How to copy an object in Objective-C

Question

I need to deep copy a custom object that has objects of its own  I ve been reading around and am a bit confused as to how to inherit NSCopying and how to use NSCopyObject

User · Answer

This is probably unpopular way  But here how I do it  object1      object to copy  YourClass  object2     YourClass alloc  init   object2 property1   object1 property1  object2 property2   object1 property2     etc   Quite simple and straight forward   P

User · Answer

There is also the use of the -  operator for copying  For Example   - id copyWithZone  NSZone  zone       MYClass  copy    MYClass new       copy- gt  property1   self- gt  property1              copy- gt  propertyN   self- gt  propertyN      return copy      The reasoning here is the resulting copied object should reflect the state of the original object  The     operator could introduce side effects as this one calls getters which in turn may contain logic

User · Answer

I don t know the difference between that code and mine  but I have problems with that solution  so I read a little bit more and found that we have to set the object before return it  I mean something like    import  lt Foundation Foundation h gt    interface YourObject   NSObject  lt NSCopying gt    property  strong  nonatomic  NSString  name   property  strong  nonatomic  NSString  line   property  strong  nonatomic  NSMutableString  tags   property  strong  nonatomic  NSString  htmlSource   property  strong  nonatomic  NSMutableString  obj   - id  copyWithZone   NSZone    zone    end    implementation YourObject   - id  copyWithZone   NSZone    zone       YourObject  copy     YourObject allocWithZone  zone  init         copy setNombre  self name        copy setLinea  self line        copy setTags  self tags        copy setHtmlSource  self htmlSource        return copy      I added this answer because I have a lot of problems with this issue and I have no clue about why is it happening  I don t know the difference  but it s working for me and maybe it can be useful for others too

User · Answer

As always with reference types  there are two notions of  copy   I m sure you know them  but for completeness    A bitwise copy  In this  we just copy the memory bit for bit - this is what NSCopyObject does  Nearly always  it s not what you want  Objects have internal state  other objects  etc  and often make assumptions that they re the only ones holding references to that data  Bitwise copies break this assumption  A deep  logical copy  In this  we make a copy of the object  but without actually doing it bit by bit - we want an object that behaves the same for all intents and purposes  but isn t  necessarily  a memory-identical clone of the original - the Objective C manual calls such an object  functionally independent  from it s original  Because the mechanisms for making these  intelligent  copies varies from class to class  we ask the objects themselves to perform them  This is the NSCopying protocol    You want the latter  If this is one of your own objects  you need simply adopt the protocol NSCopying and implement - id copyWithZone  NSZone   zone  You re free to do whatever you want  though the idea is you make a real copy of yourself and return it  You call copyWithZone on all your fields  to make a deep copy  A simple example is   interface YourClass   NSObject  lt NSCopying gt        SomeOtherObject  obj        In the implementation - id copyWithZone  NSZone   zone        We ll ignore the zone for now   YourClass  another     YourClass alloc  init     another obj    obj copyWithZone  zone      return another

User · Answer

Apple documentation says     A subclass version of the copyWithZone  method should send the message   to super first  to incorporate its implementation  unless the subclass   descends directly from NSObject    to add to the existing answer    interface YourClass   NSObject  lt NSCopying gt        SomeOtherObject  obj        In the implementation - id copyWithZone  NSZone   zone     YourClass  another    super copyWithZone zone     another obj    obj copyWithZone  zone      return another

User · Answer

another obj    obj copyWithZone  zone     I think  that this line causes memory leak  because you access to obj through property which is  I assume  declared as retain  So  retain count will be increased by property and copyWithZone   I believe it should be   another obj     obj copyWithZone  zone  autorelease     or   SomeOtherObject  temp    obj copyWithZone  zone   another obj   temp   temp release

[objective-c] How to copy an object in Objective-C

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