synthesize vs dynamic what are the differences

Question

What are the differences between implementing a  property with  dynamic or  synthesize

User · Answer

As others have said  in general you use  synthesize to have the compiler generate the getters and  or settings for you  and  dynamic if you are going to write them yourself   There is another subtlety not yet mentioned   synthesize will let you provide an implementation yourself  of either a getter or a setter  This is useful if you only want to implement the getter for some extra logic  but let the compiler generate the setter  which  for objects  is usually a bit more complex to write yourself    However  if you do write an implementation for a  synthesize d accessor it must still be backed by a real field  e g   if you write - int  getFoo    you must have an int foo  field   If the value is being produce by something else  e g  calculated from other fields  then you have to use  dynamic

User · Answer

here is  example of  dynamic   import  lt Foundation Foundation h gt    interface Book   NSObject      NSMutableDictionary  data     property  retain  NSString  title   property  retain  NSString  author   end   implementation Book  dynamic title  author   -  id init       if   self    super init              data     NSMutableDictionary alloc  init            data setObject   Tom Sawyer  forKey   title             data setObject   Mark Twain  forKey   author              return self     -  void dealloc        data release        super dealloc      -  NSMethodSignature   methodSignatureForSelector  SEL selector       NSString  sel   NSStringFromSelector selector       if   sel rangeOfString   set   location    0            return  NSMethodSignature signatureWithObjCTypes  v             else           return  NSMethodSignature signatureWithObjCTypes                   -  void forwardInvocation  NSInvocation   invocation       NSString  key   NSStringFromSelector  invocation selector        if   key rangeOfString   set   location    0            key     key substringWithRange NSMakeRange 3   key length -4   lowercaseString           NSString  obj           invocation getArgument  amp obj atIndex 2            data setObject obj forKey key         else           NSString  obj    data objectForKey key            invocation setReturnValue  amp obj             end  int main int argc  char   argv        NSAutoreleasePool  pool     NSAutoreleasePool alloc  init        Book  book     Book alloc  init       printf   s is written by  s n    book title UTF8String    book author UTF8String        book title     1984       book author     George Orwell       printf   s is written by  s n    book title UTF8String    book author UTF8String         book release       pool release      return 0

User · Answer

One thing want to add is that if a property is declared as  dynamic it will not occupy memory  I confirmed with allocation instrument   A consequence is that you can declare property in class category

User · Answer

synthesize will generate getter and setter methods for your property   dynamic just tells the compiler that the getter and setter methods are implemented not by the class itself but somewhere else  like the superclass or will be provided at runtime    Uses for  dynamic are e g  with subclasses of NSManagedObject  CoreData  or when you want to create an outlet for a property defined by a superclass that was not defined as an outlet    dynamic also can be used to delegate the responsibility of implementing the accessors  If you implement the accessors yourself within the class then you normally do not use  dynamic   Super class     property  nonatomic  retain  NSButton  someButton       synthesize someButton    Subclass    property  nonatomic  retain  IBOutlet NSButton  someButton       dynamic someButton

User · Answer

As per the documentation   https   developer apple com library mac documentation cocoa conceptual ObjCRuntimeGuide Articles ocrtDynamicResolution html   dynamic tells the compiler that the accessor methods are provided at runtime   With a little bit of investigation I found out that providing accessor methods override the  dynamic directive    synthesize tells the compiler to create those accessors for you  getter and setter    property tells the compiler that the accessors will be created  and that can be accessed with the dot notation or  object message

User · Answer

As per the Apple documentation   You use the  synthesize statement in a class   s implementation block to tell the compiler to create implementations that match the specification you gave in the  property declaration   You use the  dynamic statement to tell the compiler to suppress a warning if it can   t find an implementation of accessor methods specified by an  property declaration   More info -  https   developer apple com library ios documentation General Conceptual DevPedia-CocoaCore DeclaredProperty html

User · Answer

dynamic is typically used  as has been said above  when a property is being dynamically created at runtime   NSManagedObject does this  why all its properties are dynamic  -- which suppresses some compiler warnings   For a good overview on how to create properties dynamically  without NSManagedObject and CoreData   see  http   developer apple com library ios  documentation Cocoa Conceptual ObjCRuntimeGuide Articles ocrtDynamicResolution html   apple ref doc uid TP40008048-CH102-SW1

User · Answer

Take a look at this article  under the heading  Methods provided at runtime       Some accessors are created dynamically at runtime  such as certain ones used in CoreData s NSManagedObject class  If you want to declare and use properties for these cases  but want to avoid warnings about methods missing at compile time  you can use the  dynamic directive instead of  synthesize                Using the  dynamic directive essentially tells the compiler  don t worry about it  a method is on the way     The  synthesize directive  on the other hand  generates the accessor methods for you at compile time  although as noted in the  Mixing Synthesized and Custom Accessors  section it is flexible and does not generate methods for you if either are implemented

[objective-c] @synthesize vs @dynamic, what are the differences?

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