    int tireCount;
    CarClass *newCar_001;
    EngineClass *newEngine_001;
    TireBasic *newTire_BASIC;
    TireSnow  *newTire_SNOW;

    // Setup
    NSLog(@"COMPOSITION Start");
    newCar_001 = [[CarClass alloc] init];

    // Engine
    newEngine_001 = [[EngineClass alloc] init];
    [newCar_001 setEngine: newEngine_001];

    // TIRES (BASIC)
    for(tireCount=0; tireCount<2; tireCount++) {
        newTire_BASIC = [[TireBasic alloc] init];
        [newCar_001 setTire:newTire_BASIC];
        [newTire_BASIC release];
    }
    // TIRES (SNOW)
    for(tireCount=0; tireCount<2; tireCount++) {
        newTire_SNOW = [[TireSnow alloc] init];
        [newCar_001 setTire:newTire_SNOW];
        [newTire_SNOW release];
    }
    // Display
    [newCar_001 printCar];

    // Clean up
    [newCar_001 release];
    [newEngine_001 release];
    [pool drain];
    return 0;
}

OUTPUT

> COMPOSITION Start
> _init: CarClass
> _init: EngineClass
> TB_init: TireBasic
> TB_init: TireBasic
> TB_init: TireSnow *****
> TS_init: TireSnow
> TB_init: TireSnow *****
> TS_init: TireSnow
>  
> This is a BASIC TIRE.
> This is a BASIC TIRE.
> This is a SNOW TIRE.
> This is a SNOW TIRE.
> 
> _deal: CarClass
> TB_deal: TireBasic
> TB_deal: TireBasic
> TS_deal: TireSnow
> TB_deal: TireSnow ******
> TS_deal: TireSnow
> TB_deal: TireSnow ******
> _deal: EngineClass

The line with stars are coming from the TireSnow [super init] and [super dealloc], it seems to be [self class] that is returning “TireSnow” each time, can anyone explain why?

many thanks

gary

Answer 1

> TS_deal: TireSnow
> TB_deal: TireSnow

You’re asking why TireSnow is printed twice?

First [TireBasic init] runs, then [TireSnow init]. In both cases they print the class name, and in both cases the class name is TireSnow.

In other words, [self class] will always return the same object, regardless of which method it’s executing in.