Difference between objectForKey and valueForKey

Question

What is the difference between objectForKey and valueForKey  I looked both up in the documentation and they seemed the same to me

User · Accepted Answer

objectForKey  is an NSDictionary method  An NSDictionary is a collection class similar to an NSArray  except instead of using indexes  it uses keys to differentiate between items  A key is an arbitrary string you provide  No two objects can have the same key  just as no two objects in an NSArray can have the same index     valueForKey  is a KVC method  It works with ANY class  valueForKey  allows you to access a property using a string for its name  So for instance  if I have an Account class with a property accountNumber  I can do the following   NSNumber  anAccountNumber    NSNumber numberWithInt 12345   Account  newAccount     Account alloc  init     newAccount setAccountNumber anAccountNUmber    NSNumber  anotherAccountNumber    newAccount accountNumber     Using KVC  I can access the property dynamically   NSNumber  anAccountNumber    NSNumber numberWithInt 12345   Account  newAccount     Account alloc  init     newAccount setValue anAccountNumber forKey   accountNumber     NSNumber  anotherAccountNumber    newAccount valueForKey   accountNumber      Those are equivalent sets of statements   I know you re thinking  wow  but sarcastically  KVC doesn t look all that useful  In fact  it looks  wordy   But when you want to change things at runtime  you can do lots of cool things that are much more difficult in other languages  but this is beyond the scope of your question    If you want to learn more about KVC  there are many tutorials if you Google especially at Scott Stevenson s blog  You can also check out the NSKeyValueCoding Protocol Reference   Hope that helps

User · Answer

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

User · Answer

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 lt 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 lt 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 lt 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  gt  ototal      valueForKey      objectForKey       NSString  faster    vtotal  gt  ototal      objectForKey      valueForKey       NSLog       takes  3 1f percent longer then      slower  100 0   ABS vtotal-ototal    MAX ototal vtotal   faster

User · Answer

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

User · Answer

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

[objective-c] Difference between objectForKey and valueForKey?

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