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Re: verify input parameter of init method and return nil before invoke [super init]
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Re: verify input parameter of init method and return nil before invoke [super init]


  • Subject: Re: verify input parameter of init method and return nil before invoke [super init]
  • From: Quincey Morris <email@hidden>
  • Date: Tue, 29 Jan 2013 01:22:50 -0800

On Jan 29, 2013, at 00:51 , Bob Cromwell <email@hidden> wrote:

> if below code valid ?   All sample codes I could find invoke "self = [super init]" first and then check the input parameter inside "if (self)" block.
>
> @implementaion Bla
>
> - (id)initWithFoo:(NSString *)foo
> {
>    if (foo == nil) {
>        return nil;
>    }
>    self = [super init];
>    if (self) {
>        _foo = foo;
>        .....
>    }
>    return self;
> }
>
> I had thought there would be memory leak.   "Bla * b =  [Bla alloc ] initWithFoo:nil];" .  However under ARC using Instruments Leaks, there are no leak found.

No, it's not valid. You can see why if you rewrite this using manual retain/release conventions:

   if (foo == nil) {
       [self release];
       return nil;
   }
   self = [super init];

You must (and ARC does) release 'self' before returning nil, to avoid a memory leak. The problem is that releasing 'self' will invoke 'dealloc', and you have no idea whether it's safe to invoke 'dealloc' if you haven't invoked done '[super init]' yet.

Even if your class doesn't have a 'dealloc', you don't know whether a superclass or a subclass might have one, now or in the future.

In practice, you *may* get away with doing this, but it's a dangerous idiom to use. Much better to check the parameter a bit later:

   if (self) {
       if (foo == nil)
           return nil;
       _foo = foo;
       .....
   }

Note that this isn't guaranteed safe either. The class, or a subclass, may have a dealloc that isn't safe if the initialization wasn't completed**. However, most dealloc's are written to avoid such problems.


** Here's an example that will crash with an early return:

   if (self) {
       if (foo == nil)
           return nil;
       _foo = foo;
       _bar = malloc (100);
       .....
   }
…..
- (void) dealloc {
    free (_bar);
}


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  • Follow-Ups:
    • Re: verify input parameter of init method and return nil before invoke [super init]
      • From: Ken Thomases <email@hidden>
    • Re: verify input parameter of init method and return nil before invoke [super init]
      • From: Jens Alfke <email@hidden>
References: 
 >verify input parameter of init method and return nil before invoke [super init] (From: Bob Cromwell <email@hidden>)

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