init返回nil会导致内存泄漏吗？

Question

Will returning nil under ARC in init lead to memory leak, when [super init] was already called, but then returned nil? Is this a legit usage?

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

       ...
       return nil;
       ...

   }
   return self;
}

Answer 1

First of all: For Qs referring to ARC never read Apple's documentation, but clang's.

Apple's documentation simply conceals (concealed?) the execution of -init and have wrong examples like that:

id ref = [[Class alloc] init]

You will find there a statement like " +alloc transfers ownership". This is at least misleading, because the return value auf -init and not the return value of +alloc is stored.

clang's documentation is better and more precise by far. Basically they say, that +alloc is an ownership transfer and that -init is ownership consuming and ownership transferring.

http://clang.llvm.org/docs/AutomaticReferenceCounting.html#semantics-of-init

Methods in the init family implicitly consume their self parameter and return a retained object.

So let's have a look to the usual code:

id ref = [Class alloc]; 
// ref is strong: RC is +1;
id ref = [ref init];    
  // Three things happen her:
  // 1. -init is executed
  // 2. The new value is stored
  // 3. The old value of ref is lost

  // 1. -init is executed
  // self is (silently) passed to the receiver
  - (id)init           
  {
    // Since self is strong (+1), RC is +2 now
    // Since -init is ownership consuming (-1), RC is +1 now
    …
    return self;
    // Since -init is ownership transferring (+1), RC is +2 now
  }

  // 2. The new value is stored
  // The value comes from an ownership transfer method, so the assignment to strong ref is neutral (0), RC is still +2

  // 3. The old value is lost (-1), RC is +1

The result is that the object has an RC of +1 and you have one strong reference to it in that code area. Everything is fine. (Of course there is a high potential for optimization, because in most cases neither self nor ref is changed, but let's keep on the regular track.)

Let's change self inside -init :

id ref = [Class alloc]; // Ownership transfer. RC is +1;
id ref = [ref init];    
  // Three things happen her:
  // 1. -init is executed
  // 2. The new value is stored
  // 3. The old value of ref is lost

  // 1. -init is executed
  //     self is (silently) passed to the receiver
  - (id)init           
  {
    // Since self is strong (+1), RC is +2 now
    // Since -init is ownership consuming (-1), RC is +1 now

    // Let's return nil as in your example
    return nil;
    // Because nil is returned, virtually the RC of nil is increased. self's RC == +1 is unchanged.
  }

  // 2. The new value is stored
  // The new value is nil.
  // However the value comes from an ownership transfer method, so the assignment to strong ref is neutral (0), RC is still +1

  // 3. The old value is lost (your old ref and the self while executing -init) (-1), RC is 0
  // The old object is dealloced, if you do not have another ref to it. Nothing leaks.

Answer 2

我只是在乐器中检查过-没有内存泄漏