ios - init中返回nil会不会导致内存泄露？

Question

当[super init]已经被调用，但随后返回nil时，init中ARC下返回nil是否会导致内存泄漏？这是合法的用法吗？

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

       ...
       return nil;
       ...

   }
   return self;
}

Answer 1

首先:对于提到 ARC 的问题，从来没有阅读过 Apple 的文档，而是 clang 的。

Apple 的文档简单地隐藏(隐藏？)-init 的执行，并有这样的错误示例:

id ref = [[Class alloc] init]

你会发现那里有类似“+alloc transfers ownership”的语句。这至少是误导性的，因为 -init 的返回值和 not +alloc 的返回值被存储。

到目前为止，clang 的文档更好、更精确。基本上他们说，+alloc 是所有权转移并且 -init 是所有权消耗和所有权转移。

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.

那么让我们看一下通常的代码:

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

结果是该对象的 RC 为 +1，并且您在该代码区域中有一个对它的强引用。一切都好。 (当然有很大的优化潜力，因为在大多数情况下 self 和 ref 都没有改变，但让我们保持正常轨道。)

让我们在 -init 中更改 self:

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.