Question

Here is 2 code snapshot-

@interface A:NSObject
{
     NSMutableArray *a;
}

@property (retain)   NSMutableArray *a;


@implementation

@synthesize a;

-(id)init
{
      if(self=[super init])
     {
            a=[[NSMutableArray alloc] init];
     }
}

@end


@interface A:NSObject
{
     NSMutableArray *_a;

}

@property (retain)   NSMutableArray *a;



@implementation

@synthesize  a=_a;

-(id)init
{
      if(self=[super init])
     {
            _a=[[NSMutableArray alloc] init];
     }
}

@end

Now what i need to know, is in both code instance variable assigned value directly rather than using accessor and retain count is 1? Or there is difference between them. Thanks.

And one more things, apple recommended not to use accessor in init/dealloc, but at the same time ask not to directly set iVar. So what is the best way to assign value of ivar in init()??

Answer 1

ARC vs non-ARC

First, you should decide whether you want to use automatic reference counting (ARC) or manual reference counting. It looks like you’ve chosen the latter – which is fine because you can always transition to ARC later.

Property attributes/setter semantics

You have marked the setter semantics for the property a as retain. This will work; but since the property is exposed in the class interface, class users may use it to set the _a ivar. Since class users may not expect your class A to directly modify the the NSMutableArray instance they passed, better to use copy semantics for mutable object properties.

Property access in init and dealloc

Use of declared properties in init and dealloc is often discouraged because doing so may have side effects due to KVO. These side effects may not work well with a partially constructed or partially deconstructed object.

Your questions

Here’s what I would do:

// assumes Xcode 4.4+ for autosynthesize
@interface A:NSObject
@property (nonatomic,copy) NSMutableArray *a;
@end

@implementation A

- (id)init {
    self = [super init];
    if( !self ) return nil;

    _a = [[NSMutableArray alloc] init];

    return self;
}
@end

EDIT:

Or, if auto synthesize is not available:

@interface A:NSObject
@property (nonatomic,copy) NSMutableArray *a;
@end

@implementation A

@synthesize a = _a;

- (id)init {
    self = [super init];
    if( !self ) return nil;

    _a = [[NSMutableArray alloc] init];

    return self;
}
@end