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Class methods which create new instances

How would you declare a constructor in objective-c which would allow you to skip the [[class alloc] init] step during a declaration; Instead of saying for example Fraction* somefrac=[[Fraction alloc] init];, just say Fraction* somefrac and the constructor would do the rest.

Community
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Iowa15
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2 Answers2

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This would instantiate the object and return it. Following naming conventions you would need to make it an autorelease'd object that gets returned. 

+ (id)fraction
{
    return [[[self alloc] init] autorelease];
}

To use it

Fraction *fraction = [Fraction fraction];

this follows the same pattern as the apple provided classes e.g.

NSArray *myArray = [NSArray array];
Paul.s
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+(Fraction *) fraction
{
    return [[[Fraction alloc] init] autorelease];
}

and then you can use

Fraction *frac = [Fraction fraction];

This style is used a lot in objective c

grahamparks
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Daniel
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  • What happens if you want to subclass `Fraction`? – Paul.s Nov 20 '11 at 18:51
  • @Paul.s: then you have to provide such a class method yourself. its not logical to call `[Derived base]` but rather `[Derived derived]` – Daniel Nov 20 '11 at 18:53
  • That doesn't sounds very DRY to me. As you would then be writing the same method again? – Paul.s Nov 20 '11 at 18:57
  • @Paul.s: I know its a bit tedious, but I rather get derived object through derived function rather than base function. if anything, it would help if objective c would allow some kind of `@synthesize` syntax for converting `init`s to this. – Daniel Nov 20 '11 at 19:00
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    Indeed it would. One suggestion to keep it a little DRY'er. Define it as I have in my answer then at least you could have `- (id)derived; { return [self fraction]; }` then you get all the normal benefits of changes bubbling up the inheritance tree – Paul.s Nov 20 '11 at 19:02