init中ARC下返回nil会导致内存泄漏，当[super init]已经被调用，然后返回nil？这是合法的用法吗？
- (id)init {
self = [super init];
if (self) {
...
return nil;
...
}
return self;
}
首先:对于引用 ARC 的 Q 从未阅读过 Apple 的文档，而是 clang 的。
Apple 的文档只是隐藏(隐藏？)-init 的执行，并且有这样的错误示例:
id ref = [[Class alloc] init]
你会发现像“+alloc 转移所有权”这样的语句。这至少具有误导性，因为 -init 的返回值而不是 +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 是所有权转移和 -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。一切都很好。 (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，但让我们保持正常的轨道。)
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.
关于ios - 在 init 中返回 nil 会导致内存泄漏吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29603467/
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