What is the difference between objectForKey
and valueForKey
?
I looked both up in the documentation and they seemed the same to me.
This question is related to
objective-c
cocoa
key-value-coding
When you do valueForKey:
you need to give it an NSString, whereas objectForKey:
can take any NSObject subclass as a key. This is because for Key-Value Coding, the keys are always strings.
In fact, the documentation states that even when you give valueForKey:
an NSString, it will invoke objectForKey:
anyway unless the string starts with an @
, in which case it invokes [super valueForKey:]
, which may call valueForUndefinedKey:
which may raise an exception.
Here's a great reason to use objectForKey:
wherever possible instead of valueForKey:
- valueForKey:
with an unknown key will throw NSUnknownKeyException
saying "this class is not key value coding-compliant for the key ".
I'll try to provide a comprehensive answer here. Much of the points appear in other answers, but I found each answer incomplete, and some incorrect.
First and foremost, objectForKey:
is an NSDictionary
method, while valueForKey:
is a KVC protocol method required of any KVC complaint class - including NSDictionary.
Furthermore, as @dreamlax wrote, documentation hints that NSDictionary
implements its valueForKey:
method USING its objectForKey:
implementation. In other words - [NSDictionary valueForKey:]
calls on [NSDictionary objectForKey:]
.
This implies, that valueForKey:
can never be faster than objectForKey:
(on the same input key) although thorough testing I've done imply about 5% to 15% difference, over billions of random access to a huge NSDictionary. In normal situations - the difference is negligible.
Next: KVC protocol only works with NSString *
keys, hence valueForKey:
will only accept an NSString *
(or subclass) as key, whilst NSDictionary
can work with other kinds of objects as keys - so that the "lower level" objectForKey:
accepts any copy-able (NSCopying protocol compliant) object as key.
Last, NSDictionary's
implementation of valueForKey:
deviates from the standard behavior defined in KVC's documentation, and will NOT emit a NSUnknownKeyException
for a key it can't find - unless this is a "special" key - one that begins with '@' - which usually means an "aggregation" function key (e.g. @"@sum, @"@avg"
). Instead, it will simply return a nil when a key is not found in the NSDictionary - behaving the same as objectForKey:
Following is some test code to demonstrate and prove my notes.
- (void) dictionaryAccess {
NSLog(@"Value for Z:%@", [@{@"X":@(10), @"Y":@(20)} valueForKey:@"Z"]); // prints "Value for Z:(null)"
uint32_t testItemsCount = 1000000;
// create huge dictionary of numbers
NSMutableDictionary *d = [NSMutableDictionary dictionaryWithCapacity:testItemsCount];
for (long i=0; i<testItemsCount; ++i) {
// make new random key value pair:
NSString *key = [NSString stringWithFormat:@"K_%u",arc4random_uniform(testItemsCount)];
NSNumber *value = @(arc4random_uniform(testItemsCount));
[d setObject:value forKey:key];
}
// create huge set of random keys for testing.
NSMutableArray *keys = [NSMutableArray arrayWithCapacity:testItemsCount];
for (long i=0; i<testItemsCount; ++i) {
NSString *key = [NSString stringWithFormat:@"K_%u",arc4random_uniform(testItemsCount)];
[keys addObject:key];
}
NSDictionary *dict = [d copy];
NSTimeInterval vtotal = 0.0, ototal = 0.0;
NSDate *start;
NSTimeInterval elapsed;
for (int i = 0; i<10; i++) {
start = [NSDate date];
for (NSString *key in keys) {
id value = [dict valueForKey:key];
}
elapsed = [[NSDate date] timeIntervalSinceDate:start];
vtotal+=elapsed;
NSLog (@"reading %lu values off dictionary via valueForKey took: %10.4f seconds", keys.count, elapsed);
start = [NSDate date];
for (NSString *key in keys) {
id obj = [dict objectForKey:key];
}
elapsed = [[NSDate date] timeIntervalSinceDate:start];
ototal+=elapsed;
NSLog (@"reading %lu objects off dictionary via objectForKey took: %10.4f seconds", keys.count, elapsed);
}
NSString *slower = (vtotal > ototal) ? @"valueForKey" : @"objectForKey";
NSString *faster = (vtotal > ototal) ? @"objectForKey" : @"valueForKey";
NSLog (@"%@ takes %3.1f percent longer then %@", slower, 100.0 * ABS(vtotal-ototal) / MAX(ototal,vtotal), faster);
}
As said, the objectForKey:
datatype is :(id)aKey
whereas the valueForKey:
datatype is :(NSString *)key
.
For example:
NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:[NSArray arrayWithObject:@"123"],[NSNumber numberWithInteger:5], nil];
NSLog(@"objectForKey : --- %@",[dict objectForKey:[NSNumber numberWithInteger:5]]);
//This will work fine and prints ( 123 )
NSLog(@"valueForKey : --- %@",[dict valueForKey:[NSNumber numberWithInteger:5]]);
//it gives warning "Incompatible pointer types sending 'NSNumber *' to parameter of type 'NSString *'" ---- This will crash on runtime.
So, valueForKey:
will take only a string value and is a KVC method, whereas objectForKey:
will take any type of object.
The value in objectForKey
will be accessed by the same kind of object.
Source: Stackoverflow.com