Question

在nil的ARC下返回init会导致内存泄漏，而[super init]已被调用，但之后返回nil？这是合法用法吗？

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

       ...
       return nil;
       ...

   }
   return self;
}

Answer 1

首先：对于提到ARC的Q，从不读Apple的文档，而是铿锵的。

Apple的文档只是隐藏（隐藏？）-init的执行，并且有错误的例子：

id ref = [[Class alloc] init]

您会在那里找到类似“+alloc转让所有权”的声明。这至少具有误导性，因为存储了返回值auf -init和不 +alloc的返回值。

到目前为止，clang的文档更好，更精确。基本上他们说，+alloc是所有权转移和，-init是所有权消费和所有权转移。

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

init系列中的方法隐式使用其self参数并返回保留的对象。

让我们来看看通常的代码：

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强引用。一切都好。（当然，优化的可能性很大，因为在大多数情况下，self和ref都没有改变，但让我们保持常规轨道。）

让我们更改self内的-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

我刚在仪器中检查过 - 没有内存泄漏

在init中返回nil会导致内存泄漏吗？

2 个答案: