Do I need to override the getter for a property to return an immutable copy?

Suppose that I have a class that holds a mutable array. I want to make sure that if other classes ask for the array they will get a non-mutable type, but in the owning class, it is actually an instance of NSMutableArray, so that I can add and remove items.

#import "Person.h"

@class Asset;

@interface Employee : Person

{
    NSMutableArray *_assets;
}

@property (nonatomic,copy) NSArray *assets;

-(void)addAssets:(Asset *)a;

The question is, do I have to modify the accessor methods into something like this, or will it automatically behave like I want?

#import "Employee.h"
#import "Asset.h"

@implementation Employee

/* Accessors for assets properties
-(NSArray *)assets
{
    return [_assets copy];
}

-(void)setAssets:(NSArray *)assets
{
    _assets = [assets mutableCopy ];

}
*/

-(void)addAssets:(Asset *)a
{
    //is assets nil?

    if (!_assets) {
        //Create Array
        _assets = [[NSMutableArray alloc]init];
    }
    [_assets addObject:a];

}

Solution

ppalancica's answer is incorrect. The copy attribute means only that the setter will take a copy when the property is set. The synthesized getter will not return a copy. You must implement that behavior yourself:

- (NSArray *)assets
{
    return [_assets copy];
}

You might want to make an internal-only accessor that doesn't make a copy. You could also redeclare the property privately; client code will then be contracted to treat the array it requests as immutable.

This code demonstrates that the synthesized getter returns the uncopied object:

#import <Foundation/Foundation.h>

@interface ArrayReturner : NSObject

@property (copy, nonatomic) NSArray * array;

@end

@implementation ArrayReturner
{
    NSMutableArray * _array;
}

- (BOOL)myArrayIsIdenticalTo:(NSArray *)otherArray
{
    return _array == otherArray;
}

@end

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

        ArrayReturner * a = [ArrayReturner new];
        [a setArray:@[@1, @2]];
        NSArray * returnedArray = [a array];
        // Does not throw
        NSCAssert([a myArrayIsIdenticalTo:returnedArray],
                  @"Returned array is a separate instance.");
    }

    return 0;
}