Question

I’ve always used NSDictionaries with strings as keys, and pretty much all the examples on the web/books/etc. are the same. I figured that I’d try it with a custom object for a key. I’ve read up on implementing the “copyWithZone” method and created the following basic class:

@interface CustomClass : NSObject
{
    NSString *constString;
}

@property (nonatomic, strong, readonly) NSString *constString;

- (id)copyWithZone:(NSZone *)zone; 

@end

@implementation CustomClass

@synthesize constString;

- (id)init
{
    self = [super init];
    if (self) {
        constString = @"THIS IS A STRING";
    }
    return self;
}

- (id)copyWithZone:(NSZone *)zone
{
    CustomClass *copy = [[[self class] allocWithZone: zone] init];
    return copy;
}

@end

Now I’m trying to just add one of these objects with a simple string value, and then getting the string value back out to log to the console:

CustomClass *newObject = [[CustomClass alloc] init];
NSString *valueString = @"Test string";
NSMutableDictionary *dict =
[[NSMutableDictionary alloc] initWithObjectsAndKeys:valueString, newObject, nil];

    NSLog(@"Value in Dictionary: %@", [dict objectForKey: newObject]);
    // Should output "Value in Dictionary: Test string"

Unfortunately the log displays a (null). I’m pretty sure I’m missing something really obvious, and feel like I need another set of eyes.

Answer 1

NSDictionary key objects work off three methods:

• -(NSUInteger)hash
• -(BOOL)isEqual:(id)other
• -(id)copyWithZone:(NSZone*)zone

The default NSObject implementation of hash and isEqual: only use the object’s pointer, so when your object is copied via copyWithZone: the copy and the original object are no longer equal.

What you need is something like this:

@implementation CustomClass

-(NSUInteger) hash;
{
    return [constString hash];
}

-(BOOL) isEqual:(id)other;
{
    if([other isKindOfClass:[CustomClass class]])
        return [constString isEqualToString:((CustomClass*)other)->constString];
    else
        return NO;
}
- (id)copyWithZone:(NSZone *)zone
{
    CustomClass *copy = [[[self class] allocWithZone: zone] init];
    copy->constString = constString; //might want to copy or retain here, just incase the string isn't a constant
    return copy;
}

@end

It’s a little bit difficult to find this out from the documentation. The overview for NSDictionary tells you about isEqual: and NSCopying:

Within a dictionary, the keys are unique. That is, no two keys in a
single dictionary are equal (as determined by isEqual:). In general, a
key can be any object (provided that it conforms to the NSCopying
protocol—see below), but note that when using key-value coding the key
must be a string (see “Key-Value Coding Fundamentals”).

And if you have a look at the documentation for -[NSObject isEqual:] it tells you about hash:

If two objects are equal, they must have the same hash value. This
last point is particularly important if you define isEqual: in a
subclass and intend to put instances of that subclass into a
collection. Make sure you also define hash in your subclass.