How can I make a deep copy in Objective-C?

StackOverflow https://stackoverflow.com/questions/20072729

  •  31-07-2022
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Question

I'm learning ios development and I'm confused with deep copying in Objective-C. For example,I have three class below. Now I want to deep copy ClassA, can anybody teach me to finish the copy method?

A:

@interface ClassA : NSObject <NSCopying>

@property (nonatomic, assign) int aInt;
@property (nonatomic, retain) ClassB *bClass;

@end

B:

@interface ClassB : NSObject <NSCopying>

@property (nonatomic, assign) int bInt;
@property (nonatomic, retain) ClassC *cClass;

@end

C:

@interface ClassC : NSObject <NSCopying>

@property (nonatomic, assign) int cInt;
@property (nonatomic, copy) NSString *str;

@end
Was it helpful?

Solution 2

You should add the copyWithZone: method in each class you want to be copiable.

NB: I wrote this by hand, watch out for typos.

-(id) copyWithZone:(NSZone *) zone
{
    ClassA *object = [super copyWithZone:zone];
    object.aInt = self.aInt;
    object.bClass = [self.bClass copyWithZone:zone];
    return object;
}

-(id) copyWithZone:(NSZone *) zone
{
    ClassB *object = [super copyWithZone:zone];
    object.bInt = self.bInt;
    object.cClass = [self.cClass copyWithZone:zone];
    return object;
}

-(id) copyWithZone:(NSZone *) zone
{
    ClassC *object = [super copyWithZone:zone];
    object.cInt = self.cInt;
    object.str = [self.str copy];
    return object;
}

OTHER TIPS

Following the explanation at http://www.techotopia.com/index.php/Copying_Objects_in_Objective-C

"This can be achieved by writing the object and its constituent elements to an archive and then reading back into the new object."

@implementation ClassA

- (id)copyWithZone:(NSZone*)zone{
    NSData *buffer;
    buffer = [NSKeyedArchiver archivedDataWithRootObject:self];
    ClassA *copy = [NSKeyedUnarchiver unarchiveObjectWithData: buffer];
    return copy;
}
@end

Objective-C on iOS doesn’t offer any direct language or library construct to switch between a shallow and a deep copy. Each class defines what it means to “get its copy”:

@implementation ClassA

- (id) copyWithZone: (NSZone*) zone
{
    ClassA *copy = [super copyWithZone:zone];
    [copy setBClass:bClass]; // this would be a shallow copy
    [copy setBClass:[bClass copy]]; // this would be a deep copy
    return copy;
}

@end

Of course you would have to do the same decision in ClassB and ClassC. If I am not mistaken, the usual semantics for a copy in Objective-C is to return a shallow copy. See also this question about copying arrays for more discussion of the topic.

I had custom classes with long lists of properties, so I iterated over them:

@interface MyClass : NSObject <NSCopying>

#import <objc/runtime.h>

-(id) copyWithZone: (NSZone *) zone {

    MyClass *myCopy = [[MyClass alloc] init];

    //deepCopy
    unsigned int numOfProperties;
    objc_property_t *properties = class_copyPropertyList([self class], &numOfProperties);

    for (int i = 0; i < numOfProperties; i++) {

       objc_property_t property = properties[i];
       NSString *propertyName = [[NSString alloc]initWithCString:property_getName(property) encoding:NSUTF8StringEncoding];
       [adressCopy setValue:[[self valueForKey:propertyName] copy] forKey:propertyName];
    }
    return myCopy;
}

All customClassProperties will need to implement this as well.

This could be of some help. The link shows how to do the deep copy using NSKeyedArchiver

http://iphonecodecenter.wordpress.com/2013/08/26/difference-between-shallow-copy-and-deep-copy/

Objective-C's copy and copyWithZone specifications are bogus and dangerous and should not be used. --!-- At least not when used with ARC (Automatic Reference Counting) (2016-08-23) --!-- The code will lead to writing out of the bounds of memory / buffer overflows. Instead I present a method to safely copy objects initAsShallowCopy and deepCopy.

See my test results in code below:

#import <Foundation/Foundation.h>

@interface ClassA : NSObject
{
    @public
    NSMutableString*    A_Name;
    NSInteger           A_NSInteger;
    long int            A_int;
    float               A_float;
}
    -(id)init;
    -(id)copyWithZone:(NSZone *) zone;      // DON'T USE copy OR copyWithZone, unless you ignore Apple's guidelines and always make shallow copies in line with the correct example code here for initAsShallowCopy (but you return a copy instead of being a copy)
    -(id)initAsShallowCopy:(ClassA *)original;  // Correct way to make a shallow copy
    -(void)deepCopy;                            // Correct way to make a deep copy (Call initAsShallowCopy first)
@end

@interface ClassB : ClassA
{
    @public
    NSMutableString*    B_Name;
    NSInteger           B_NSInteger;
    long int            B_int;
    float               B_float;
}
    -(id)init;
    -(id)copyWithZone:(NSZone *) zone;      // DON'T USE copy OR copyWithZone, unless you ignore Apple's guidelines and always make shallow copies in line with the correct example code here for initAsShallowCopy (but you return a copy instead of being a copy)
    -(id)initAsShallowCopy:(ClassB *)original;  // Correct way to make a shallow copy
    -(void)deepCopy;                            // Correct way to make a deep copy (Call initAsShallowCopy first)
    -(void)print;
@end

@interface ClassCWithoutCopy : NSObject
{
    @public
    NSMutableString*    C_Name;
    NSInteger           C_NSInteger;
    long int            C_int;
    float               C_float;
}
-(id)init;
-(void)print;

@end

@implementation ClassA


    -(id)init
    {
        if ( self = [super init] ) {    // initialize NSObject
            //A_Name        = [[NSMutableString alloc] init];
            //[A_Name setString:@"I am inited to A"];
            A_Name      = [NSMutableString stringWithString:@"I am inited to A"];
            A_NSInteger = 1;
            A_int       = 1;
            A_float     = 1.0;

            return self;
        }
        return nil;
    }

    /*
    FROM https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/#//apple_ref/occ/instm/NSObject/copy

    -- NSObject Class Reference --


    - (id)copy

    Discussion
    This is a convenience method for classes that adopt the NSCopying protocol. An exception is raised if there is
    no implementation for copyWithZone:.

    NSObject does not itself support the NSCopying protocol. Subclasses must support the protocol and
    implement the copyWithZone: method. 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.


    + copyWithZone:

    Discussion
    This method exists so class objects can be used in situations where you need an object that conforms to the NSCopying protocol.
    For example, this method lets you use a class object as a key to an NSDictionary object.
    You should not override this method.

    CONCLUSION

    copy says we should incorporate the implementation of copyWithZone, while copyWithZone says we should not override it.. So what is it?
    Looking at copyWithZone, we see that it is a class method (+), meaning it has not access to its instantiated members.
    So maybe they mean, we should not override the class method (+), but we should implement its instance method -copyWithZone:
    !!In any case we should not implement copy, because it is just made for convenience by Apple!!

    FROM: https://developer.apple.com/library/tvos/documentation/Cocoa/Reference/Foundation/Protocols/NSCopying_Protocol/index.html

    -- NSCopying --

    Your options for implementing this protocol are as follows:

    1) Implement NSCopying using alloc and init... in classes that don’t inherit copyWithZone:.

    2) Implement NSCopying by invoking the superclass’s copyWithZone: when NSCopying behavior is inherited.
    If the superclass implementation might use the NSCopyObject function, make explicit assignments to
    pointer instance variables for retained objects.

    3) Implement NSCopying by retaining the original instead of creating a new copy when the class and its contents are immutable.

    CONCLUSION:

    From 1) NSObject does not implement copyWithZone so any class that you make that should support copying should call [[Class alloc] init].

    From 2) Any subclass of a copyable object should call [super copyWithZone:zone], but NOT [[Class alloc] init] !!!!!!
    */

    -(id) copyWithZone:(NSZone *) zone
    {
        ClassA *CopiedObject = [[ClassA alloc] init];

        if(CopiedObject){
            CopiedObject->A_Name        = [A_Name copy];
            CopiedObject->A_NSInteger   = A_NSInteger;
            CopiedObject->A_int         = A_int;
            CopiedObject->A_float       = A_float;
            return CopiedObject;
        }
        return nil;
    }

    -(id)initAsShallowCopy:(ClassA *)original   // Correct way to make a shallow copy
    {
        /* Why this has to be done like this:
            It is very annoying to assign every variable explicitely.
            However this has to be done, in order for ARC (Automatic Reference Counting) (2016-08-23) to work.
            The compiler needs to be aware of any reference made to an object or reference cleared to an object in order to keep track of the
            reference counts.
            The danger is that when you add a variable to you class later on, you must not forget to update your initAsShallowCopy function and 
            possibly your DeepCopy function.
            It would be much nicer if you could just do:
            *self = *original;
            But that gives compiler error:
            /DeepCopyTest/main.m:135:9: Cannot assign to class object ('ClassA' invalid)
            So therefore there is also no raw memory copy between objects,
            so we are stuck with writing out each member variable explicitely.
        */
        if ( self = [super init] ) {    // initialize NSObject
            A_Name      = original->A_Name;
            A_NSInteger = original->A_NSInteger;
            A_int       = original->A_int;
            A_float     = original->A_float;
            return self;
        }
        return nil;
    }

    -(void)deepCopy;                            // Correct way to make a deep copy (Call initAsShallowCopy first)
    {
        /*  Luckily now, we only have to duplicate the objects that require a deep copy.
            So we don't have to write out all the floats, ints and NSIntegers, etcetera. Thus only the pointers (*) to objects.
            */
        A_Name  = [A_Name copy];
    }

@end

@implementation ClassB


    -(id)init
    {
        if ( self = [super init] ) {    // initialize ClassA
            B_Name      = [NSMutableString stringWithString:@"I am inited to B"];
            B_NSInteger = 2;
            B_int       = 2;
            B_float     = 2.0;

            return self;
        }
        return nil;
    }

    -(id) copyWithZone:(NSZone *) zone
    {
        //ClassA *CopiedObject = [[ClassA alloc] init]; We are not a direct descendant from NSObject, so don't call alloc-init
        // instead call the super copyWithZone
        ClassB *CopiedObject = [super copyWithZone:zone];   /* Using ARC (Automatic Reference Counting) 2016-08-23:
        THIS IS A MASSIVE BUFFER OVERFLOW/WRITING OUT OF BOUNDS RISK:
        Since super now allocates the object, it will now only allocate an object of size ClassA
        and effectively allocate too little memory for the ClassB. Unless memory allocation is upgraded to work with magic for
        Objective-C, DON'T USE copy or copyWithZone!!!!
        */

        if(CopiedObject){
            CopiedObject->B_Name        = [B_Name copy];
            CopiedObject->B_NSInteger   = B_NSInteger;
            CopiedObject->B_int         = B_int;
            CopiedObject->B_float       = B_float;
            return CopiedObject;
        }
        return nil;
    }

    -(id)initAsShallowCopy:(ClassB *)original   // Correct way to make a shallow copy
    {
        if ( self = [super initAsShallowCopy:original] ) {  // initialize ClassA
            B_Name      = original->B_Name;
            B_NSInteger = original->B_NSInteger;
            B_int       = original->B_int;
            B_float     = original->B_float;
            return self;
        }
        return nil;
    }

    -(void)deepCopy;                            // Correct way to make a deep copy (Call initAsShallowCopy first)
    {
        /*  Luckily now, we only have to duplicate the objects that require a deep copy.
            So we don't have to write out all the floats, ints and NSIntegers, etcetera. Thus only the pointers (*) to objects.
            */
        [super deepCopy];
        B_Name  = [B_Name copy];
    }

    -(void)print
    {
        NSLog(@"A_Name=\"%@\", A_NSInteger=%ld,A_int=%ld,A_float=%f",A_Name,A_NSInteger,A_int,A_float);
        NSLog(@"B_Name=\"%@\", B_NSInteger=%ld,B_int=%ld,B_float=%f",B_Name,B_NSInteger,B_int,B_float);
    }

@end

@implementation ClassCWithoutCopy


    -(id)init
    {
        if ( self = [super init] ) {    // initialize NSObject
            C_Name      = [NSMutableString stringWithString:@"I am inited to C"];
            C_NSInteger = 3;
            C_int       = 3;
            C_float     = 3.0;

            return self;
        }
        return nil;
    }

    -(void)print
    {
        NSLog(@"C_Name=\"%@\", C_NSInteger=%ld,C_int=%ld,C_float=%f",C_Name,C_NSInteger,C_int,C_float);
    }
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        ClassB      *OriginalB;
        ClassB      *CopiedB;

    #define USE_CORRECT_DEEP_COPY_AND_SHALLOW_COPY  1
    #define USE_CLASSC_WITHOUT_COPY_TEST    0

    #if(USE_CLASSC_WITHOUT_COPY_TEST)

        ClassCWithoutCopy   *OriginalC;
        ClassCWithoutCopy   *CopiedC;

        OriginalC   = [[ClassCWithoutCopy alloc] init];
        CopiedC     = [OriginalC copy]; /* Thread 1: signal SIGABRT: libc++abi.dylib: terminating with uncaught exception of type NSException
        *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ClassCWithoutCopy copyWithZone:]: unrecognized selector sent to instance 0x100100450' */
        //CopiedC       = [ClassCWithoutCopy copyWithZone:nil]; /* DeepCopyTest/main.m:283:33: 'copyWithZone:' is unavailable: not available in automatic reference counting mode
        */

        NSLog(@"OriginalC print:1");
        [OriginalC print];
        NSLog(@"CopiedC print:1");
        [CopiedC print];

        [OriginalC->C_Name appendString:@" and Appended as the original"];
        OriginalC->C_NSInteger = 30;
        OriginalC->C_int = 30;
        OriginalC->C_float = 30.0;

        NSLog(@"OriginalC print:2");
        [OriginalC print];
        NSLog(@"CopiedC print:2");
        [CopiedC print];
    #endif

    #if(USE_CORRECT_DEEP_COPY_AND_SHALLOW_COPY)
        OriginalB   = [[ClassB alloc] init];
        CopiedB     = [[ClassB alloc] initAsShallowCopy:OriginalB];

        NSLog(@"OriginalB print:1");
        [OriginalB print];
        NSLog(@"CopiedB print:1");
        [CopiedB print];

        [OriginalB->A_Name appendString:@" and Appended as the original"];
        OriginalB->A_NSInteger = 10;
        OriginalB->A_int = 10;
        OriginalB->A_float = 10.0;
        [OriginalB->B_Name appendString:@" and Appended as the original"];
        OriginalB->B_NSInteger = 20;
        OriginalB->B_int = 20;
        OriginalB->B_float = 20.0;



        NSLog(@"OriginalB print:2");
        [OriginalB print];
        NSLog(@"CopiedB print:2");
        [CopiedB print];
        // This works as expected: The values of OriginalB and CopiedB differ, but the shallow copied strings are the same.

        // Now make a deep copy of CopiedB
        [CopiedB deepCopy];

        [OriginalB->A_Name appendString:@" and Appended twice as the original"];
        OriginalB->A_NSInteger = 100;
        OriginalB->A_int = 100;
        OriginalB->A_float = 100.0;
        [OriginalB->B_Name appendString:@" and Appended twice as the original"];
        OriginalB->B_NSInteger = 200;
        OriginalB->B_int = 200;
        OriginalB->B_float = 200.0;

        NSLog(@"OriginalB print:3");
        [OriginalB print];
        NSLog(@"CopiedB print:3");
        [CopiedB print];
        // This works as expected: The values of OriginalB and CopiedB differ and als the deep copied strings are different.

    #else
        OriginalB   = [[ClassB alloc] init];
        CopiedB     = [OriginalB copy];             // Undefined behaviour. You will write unallocated memory

        NSLog(@"OriginalB print:1");
        [OriginalB print];
        NSLog(@"CopiedB print:1");
        /*[CopiedB print];  / * Thread 1: signal SIGABRT: libc++abi.dylib: terminating with uncaught exception of type NSException
        *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ClassA print]: unrecognized selector sent to instance 0x10010ad60' */
        NSLog(@"A_Name=\"%@\", A_NSInteger=%ld,A_int=%ld,A_float=%f",CopiedB->A_Name,CopiedB->A_NSInteger,CopiedB->A_int,CopiedB->A_float);
        NSLog(@"B_Name=\"%@\", B_NSInteger=%ld,B_int=%ld,B_float=%f",CopiedB->B_Name,CopiedB->B_NSInteger,CopiedB->B_int,CopiedB->B_float); // Undefined behaviour. You will read unallocated memory


        [OriginalB->A_Name appendString:@" and Appended as the original"];
        OriginalB->A_NSInteger = 10;
        OriginalB->A_int = 10;
        OriginalB->A_float = 10.0;
        [OriginalB->B_Name appendString:@" and Appended as the original"];
        OriginalB->B_NSInteger = 20;
        OriginalB->B_int = 20;
        OriginalB->B_float = 20.0;
        // This at least works: Changing Original, does not alter the values of Copy.


        NSLog(@"OriginalB print:2");
        [OriginalB print];
        NSLog(@"CopiedB print:2");
        NSLog(@"A_Name=\"%@\", A_NSInteger=%ld,A_int=%ld,A_float=%f",CopiedB->A_Name,CopiedB->A_NSInteger,CopiedB->A_int,CopiedB->A_float);
        //NSLog(@"B_Name=\"%@\", B_NSInteger=%ld,B_int=%ld,B_float=%f",CopiedB->B_Name,CopiedB->B_NSInteger,CopiedB->B_int,CopiedB->B_float);   // Undefined behaviour. You will read unallocated memory

        /*[CopiedB->A_Name appendString:@" and Appended as the copy"];  / * Thread 1: signal SIGABRT: libc++abi.dylib: terminating with uncaught exception of type NSException
        *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:' */
        CopiedB->A_NSInteger = 100;
        CopiedB->A_int = 100;
        CopiedB->A_float = 100.0;
        /*[CopiedB->B_Name appendString:@" and Appended as the copy"];  / * Thread 1: signal SIGABRT: libc++abi.dylib: terminating with uncaught exception of type NSException
         *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:'*/
        CopiedB->B_NSInteger = 200;                                     // Undefined behaviour. You will write unallocated memory
        CopiedB->B_int = 200;                                           // Undefined behaviour. You will write unallocated memory
        CopiedB->B_float = 200.0;                                       // Undefined behaviour. You will write unallocated memory

        /* Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
        DeepCopyTest(2376,0x7fff7edda310) malloc: *** error for object 0x10010ad98: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug */

        NSLog(@"OriginalB print after modification of CopiedB:");
        [OriginalB print];
        NSLog(@"CopiedB print after modification of CopiedB:");
        /*[CopiedB print];; / * Thread 1: signal SIGABRT: libc++abi.dylib: terminating with uncaught exception of type NSException
        *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ClassA print]: unrecognized selector sent to instance 0x10010ad60' */

    #endif
    }
    return 0;
}

PS-1: FROM:

https://developer.apple.com/library/mac/documentation/General/Conceptual/DevPedia-CocoaCore/ObjectCopying.html

-- Object copying --

A deep copy duplicates the objects referenced while a shallow copy duplicates only the references to those objects. So if object A is shallow-copied to object B, object B refers to the same instance variable (or property) that object A refers to. Deep-copying objects is preferred to shallow-copying, especially with value objects.

NOTE:

This is unclear formulation, especially with the accompanied illustration, which suggests a wrong explanation.

This formulation makes it appear that two references to the same object count as a shallow copy. This is not true. It isn't a copy at all.

The clear formulation would be that: -A shallow copy of an object has all the values and references copied from its parent, but is itself a unique object in memory. -A deep copy of an object has all the values copied from its parent and is itself a unique object in memory, but all the references now reference to -deep themselves - copies of the original references objects.

Although the exact implementation of deep copying might not 100% give deep copies. Objects that point to external references (suchs as a hardware item or graphics driver can't be duplicated, but only increase the reference count) Some deep copying has no functional sense. An object might reference its window it is in, but it makes no sense to duplicate the window. An object might also reference data that is considered immutable, so it would not be efficient to duplicate that.

PS-2: You could have give me the hint of ctrl-K before I tried to format all my code manually.

PS-3: Apple-Z (undo) undoes all my formatting instead of the last one and I can't redo it.

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