isEqual在Xcode中无法正常工作

Question

My code looks like proper but isEqual used to compare two object is not working. i'm new to iOS. there is no much resource to check. [box2 isEqual:box1] is always gives me No. but it supposed to give Yes. anything wrong in my code, please suggest me the correct thing. thanks in advance. expecting best suggestion .

#import <Foundation/Foundation.h>
@interface Box:NSObject
{
    double length;   // Length of a box
    double breadth;  // Breadth of a box
    double height;   // Height of a box
}
@property(nonatomic, readwrite) double height; // Property

-(double) volume;
//-(id)initWithLength:(double)l andBreadth:(double)b;
@end

@implementation Box

@synthesize height; 

-(id)init
{
   self = [super init];
   if(self)
   {
   length = 2.0;
   breadth = 3.0;
   }

   return self;
}

-(double) volume
{
   return length*breadth*height;
}

@end
@class Box;
int main (int argc, const char * argv[])
{
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   Box *box1 = [[Box alloc]init];    // Create box1 object of type Box
   Box *box2 = [[Box alloc]init];
   NSLog (@"hello world");
   box1.height = 5.0;
   NSLog (@"%ld",[box1 volume]);

   if([box2 isEqual:box1])
   {
   NSLog(@"%b",[box2 isEqual:box1]);
   }
   [pool drain];
   return 0;
}

Answer 1

You have to override isEqual method in your object.

eg in your "@implementation Box"

- (BOOL)isEqual:(id)object
{
    if ([object isKindOfClass:[Box class]]) {
        Box*box = (Box*)object;
        return self.height == box.height; // compare heights values
    }
    return [super isEqual:object];
}

Answer 2

isEqual: works just fine and as documented, but not as you expected.

isEqual: is a property of NSObject. It returns YES if two object pointers are the same, and NO if they are not. If you want a different result, you need to override isEqual for your Box class. Note that the argument to isEqual: could be anything, not just a Box, so you need to be careful.

Obviously you shouldn't declare instance variables put properties, so the code below assumes that. The way your code is written, you will have some confusion with an instance variable height, a property height, and an instance variable _height, which will give you headaches. It also assumes that you are not subclassing Box (for example you could have a coloured box, which should not be equal to a Box with same width, height and breadth).

- (BOOL)isEqual:(id)other
{
    if (! [other isKindOfClass:[Box class]]) return NO;
    Box* otherBox = other;
    return _length == other.length && _breadth == other.breadth 
           && _height == other.height;
}