is it true that the key values for NSMutableDictionary can only be strings?
I was trying to use objects, but I am getting a warning.
From the docs:
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).
What warning are you getting?
You can use any object, but the object must implement -[NSObject hash], -[NSObject isEqual:], and the NSCopying protocol.
If you take a look at header file of NSMutableDictionary, the add function can take id as the key:
- (void)setObject:(id)anObject forKey:(id)aKey;
- (void)removeObjectForKey:(id)aKey;
So you can use virtually anything as the key and value.
Related
I would like to use a custom object as a key in a hash-like structure. I've tried using NS[Mutable]Dictionary but in order for my object to be a key it has to implement the NSCopying protocol. NSDictionary is sending a copy message to all of it's keys as far as I've read. I don't want to implement the protocol (my object is quite complex) nor do I want it to be copied. What are my options? Do I have any?
NSDictionary is toll-free bridged with CFDictionaryRef, but they actually differ in behavior when adding objects. Specifically, NSDictionary's -setObject:forKey: will copy the key, but CFDictionaryRef's CFDictionarySetValue() will not copy the key. This means that if you want to use non-copyable keys, you can use CFDictionarySetValue() instead to add it to the dictionary.
CFDictionarySetValue((CFMutableDictionaryRef)myDict, myKey, myValue);
This will still retain the key, but it won't copy it. And you can use the normal NSDictionary methods for everything else.
Do you need the NSDictionary to retain the object? If not, you can turn it into an NSValue and use that as the key:
NSValue *value = [NSValue valueWithNonretainedObject:yourCustomObject];
[dictionary setObject:someObject forKey:value];
This can get a bit messy but is in alternative to implementing NSCopying.
You can roll your own dictionary. Not really that hard.
Another option is to use a surrogate object, containing a pointer to "the" object. The surrogate would implement the hash and either copy or reference the fields to be compared for isEqual. It could do a basic sanity check to assure the compared fields have not been changed when it's referenced.
You could just do this:
- (id)copyWithZone:(NSZone *)zone {
return [self retain];
}
Can we use only NSString objects as key in an NSDictionary? How do we know which objects can be used and which cannot?
From the documentation:
In general, a key can be any object (provided that it conforms to the NSCopying protocol), but note that when using key-value coding the key must be a string (see “Key-Value Coding Fundamentals”).
So you can use anything copyable besides strings, but they'll be problematic with KVC. I just use strings for keys to keep things safe, consistent and simple.
You can use anything that conforms to NSCopying. That is, you can use id - type objects, as long as they conform to NSCoding protocol.
In instances where the key is NSString, then isEqualToString: is called for retrieval. Otherwise, isEqual: is called on the object to determine whether the key matches the requested key.
The key (and value for that matter) cannot be nil or NULL. They can, however, be [NSNull null].
I need to use non-copyable objects as keys in the NSMutableDictionary which, by default, is not allowed. I understand the reasons for this not being allowed (retaining the key object, unlike the value object, is undesirable), but it seems like in my particular situation there could be a way around this.
The thing is that I'm only need to query the dictionary using the key's address, i.e. having the lookup predicate
if (providedKey == storedKey)
instead of
if ([providedKey isEqual:storedKey])
would be perfectly sufficient.
Is there a way of extracting the object's reference address (or other form of identity) as a copyable comparable object which I could use as a dictionary key instead of the object itself?
It seems that +[NSValue valueWithPointer:] might be what you want. It stores the pointer itself in an object that conforms to NSCopying, so that you can use it as a dictionary key. Retrieve the pointer using pointerValue.
You could use [NSString stringWithFormat:#"%p", someObject] as your key.
This will create a string with the object's address as a hex value.
You could use an nsnumber that represents the hash of the object.
you can drop down to the CoreFoundation APIs and define you own callbacks (among other things).
NSMutableDictionary is a CFMutableDictionary.
specifically, you want to create a CFMutableDictionary, and define your own CFDictionaryKeyCallBacks.
I have a NSMutableDictionary, and my keys are objects (and the class implement the NSCopying protocol).
I add a value in the dictionary associated to a key, then, when I call the objectFoKey: method for the key which is in the dictionary, randomly, sometimes the method call "isEqual" and the key is found, and sometimes, the method "isEqual" is not called and the key is not found.
I can understand I do something wrong and the key is not found, but why sometimes the key is found but not always ?
Thank you very much if you have some hints to fix that problem, it becomes to make me crazy.
Oups, ok, I found my mistake:
I must implement the hash method, that I didn't do.
I want to add a selector into a dictionary (the main purpose is for identifying the callback method and delegate after finish doing something)
But I find that I can not do that, the program will get an error "EXC_BAD_ACCESS".
Are there any other way for me to add that method selector to a dictionary?
Thanks for your help.
I know this question was answered a long time ago, but just in case anyone stumbles upon it like I did...
The combination of NSStringFromSelector and NSSelectorFromString as answered above is probably the best way to go. But if you really want to, you can use a selector as a value or key in an NSDictionary.
A selector (type SEL) is implemented as a pointer to a struct in Apple's Objective-C runtimes. A pointer cannot be used directly in a dictionary, but a pointer can be wrapped in an NSValue object that can be used.
Using this method you can store a selector as a value in a dictionary using code like this:
dictionary =
[NSDictionary dictionaryWithObject:[NSValue valueWithPointer:selector]
forKey:key];
A selector can be retrieved using code like this:
SEL selector = [[dictionary objectForKey:key] pointerValue];
Similarly for using a selector as a key:
dictionary =
[NSDictionary dictionaryWithObject:value
forKey:[NSValue valueWithPointer:selector]];
value = [dictionary objectForKey:[NSValue valueWithPointer:selector]];
Adding a new entry to a dictionary does two things (in addition to adding it to the dictionary, obviously):
It takes a copy of the key value. This means that the the key object must implement the NSCopying protocol
retains the value. This means that it needs to implement the NSObject protocol
It's probably the second that's causing your EXC_BAD_ACCESS.
There are at least two ways around this.
Firstly, rather than adding the selector you could add the instance of the class that implements the selector to your dictionary. Usually your class will inherit from NSObject and it will work fine. Note that it will retain the class though, maybe not what you want.
Secondly, you can convert a selector to a string (and back again) using NSSelectorFromString and NSStringFromSelector (docs are here).
I get my answer based on the comment of Zydeco:
You can convert between SEL and
NSString using NSSelectorFromString
and NSStringFromSelector
The common idiom in Obj-C is to have specific names for callbacks for specific events. (Such parserDidBeginDocument: from NSXMLParserDelegate). If you really need to be able to specify the names, it is likely that your only recourse is to add the names of the selectors as #"mySelector:withArgument:context:" or somesuch.