Question

Help please! my first program in objective c. Followed a tutorial word for word but it gives me this error that I don’t know quite how to read for objective c.

SimpleCar.h:

#import <Cocoa/Cocoa.h>


@interface SimpleCar : NSObject {
    NSString* make;
    NSString* model;
    NSNumber* vin;
}

// set methods
- (void) setVin:   (NSNumber*)newVin;
- (void) setMake:  (NSString*)newMake;
- (void) setModel: (NSString*)newModel;

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel;

// get methods
- (NSString*) make;
- (NSString*) model;
- (NSNumber*) vin;

@end

SimpleCar.m:

#import "SimpleCar.h"


@implementation SimpleCar
// set methods
- (void) setVin: (NSNumber*)newVin {

    [vin release];
    vin = [[NSNumber alloc] init];
    vin = newVin;

}

- (void) setMake: (NSString*)newMake {

    [make release];
    make = [[NSString alloc] initWithString:newMake];

}

- (void) setModel: (NSString*)newModel {

    [model release];
    model = [[NSString alloc] initWithString:newModel];

}

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel {

    // Reuse our methods from earlier
    [self setMake:newMake];
    [self setModel:newModel];

}

- (NSString*) make {
    return make;
}

- (NSString*) model {
    return model;
}

- (NSNumber*) vin {
    return vin;
}

-(void) dealloc
{
    [vin release];
    [make release];
    [model release];
    [super dealloc];
}

@end

CarApp.m:

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

int main (int argc, const char * argv[]) {

  NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    SimpleCar *myCar = [[SimpleCar alloc] init];

    NSNumber *newVin = [NSNumber numberWithInt:123];

    [myCar setVin:newVin];
    [myCar setMake:@"Honda" andModel:@"Civic"];

    NSLog(@"The car is: %@ %@", [myCar make], [myCar model]);
    NSLog(@"The vin is: %@", [myCar vin]);

    [myCar release];

  [pool drain];

  return 0;
}

compiler call:

bash-3.2$ gcc CarApp.m SimpleCar.m -g -m64

error:

Undefined symbols for architecture x86_64:
  "___CFConstantStringClassReference", referenced from:
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
      CFString in ccR0Zlgm.o
  "_objc_msgSend", referenced from:
      _main in ccR0Zlgm.o
      -[SimpleCar setVin:] in ccJfVliU.o
      -[SimpleCar setMake:] in ccJfVliU.o
      -[SimpleCar setModel:] in ccJfVliU.o
      -[SimpleCar setMake:andModel:] in ccJfVliU.o
     (maybe you meant: l_objc_msgSend_fixup_release, l_objc_msgSend_fixup_alloc )
  "_NSLog", referenced from:
      _main in ccR0Zlgm.o
  "_objc_msgSend_fixup", referenced from:
      l_objc_msgSend_fixup_alloc in ccR0Zlgm.o
      l_objc_msgSend_fixup_release in ccR0Zlgm.o
      l_objc_msgSend_fixup_release in ccJfVliU.o
      l_objc_msgSend_fixup_alloc in ccJfVliU.o
     (maybe you meant: l_objc_msgSend_fixup_release, l_objc_msgSend_fixup_alloc )
  "_OBJC_CLASS_$_NSAutoreleasePool", referenced from:
      objc-class-ref in ccR0Zlgm.o
  "_OBJC_CLASS_$_NSNumber", referenced from:
      objc-class-ref in ccR0Zlgm.o
      objc-class-ref in ccJfVliU.o
  "_objc_msgSendSuper2", referenced from:
      -[SimpleCar dealloc] in ccJfVliU.o
  "_OBJC_METACLASS_$_NSObject", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
  "__objc_empty_cache", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "__objc_empty_vtable", referenced from:
      _OBJC_METACLASS_$_SimpleCar in ccJfVliU.o
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "_OBJC_CLASS_$_NSObject", referenced from:
      _OBJC_CLASS_$_SimpleCar in ccJfVliU.o
  "_OBJC_CLASS_$_NSString", referenced from:
      objc-class-ref in ccJfVliU.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Answer 1

It looks like you just created an Xcode Workspace without an Xcode project. Here’s a project that you could use:

http://www.markdouma.com/developer/CarApp.zip

Generally, you just choose File > New Project to create a new project. You’d likely want a Foundation-based command-line program for this particular project.

Unfortunately, the tutorial didn’t give the best implementation of the SimpleCar class. Below is one possible rewrite:

#import "SimpleCar.h"

@implementation SimpleCar

-(void) dealloc
{
    [vin release];
    [make release];
    [model release];
    [super dealloc];
}


// set methods
- (void) setVin: (NSNumber*)newVin {
    [newVin retain];
    [vin release];
    vin = newVin;
//    [vin release];
//    vin = [[NSNumber alloc] init];
//    vin = newVin;
}

The commented code above is the original code. It is both potentially dangerous and also leaks memory. The first line of the original code releases vin before doing anything else, which is potentially dangerous. For example, what if in your CarApp.m you did this:

NSNumber *vinValue = [car vin];
[car setVin:vin];
NSLog(@"car's vin == %@", [car vin]); // unpredictable results/crash

The original code would have released the existing vin value without bothering to make sure the value being passed in wasn’t actually vin itself. By setting vin = newVin, it was setting vin to point to itself but after being released. Any subsequent attempts to send messages to vin would result in a crash or unpredictable results.

The original code also leaks memory. Instances of NSNumber are immutable, so creating a number through alloc/init doesn’t really make much sense (since it will always be zero, and can never be changed). My replacement code first retains the value that is passed in, in case it happens to be vin. It then releases vin and then assigns vin to newVin.

The setMake: and setModel: methods are problematic for the same reasons.

- (void) setMake: (NSString*)newMake {
    [newMake retain];
    [make release];
    make = newMake;
//    [make release];
//    make = [[NSString alloc] initWithString:newMake];
}

- (void) setModel: (NSString*)newModel {
    [newModel retain];
    [model release];
    model = newModel;
//    [model release];
//    model = [[NSString alloc] initWithString:newModel];
}

// convenience method
- (void) setMake: (NSString*)newMake
        andModel: (NSString*)newModel {
    // Reuse our methods from earlier
    [self setMake:newMake];
    [self setModel:newModel];
}

- (NSString*) make {
    return make;
}
- (NSString*) model {
    return model;
}
- (NSNumber*) vin {
    return vin;
}
@end