How to copy custom objects in Objective-C?

Question

I have two arrays. First contains custom objects. Now I want to copy all object of first array in another array. For that I am using below code.

Arays.

arr_post=[[NSMutableArray alloc]init];
copy_arr_user_post=[[NSMutableArray alloc]init];

am adding the objects into them like this.

for(i=0;i<[arr_main count];i++)
{
    Post *obj=[[Post alloc]init];
    obj.name=@"abc";
    obj.category=@"social";
   [arr_post addObject:obj];


}

Now I am copying to another array like this

 [arr_post addObject:user_post];
 Post *objectCopy = [user_post copy]; //create a copy of our object
 [copy_arr_user_post addObject: objectCopy]; //insert copy into other array

In Post.h

@interface Post : NSObject<NSCopying>

In Post.m

- (id)copyWithZone:(NSZone *)zone
{
    // Copying code here.
    Post *another =[[[self class] allocWithZone:zone] init];
    another.id=self.id;
    another.category=self.category;
    return another;
}

But it does not copy objects I get null value. Why?

Clarify your issue Where in the posted code are you getting a nil value? Which code is not working as expected? — rmaddy
– rmaddy, Commented Oct 23, 2015 at 5:22
I think this is a very useful link for studying about deep copying collections: developer.apple.com/library/mac/documentation/Cocoa/Conceptual/… — stosha
– stosha, Commented Oct 23, 2015 at 5:46
i am getting nil value when i get the objects from second array — TechChain
– TechChain, Commented Oct 23, 2015 at 6:47
You can't store nil objects in an array so the array itself must be nil. — rmaddy
– rmaddy, Commented Oct 23, 2015 at 13:59

matti157 · Accepted Answer · 2015-10-23 06:59:06Z

One method that I find faster than NSCopyng

-Create an NSObject category with this two method

#import <objc/runtime.h>
-(id)deepCopy
{
NSArray *tmpArray = @[self];
NSData *buffer = [NSKeyedArchiver archivedDataWithRootObject:tmpArray];
return [NSKeyedUnarchiver unarchiveObjectWithData:buffer][0];
}

- (NSMutableArray *)allProperties
{
    NSMutableArray *props = [NSMutableArray array];
    unsigned int outCount, i;
    objc_property_t *properties = class_copyPropertyList([self class], &outCount);
    for (i = 0; i < outCount; i++) {
        objc_property_t property = properties[i];

        //Excluding all readOnly properties
        unsigned int numOfAttributes;
        objc_property_attribute_t *propertyAttributes = property_copyAttributeList(property, &numOfAttributes);

        BOOL foundReadonly = NO;

        for ( unsigned int ai = 0; ai < numOfAttributes; ai++ )
        {
            switch (propertyAttributes[ai].name[0]) {
                case 'T': // type
                    break;
                case 'R': // readonly
                    foundReadonly = YES;
                    break;
                case 'C': // copy
                    break;
                case '&': // retain
                    break;
                case 'N': // nonatomic
                    break;
                case 'G': // custom getter
                    break;
                case 'S': // custom setter
                    break;
                case 'D': // dynamic
                    break;
                default:
                    break;
            }
        }
        free(propertyAttributes);

        if (!foundReadonly)
        {
            NSString *propertyName = [[NSString alloc] initWithCString:property_getName(property) encoding:NSASCIIStringEncoding];
            [props addObject:propertyName];
        }
    }
    free(properties);
    return props;
}

-Make your object conforms to NSCoding

#pragma mark - NSCoding
- (instancetype)initWithCoder:(NSCoder *)decoder {
    self = [super init];
    if (self)
    {
        NSArray *keys = [self allProperties];

        for (NSString *key in keys)
        {
            [self setValue:[decoder decodeObjectForKey:key] forKey:key] ;
        }

    }
    return self;
}
- (void)encodeWithCoder:(NSCoder *)aCoder
{
    NSArray *keys = [self allProperties];

    for (NSString *key in keys)
    {
        [aCoder encodeObject:[self valueForKey:key] forKey:key];
    }
}

-Import the category

Now you are able to copy any kinds of object

MYObject *copy = [originalObject deepCopy];
NSArray *arrayWithCopiedObjects = [originalArray deepCopy];

etc....

"Now you are able to copy any kind of object" - No. Now you can copy any object and its properties. This only works on simple objects that only need properties to be copied. I have many classes that this wouldn't work on.

Leo · Accepted Answer · 2015-10-23 05:26:29Z

Try

NSMutableArray *  arr_post=[[NSMutableArray alloc]init];
NSMutableArray * copy_arr_user_post=[[NSMutableArray alloc]init];
for(int i=0;i<3;i++)
{
    Post *obj=[[Post alloc]init];
    obj.name=@"abc";
    obj.category=@"social";
    [arr_post addObject:obj];
}

[arr_post enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
    [copy_arr_user_post addObject:[obj copy]];
}];
Post * temp = [arr_post objectAtIndex:0];
temp.name = @"123";
NSLog(@"%@",arr_post);
NSLog(@"%@",copy_arr_user_post);

And log

2015-10-23 13:25:31.994 OCTest[1784:130931] (
"123 social",
"abc social",
"abc social"
)
2015-10-23 13:25:31.995 OCTest[1784:130931] (
"abc social",
"abc social",
"abc social"
)

I add description for debugging

-(NSString *)description{
    return [NSString stringWithFormat:@"%@ %@",self.name,self.category];
}

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