__bridge方法參數不需要？

我們知道，與ARC我們需要一個__bridge一個ID轉換爲void *：__bridge方法參數不需要？

void *t = (void *)self;   // ERROR: Cast of ... requires a bridged cast 
void *t = (__bridge void *)self; // CORRECT

等都是C函數調用：

void f(void *t) { 
    .... 
} 

f((void *)self);   // ERROR 
f((__bridge void *)self); // CORRECT

我想這應該保持的方法爲好，而事實上，這Beginning ARC in iOS 5 Tutorial給出了下面的例子，並說，__bridge需要：

MyClass *myObject = [[MyClass alloc] init]; 
[UIView beginAnimations:nil context:(__bridge void *)myObject];

然而，今天我不小心在我的一個程序中調用了方法調用中的__bridge，並且編譯並運行代碼時沒有任何問題。上述示例中的__bridge似乎是不必要的：

[UIView beginAnimations:nil context:(void *)myObject]; // COMPILED OK

是這樣嗎？在這種情況下，__bridge真的不必要嗎？或者刪除它會改變代碼的含義？

來源

2014-01-26 Terry Chang

這是包括在ARC docs section 3.3.3（重點煤礦）：

3.3.3 Conversion from retainable object pointer type in certain contexts

[beginning Apple 4.0, LLVM 3.1]

If an expression of retainable object pointer type is explicitly cast to a C retainable pointer type, the program is ill-formed as discussed above unless the result is immediately used:

to initialize a parameter in an Objective-C message send where the parameter is not marked with the cf_consumed attribute, or to

initialize a parameter in a direct call to an audited function where the parameter is not marked with the cf_consumed attribute.

在你的代碼，myObject是一個「可保留的對象指針」。「C可保留指針類型」包括void*（這是一個稍微馬虎的定義，它們用作佔位符，因爲Core Foundation「對象」通常是void*）。

因此，如果用作方法參數，ObjC對象可以隱式轉換爲void*。在這種情況下，沒有額外的內存管理語義（即它等同於一個__bridge強制轉換）。 Section 7.8警告我們，void*未來可能不會這樣處理，但我不擔心這一點。如果發生這種情況，添加__bridge將是微不足道的。

有一點要記住的是myObject這裏沒有保護。在完成動畫之前，確保它以其他方式進行保留取決於您，否則您可能會崩潰。

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2014-01-26 17:21:16

非常有趣，很好的發現。 +1，因爲你給了我又一個憎恨ARC的理由;-) – 2014-01-26 20:48:50

__bridge用於傳遞變量/引用（如保留計數）的所有權，這是對API的Objective-C或Objective-C的C API。

轉到通過Clang's doc：

Bridged casts

A bridged cast is a C-style cast annotated with one of three keywords:
(__bridge T) op casts the operand to the destination type T. If T is a retainable object pointer type, then op must have a 
non-retainable pointer type. If T is a non-retainable pointer type, then op must have a retainable object pointer type. Otherwise the cast is ill-formed. There is no transfer of ownership, and ARC inserts no retain operations. (__bridge_retained T) op casts the operand, which must have retainable object pointer type, to the destination type, which must be a non-retainable pointer type. ARC retains the value, subject to the usual optimizations on local values, and the recipient is responsible for balancing that +1. (__bridge_transfer T) op casts the operand, which must have non-retainable pointer type, to the destination type, which must be a retainable object pointer type. ARC will release the value at the end of the enclosing full-expression, subject to the usual optimizations on local values.

These casts are required in order to transfer objects in and out of ARC control; see the rationale in the section on conversion of retainable object pointers.

Using a __bridge_retained or __bridge_transfer cast purely to convince ARC to emit an unbalanced retain or release, respectively, is poor form.

現在，

無效* T =（無效*）自我; //錯誤：Cast of ...需要橋接轉換爲什麼它是錯誤的，因爲您嘗試將參考從Objective-C轉換爲C.它未能通過引用的所有權。

void * t =（__bridge void *）self; //更正爲什麼它是正確的，因爲將您的客觀C參考轉移到C.根據LLVM的文檔。見上面給出的鑄造規則。

MyClass *myObject = [[MyClass alloc] init]; 
[UIView beginAnimations:nil context:(__bridge void *)myObject];

上面的紋路都是完全還行，因爲你通過C基準類型的上下文，而不是NULL

來源

2014-01-26 11:42:17 Tirth

__bridge方法參數不需要？

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