Objective-C return instances of different classes at init

Question

Is it possible at Objective C at init method to return an instance of different classes? I'm having a Class called: MyCustomClass. I also have two other different classes called Class 1 and Class2. What I'm trying to implement is: When I call [[MyCustomClass alloc] initWithSomeParameters to create instance of Class1 or Class2 depending on some condition.

MyCustomClass.m:

#import "MyCustomClass.h"
#import "Class1.h"
#import "Class2.h"

-(id) initWithSomeParameters: (id) params{
  id myClass;
  if (someCondition){
    myClass = [[Class1 alloc] initWithSomeParameters:(id) params];
    [myClass setSomething:something];
  }else{
    myClass = [[Class2 alloc] initWithSomeParameters:(id) params];
    [myClass setSomething:something];
  }
  return myClass;
}

...and later I call

id myCustomClass = [[MyCustomClass alloc] initWithSomeParameters:(id) params];

Is this a wrong approach? If so, what would be the correct one?

Answer 1

Several others have mentioned this, but the result of calling [[MyClass alloc] init] must always be nil or a kind of MyClass. It doesn't have to specifically be an instance of MyClass; one of its descendants is possible, as with NSArray or NSString. In code, this requirement would look like:

MyClass *a = [[MyClass alloc] init];
NSAssert((a==nil) || [a isKindOfClass:[MyClass class]], @"This must always hold true.");

I've never attempted to implement this, but it would probably have to look something like this:

- (id)initAsSubclass:(NSString *)sublcassName
{
    Class c = NSClassFromString(subclassName);
    self = [[c alloc] init];
    if (self) {
        // Do Custom Init Here
    }
    return self;
}

The keys would be:

• DO NOT perform [super init].
• Create a completely new object with +alloc.
• Assign the newly created object to self.
• If not using ARC, perform [self autorelease], before replacing the value. (If the object that is currently executing code becomes deallocated, it can cause issues. -autorelease will defer that until this section is complete.)

Answer 2

Its not a good approach. Its better use some helper class or us factory pattern and provide parameters to method. Then depending on parameters create an object of class and return.

Its not good approach to create object of different class in init method of different class.

Edit: if You want to show UIView or UIAlertView depending on iOS version do like this.

@interface AlertHelper : NSObject
+ (id)getAlert;
@end

///
@implementation AlertHelper
+(id)getAlert{
NSString *version = [[UIDevice currentDevice] systemVersion];
int ver = [version intValue];
if (ver < 7){
//For iOS 6
return something;
}
else{
//for ios 7
return something
}
}
@end

Answer 3

You should make some kind of controller, which initializes correct classes. You can also achieve same that using class methods.

ANd in genreal this given implementation is bad, because you alloc memory once [MyCustomClass alloc] and then in -(id)initWithSomeParameters:(id)params you are allocating memory again. So, even different address will be retruned, that isn't agains apple guidelines, some apple classes also have such behavior, but they do it because of optimizations. But here it is wrong.

Answer 4

The way to do it is like this:

Create Base class like:

#import "Base.h"
#import "Class1.h"
#import "Class2.h"

@implementation Base

+ (id)classWithParams:(id)params
{
     id retVal = nil;    

     if (condition_based_on_params_means_creating_class1)
     {
        retVal = [[Class1 alloc] initWithSomeParameters:params];
     }
     else
     {
         retVal = [[Class2 alloc] initWithSomeParameters:params]
     }

     return retVal;
}

@end 

Class1 inherits from Base:

@interface Class1 : Base
{
}
- (id)initWithSomeParameters:(id)parameters;
@end

Class2 inherits from Base:

@interface Class2 : Base
{
}
- (id)initWithSomeParameters:(id)parameters;
@end

Ultimately you will have:

Base* a = [Base classWithParams:yourParams];