Sending message to object that is member of array...

Question

Hi all,

Really hoping someone can help me sort out why sending objects that are members of an array seems to be the hardest thing in the world in Obj-C.

Here's the setup: I have a car class. Car has two member objects, engine and tire (of which there are four). Then I have an NSArray (also a member of car) initialized to hold the tire objects. I did this because I cannot figure out how to write or synthesize getter methods for just declaring like Tire *tires[4] (so I have to use NSArray and use objectAtIndex.

Here is the car class:

#import "Tire.h"
#import "Engine.h"

@interface Car : NSObject
{
    Engine *engine;
    Tire *tire1;
    Tire *tire2;
    Tire *tire3;
    Tire *tire4;
    NSArray *tirearray;
}

@property (nonatomic, copy) id engine;
@property (nonatomic, copy) id tire;
@property (nonatomic, copy) id tirearray;

@implementation Car

@synthesize engine;
@synthesize tire;
@synthesize tirearray;

- (void) print {

    NSLog(@"%@", engine);

}

- (id) init {

    if (self = [super init]) {
        engine = [Engine new];
        tire1 = [[tire alloc] init];
        tire2 = [[tire alloc] init];
        tire3 = [[tire alloc] init];
        tire4 = [[tire alloc] init];
        tirearray = [NSArray arrayWithObjects: tire1, tire2, tire3, tire4, nil];
    }

    return (self);

}

Then main:

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

    Car *car = [[Car alloc] init];

    [[car.tirearray objectAtIndex:0] setPressure: 32];


    [pool drain];
    return 0;
}

What I'm trying to do is figure out how to send messages to the objects in the array! That's all I want to do. The above code builds, but returns uncaught exception 'NSRangeException', reason: '* -[NSArray objectAtIndex:]: index (0) beyond bounds (0)' !!!

Just so you know, pressure is just a member variable of the tire class, and the getter methods have been synthesized.

Then I want to print something to the console like "The pressure of tire X is X PSI".

This is driving me nuts! This should be simple! AHHHH.

Thanks in advance!

Answer 1

Oh man. I feel so silly. I didn't initialize the array at all! I need to alloc then initialize with initWithObjects. Haha. Oops.

Answer 2

The code

        tire1 = [[tire alloc] init];

should be

        tire1 = [[Tire alloc] init];

Who told you to declare a property as an id? That's a very, very, very bad practice and you should stop it now. Right now.

If you bought a textbook which says so, please just burn it to ashes now.

By declaring your property

 @property (nonatomic, copy) Tire* tire; 

the compiler warn you at

        tire1 = [[tire alloc] init];

saying that tire doesn't respond to alloc.