How to write an Objective-C convenience constructor

Question

I'm trying to add a convenience constructor to my custom object. Similar to [NSArray arrayWithArray:]

I know it involves a class method that returns an auto released object. I've been googling around but all I can seem to find is the definition of a convenience constructor but not how to write one.

Answer 1

Let's say you have the following:

@class PotatoPeeler : NSObject
- (instancetype)initWithWidget: (Widget *)w;
@end

Then to add a factory method, you'd change it to this:

@class PotatoPeeler : NSObject
+ (instancetype)potatoPeelerWithWidget: (Widget *)w;
- (instancetype)initWithWidget: (Widget *)w;
@end

And your implementation would simply be:

+ (instancetype)potatoPeelerWithWidget: (Widget *)w {
    return [[[self alloc] initWithWidget: w] autorelease];
}

Edit: replaced id with instancetype. They are functionally identical, but the latter provides better hints to the compiler about the method's return type.

Answer 2

Generally my approach is the following: first I create a normal initializer method (instance method), then I create a class method that calls the normal initializer. It seems to me Apple uses the same approach most of the time. An example:

@implementation SomeObject

@synthesize string = _string; // assuming there's an 'string' property in the header

- (id)initWithString:(NSString *)string 
{
   self = [super init];
   if (self)
   {
      self.string = string;
   }
   return self;
}

+ (SomeObject *)someObjectWithString:(NSString *)string
{
   return [[[SomeObject alloc] initWithString:string] autorelease];
}

- (void)dealloc
{
   self.string = nil;

   [super dealloc];
}

@end