[blackfuse]

// ABox.h
@interface ABox : NSObject 

@property (nonatomic, copy) NSString *name;
@property float length;
@property float width;
@property float height;

- (float) computeVolume;
- (NSString *) estimateCategory;

@end


// ABox.m
@implementation ABox
@synthesize name, length, width, height;

- (float) computeVolume {
   return self.length * self.width * self.height;
}
- (NSString *) estimateCategory {
   float vol = [self computeVolume];
   if(vol >= 8000)
      return @"Large";
   else if((vol < 8000) && (vol > 1000))
      return @"Medium";
   else
      return @"Small";
}

2 errors not sure why
Undefined symbols for architecture x86_64: "_main", referenced from: -u command line option ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

[JackOfAllTrades]

Don't really know Objective-C, but in most C executable programs you need an entry point, which is called main. That seems to be what it's complaining about: you don't have one.