Implementing NSCopying

Question

To implement NSCopying, your object must respond to the -copyWithZone: selector. Here’s how you declare that you conform to it:

@interface MyObject : NSObject <NSCopying> {

Then, in your object’s implementation (your .m file):

- (id)copyWithZone:(NSZone *)zone
{
    // Copying code here.
}

What should your code do? First, create a new instance of the object—you can call [[[self class] alloc] init] to get an initialized obejct of the current class, which works well for subclassing. Then, for any instance variables that are a subclass of NSObject that supports copying, you can call [thatObject copyWithZone:zone] for the new object. For primitive types (int, char, BOOL and friends) just set the variables to be equal. So, for your obejct Vendor, it’d look like this:

- (id)copyWithZone:(NSZone *)zone
{
    id copy = [[[self class] alloc] init];

    if (copy) {
        // Copy NSObject subclasses
        [copy setVendorID:[[self.vendorID copyWithZone:zone] autorelease]];
        [copy setAvailableCars:[[self.availableCars copyWithZone:zone] autorelease]];

        // Set primitives
        [copy setAtAirport:self.atAirport];
    }

    return copy;
}

This answer is similar to the accepted, but uses allocWithZone: and is updated for ARC. NSZone is foundation class for allocating memory. While ignoring NSZone might work for most cases, it is still incorrect.

To correctly implement NSCopying you must implement a protocol method which allocates a new copy of the object, with properties that match the values of the original.

In the interface declaration in the header, specify that your class implements the NSCopying protocol:

@interface Car : NSObject<NSCopying>
{
 ...
}

In the .m implementation add a -(id)copyWithZone method which looks something like the following:

- (id)copyWithZone:(NSZone*)zone
{
    Car* carCopy = [[[self class] allocWithZone:zone] init];

    if (carCopy)
    {
        carCopy.isAvailable = _isAvailable;
        carCopy.transmissionType = _transmissionType;
        ... // assign all other properties.
    }

    return carCopy;
}

Swift Version

Just call object.copy() to create the copy.

I didn't use copy() for value types since those are copied "automatically." But I had to use copy() for class types.

I ignored the NSZone parameter because docs say it is deprecated:

This parameter is ignored. Memory zones are no longer used by Objective-C.

Also, please note that this is a simplified implementation. If you have subclasses it gets a bit tricker and you should use dynamic type: type(of: self).init(transmissionType: transmissionType).

class Vendor {
    let vendorId: String
    var availableCars: [Car] = []

    init(vendorId: String) {
        self.vendorId = vendorId
    }
}

extension Vendor: NSCopying {
    func copy(with zone: NSZone? = nil) -> Any {
        let copy = Vendor(vendorId: vendorId)
        if let availableCarsCopy = availableCars.map({$0.copy()}) as? [Car] {
            copy.availableCars = availableCarsCopy
        }
        return copy
    }
}

class Car {
    let transmissionType: String
    var isAvailable: Bool = false
    var fees: [Double] = []

    init(transmissionType: String) {
        self.transmissionType = transmissionType
    }
}

extension Car: NSCopying {
    func copy(with zone: NSZone? = nil) -> Any {
        let copy = Car(transmissionType: transmissionType)
        copy.isAvailable = isAvailable
        copy.fees = fees
        return copy
    }
}