How do I compare two objects of a custom class? My idea was to add an additional method to the class in which I can compare the current object with another object of the same kind.
So I can write my own code how each field of the class is compared.
This is how I would do it. Or are there some predefined methods to do that? Like "isEqualTo" of the NSString class?
The pointers to -isEqual:
are good, but if you implement -isEqual:
, you absolutely must also implement -hash
in such a way that if two objects return YES
for -isEqual:
they will also return the same value for -hash
. Implementing isEqual:
without also implementing -hash
leads to some very surprising bugs when you use Collections like NSArray.
For new developers, I tend to recommend against overloading -isEqual:
. I recommend instead using the same technique as NSString, and create a custom -isEqualToFoo:
(where Foo
is your class) until you understand the impact of -isEqual:
on collections and specifically want this behavior. Overloading -isEqual:
powerful, but the bugs you can create are subtle. Creating your own custom comparator is safer and clearer in many cases.
The standard way is to override - (BOOL)isEqual:(id)anObject
and - (NSUInteger)hash
.
You should read the documentation for NSObject protocol and this SO question has some interesting answers on how to write your hash method.
查看isEqual:
和compare:
方法。
I have the following object:
#import <Foundation/Foundation.h>
typedef NS_ENUM(NSUInteger, SeasonType) {
kWinter,
kSpring,
kSummer,
kFall
};
@interface Season : NSObject
@property (nonatomic) SeasonType season;
@property (nonatomic) NSUInteger year;
+(id) seasonWithYear:(NSInteger)year season:(SeasonType)season;
-(id) initWithYear:(NSInteger)year season:(SeasonType)season;
@end
What I do is overwrite base NSObject comparison methods, there's no need of reinventing the wheel and code keeps cleaner as well:
#import "Season.h"
@interface Season()
@end
@implementation Season
+(id) seasonWithYear:(NSInteger)year season:(SeasonType)season{
return [[self alloc] initWithYear:year season:season];
}
-(id) initWithYear:(NSInteger)year season:(SeasonType)season{
self = [super init];
if (self)
{
_year = year;
_season=season;
_baseDate=nil;
}
return self;
}
#pragma mark - NSObject
- (BOOL)isEqual:(id)object {
if (self == object) {
return YES;
}
if (![object isKindOfClass:[Season class]]) {
return NO;
}
return [self _isEqualToSeason:(Season *)object];
}
- (NSUInteger)hash {
return self.season ^ self.year;
}
#pragma mark - Private/Internal
- (BOOL)_isEqualToSeason:(Season *)season {
if (!season) {
return NO;
}
return ((!self.season && !season.season) || self.season == season.season) &&
((!self.year && !season.year) || self.year == season.year) ;
}
@end
Usage:
Season *season2 = [Season seasonWithYear:2010 season:kFall];
Season *season3 = [Season seasonWithYear:2009 season:kFall];
[season2 isEqual:season3];
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