how to do true deep copy for NSArray and NSDictionary with have nested arrays/dictionary?

Question

Question: Is there a way to use existing objective-c methods to do a full deep copy of a NSDictionary or NSArray, that themselves have nested dictionaries or arrays within them?

That is I have read the problem may be when it hits a nested dictionary or array it only copies the pointer to the nested item, and not copy the item truely.

Background: So as an example for me I'm trying to load/save the following config with NSUserDefaults and when loading need to convert the immutable copies one gets from NSUserDefault to mutable prior to making changes.

• Items (NSDictionary)
  • Item (NSDictionary)
    • aString: NSString
    • aString2: NSString
    • aDate: NSDate
    • aDate2: NSDate
    • aBool: BOOL
    • aTI1: NSTimeInterval
    • aTI2: NSTimeInterval
    • Keywords (NSArray)
      • keyword: NSString
      • keyword: NSString

Answer 1

A couple of years ago, I wrote a few category methods for exactly the same reason, transforming a whole tree of user defaults to mutable. Here they are - use them at your own risk! :-)

//
//  SPDeepCopy.h
//
//  Created by Sherm Pendley on 3/15/09.
//

#import <Cocoa/Cocoa.h>

// Deep -copy and -mutableCopy methods for NSArray and NSDictionary

@interface NSArray (SPDeepCopy)

- (NSArray*) deepCopy;
- (NSMutableArray*) mutableDeepCopy;

@end

@interface NSDictionary (SPDeepCopy)

- (NSDictionary*) deepCopy;
- (NSMutableDictionary*) mutableDeepCopy;

@end

---

//
//  SPDeepCopy.m
//
//  Created by Sherm Pendley on 3/15/09.
//

#import "SPDeepCopy.h"


@implementation NSArray (SPDeepCopy)

- (NSArray*) deepCopy {
    unsigned int count = [self count];
    id cArray[count];

    for (unsigned int i = 0; i < count; ++i) {
        id obj = [self objectAtIndex:i];
        if ([obj respondsToSelector:@selector(deepCopy)])
            cArray[i] = [obj deepCopy];
        else
            cArray[i] = [obj copy];
    }

    NSArray *ret = [[NSArray arrayWithObjects:cArray count:count] retain];

    // The newly-created array retained these, so now we need to balance the above copies
    for (unsigned int i = 0; i < count; ++i)
        [cArray[i] release];

    return ret;
}
- (NSMutableArray*) mutableDeepCopy {
    unsigned int count = [self count];
    id cArray[count];

    for (unsigned int i = 0; i < count; ++i) {
        id obj = [self objectAtIndex:i];

        // Try to do a deep mutable copy, if this object supports it
        if ([obj respondsToSelector:@selector(mutableDeepCopy)])
            cArray[i] = [obj mutableDeepCopy];

        // Then try a shallow mutable copy, if the object supports that
        else if ([obj respondsToSelector:@selector(mutableCopyWithZone:)])
            cArray[i] = [obj mutableCopy];

        // Next try to do a deep copy
        else if ([obj respondsToSelector:@selector(deepCopy)])
            cArray[i] = [obj deepCopy];

        // If all else fails, fall back to an ordinary copy
        else
            cArray[i] = [obj copy];
    }

    NSMutableArray *ret = [[NSMutableArray arrayWithObjects:cArray count:count] retain];

    // The newly-created array retained these, so now we need to balance the above copies
    for (unsigned int i = 0; i < count; ++i)
        [cArray[i] release];

    return ret;
}

@end

@implementation NSDictionary (SPDeepCopy)

- (NSDictionary*) deepCopy {
    unsigned int count = [self count];
    id cObjects[count];
    id cKeys[count];

    NSEnumerator *e = [self keyEnumerator];
    unsigned int i = 0;
    id thisKey;
    while ((thisKey = [e nextObject]) != nil) {
        id obj = [self objectForKey:thisKey];

        if ([obj respondsToSelector:@selector(deepCopy)])
            cObjects[i] = [obj deepCopy];
        else
            cObjects[i] = [obj copy];

        if ([thisKey respondsToSelector:@selector(deepCopy)])
            cKeys[i] = [thisKey deepCopy];
        else
            cKeys[i] = [thisKey copy];

        ++i;
    }

    NSDictionary *ret = [[NSDictionary dictionaryWithObjects:cObjects forKeys:cKeys count:count] retain];

    // The newly-created dictionary retained these, so now we need to balance the above copies
    for (unsigned int i = 0; i < count; ++i) {
        [cObjects[i] release];
        [cKeys[i] release];
    }

    return ret;
}
- (NSMutableDictionary*) mutableDeepCopy {
    unsigned int count = [self count];
    id cObjects[count];
    id cKeys[count];

    NSEnumerator *e = [self keyEnumerator];
    unsigned int i = 0;
    id thisKey;
    while ((thisKey = [e nextObject]) != nil) {
        id obj = [self objectForKey:thisKey];

        // Try to do a deep mutable copy, if this object supports it
        if ([obj respondsToSelector:@selector(mutableDeepCopy)])
            cObjects[i] = [obj mutableDeepCopy];

        // Then try a shallow mutable copy, if the object supports that
        else if ([obj respondsToSelector:@selector(mutableCopyWithZone:)])
            cObjects[i] = [obj mutableCopy];

        // Next try to do a deep copy
        else if ([obj respondsToSelector:@selector(deepCopy)])
            cObjects[i] = [obj deepCopy];

        // If all else fails, fall back to an ordinary copy
        else
            cObjects[i] = [obj copy];

        // I don't think mutable keys make much sense, so just do an ordinary copy
        if ([thisKey respondsToSelector:@selector(deepCopy)])
            cKeys[i] = [thisKey deepCopy];
        else
            cKeys[i] = [thisKey copy];

        ++i;
    }

    NSMutableDictionary *ret = [[NSMutableDictionary dictionaryWithObjects:cObjects forKeys:cKeys count:count] retain];

    // The newly-created dictionary retained these, so now we need to balance the above copies
    for (unsigned int i = 0; i < count; ++i) {
        [cObjects[i] release];
        [cKeys[i] release];
    }

    return ret;
}

@end