Question

I am trying to create a set of objects using NSMutableSet. The object is a Song, each tag has a name and an author.

code:

#import "Song.h"

@implementation Song

@synthesize name,author;

-(Song *)initWithName:(NSString *)n andAuth:(NSString *)a {
    self = [super init];

    if (self) {
        name = n;
        author = a;
    }

    return self;
}

-(void)print {
    NSLog(@"song:%@; author:%@;", name,author);
}

-(BOOL)isEqual:(id)obj {
    //NSLog(@"..isEqual");

    if([[obj name] isEqualToString:name]
       && [[obj author] isEqualToString:author]) {
        return YES;
    }

    return NO;
}

-(BOOL)isEqualTo:(id)obj {
    NSLog(@"..isEqualTo");

    if([[obj name] isEqualToString:name]
       && [[obj author] isEqualToString:author]) {
        return YES;
    }

    return NO;
}

@end

Then put this object into NSMutableSet：

int main(int argv, char *argc[]) {
    @autoreleasepool {
        Song *song1 = [[Song alloc] initWithName:@"music1" andAuth:@"a1"];
        Song *song2 = [[Song alloc] initWithName:@"music2" andAuth:@"a2"];
        Song *song3 = [[Song alloc] initWithName:@"music3" andAuth:@"a3"];

        Song *needToRemove = [[Song alloc] initWithName:@"music3" andAuth:@"a3"];

        NSMutableSet *ns = [NSMutableSet setWithObjects:song1, song2, song3, nil];

        [ns removeObject:needToRemove];

        for (Song *so in ns) {
            [so print];
        }
    }
}

But the strange thing happend,music3 is still in the NSMutableSet。But change to NSMutableArray,the music3 can delete.NSMutableArray’s removeObject call object’s isEqual method. I find the explain of the removeObject.Just a sentence：

Removes a given object from the set.

It’s not explain how it works.How to delete object like this way?NSMutableSet’s removeObject call which method?

Answer 1

The objective-c collection classes rely on - (NSUInteger)hash to figure out equal objects.

If your objects returns YES for isEqual: but a different hash, classes like NSSet will consider the objects different.

See the discussion of hash:

If two objects are equal (as determined by the isEqual: method), they must have the same hash value. This last point is particularly important if you define hash in a subclass and intend to put instances of that subclass into a collection.

Implement the hash method. Something like this should work:

- (NSUInteger)hash {
    return [self.author hash] ^ [self.name hash];
}