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CoreFoundation.framework) 是一组C语言接口，它们为iOS应用程序提供基本数据管理和服务功能。下面列举该框架支持进行管理的数据以及可提供的服务：群体数据类型 (数组、集合等)
程序包
字符串管理
日期和时间管理
原始数据块管理
偏好管理
URL及数据流操作
线程和RunLoop
端口和soket通讯
Core Foundation框架和Foundation框架紧密相关，它们为相同功能提供接口，但Foundation框架提供Objective-C接口。如果您将Foundation对象和Core Foundation类型掺杂使用，则可利用两个框架之间的 “toll-free bridging”。所谓的Toll-free bridging是说您可以在某个框架的方法或函数同时使用Core Foundatio和Foundation 框架中的某些类型。很多数据类型支持这一特性，其中包括群体和字符串数据类型。每个框架的类和类型描述都会对某个对象是否为 toll-free bridged，应和什么对象桥接进行说明。
如需进一步信息，请阅读Core Foundation 框架参考。
自 Xcode4.2 开始导入ARC机制后，为了支持对象间的转型，Apple又增加了许多转型用的关键字。这一讲我们就来了解其用法，以及产生的理由。
我们先来看一下ARC无效的时候，我们写id类型转void*类型的写法：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[[</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSObject</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> alloc</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> init</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
反过来，当把void*对象变回id类型时，只是简单地如下来写，
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> p</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">obj release</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li></ol>
但是上面的代码在ARC有效时，就有了下面的错误：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> error</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">:</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">implicit</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> conversion of an </span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">Objective</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">-</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">C pointer</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> to </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">’</span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">is</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> disallowed </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">with</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> ARC</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">^</span></li><li class="L4" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L5" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> error</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">:</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">implicit</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> conversion of a non</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">-</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">Objective</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">-</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">C pointer</span></li><li class="L6" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> type </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">’</span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> to </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">is</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> disallowed </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">with</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> ARC</span></li><li class="L7" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> id o </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> p</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L8" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">^</span></li></ol>
为了解决这一问题，我们使用 __bridge 关键字来实现id类型与void*类型的相互转换。看下面的例子。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[[</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSObject</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> alloc</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> init</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L4" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id o </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
将Objective-C的对象类型用 __bridge 转换为 void* 类型和使用 __unsafe_unretained 关键字修饰的变量是一样的。被代入对象的所有者需要明确对象生命周期的管理，不要出现异常访问的问题。
除过 __bridge 以外，还有两个 __bridge 相关的类型转换关键字：
接下来，我们将看看这两个关键字的区别。
先来看使用 __bridge_retained 关键字的例子程序：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[[</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSObject</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> alloc</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> init</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_retained </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
从名字上我们应该能理解其意义：类型被转换时，其对象的所有权也将被变换后变量所持有。如果不是ARC代码，类似下面的实现：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[[</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSObject</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> alloc</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> init</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p retain</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li></ol>
可以用一个实际的例子验证，对象所有权是否被持有。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="lit" style="color:#06666;border: 0px; margin: 0px; padding: 0px;">0</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">{</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[[</span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSObject</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> alloc</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> init</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L4" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> p </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_retained </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">void</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">obj</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L5" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">}</span></li><li class="L6" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L7" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSLog</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(@</span><span class="str" style="color:#0880;border: 0px; margin: 0px; padding: 0px;">"class=%@"</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">,</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p </span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">class</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">]);</span></li></ol>
出了大括号的范围后，p 仍然指向一个有效的实体。说明他拥有该对象的所有权，该对象没有因为出其定义范围而被销毁。
相反，当想把本来拥有对象所有权的变量，在类型转换后，让其释放原先所有权的时候，需要使用 __bridge_transfer 关键字。文字有点绕口，我们还是来看一段代码吧。
如果ARC无效的时候，我们可能需要写下面的代码。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="com" style="color:#8800;border: 0px; margin: 0px; padding: 0px;">// p 变量原先持有对象的所有权</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">obj retain</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">[(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p release</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">];</span></li></ol>
那么ARC有效后，我们可以用下面的代码来替换：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="com" style="color:#8800;border: 0px; margin: 0px; padding: 0px;">// p 变量原先持有对象的所有权</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id obj </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_transfer id</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">p</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
可以看出来，__bridge_retained 是编译器替我们做了 retain 操作，而 __bridge_transfer 是替我们做了 release1。
在iOS世界，主要有两种对象：Objective-C 对象和 Core Foundation 对象0。Core Foundation 对象主要是有C语言实现的 Core Foundation Framework 的对象，其中也有对象引用计数的概念，只是不是 Cocoa Framework::Foundation Framework 的 retain/release，而是自身的 CFRetain/CFRelease 接口。
这两种对象间可以互相转换和操作，不使用ARC的时候，单纯的用C原因的类型转换，不需要消耗CPU的资源，所以叫做 Toll-Free bridged。比如 NSArray和CFArrayRef, NSString和CFStringRef，他们虽然属于不同的 Framework，但是具有相同的对象结构，所以可以用标准C的类型转换。
比如不使用ARC时，我们用下面的代码：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">[</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> stringWithFormat</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">:...];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
同样，Core Foundation类型向Objective-C类型转换时，也是简单地用标准C的类型转换即可。
但是在ARC有效的情况下，将出现类似下面的编译错误：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">Cast</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> of </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">Objective</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">-</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">C pointer type </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">‘</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> to C pointer type </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">‘</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">aka </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">‘</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">const</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">struct</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> __CFString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*’)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> requires a bridged cast</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">Use</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> __bridge to convert directly </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">no</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> change </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">in</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> ownership</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">Use</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> __bridge_retained to make an ARC </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">object</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> available </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">as</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> a </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">+</span><span class="lit" style="border: 0px; margin: 0px; padding: 0px;">1</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">‘</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">’</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">aka </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">‘</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">const</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">struct</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> __CFString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*’)</span></li></ol>
错误中已经提示了我们需要怎样做：用 __bridge 或者 __bridge_retained 来转型，其差别就是变更对象的所有权。
正因为Objective-C是ARC管理的对象，而Core Foundation不是ARC管理的对象，所以才要特意这样转换，这与id类型向void*转换是一个概念。也就是说，当这两种类型（有ARC管理，没有ARC管理）在转换时，需要告诉编译器怎样处理对象的所有权。
上面的例子，使用 __bridge/__bridge_retained 后的代码如下：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">[</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> stringWithFormat</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">:...];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">;</span></li></ol>
只是单纯地执行了类型转换，没有进行所有权的转移，也就是说，当string对象被释放的时候，cfString也不能被使用了。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">[</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> stringWithFormat</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">:...];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_retained </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">...</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFRelease</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">cfString</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">);</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="com" style="border: 0px; margin: 0px; padding: 0px;">// 由于Core Foundation的对象不属于ARC的管理范畴，所以需要自己release</span></li></ol>
使用 __bridge_retained 可以通过转换目标处（cfString）的 retain 处理，来使所有权转移。即使 string 变量被释放，cfString 还是可以使用具体的对象。只是有一点，由于Core Foundation的对象不属于ARC的管理范畴，所以需要自己release。
实际上，Core Foundation 内部，为了实现Core Foundation对象类型与Objective-C对象类型的相互转换，提供了下面的函数。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFTypeRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFBridgingRetain</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id X</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">{</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">return</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_retained </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFTypeRef</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">X</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">}</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L4" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">id </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFBridgingRelease</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFTypeRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> X</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">{</span></li><li class="L5" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">return</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_transfer id</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">X</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L6" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">}</span></li></ol>
所以，可以用 CFBridgingRetain 替代 __bridge_retained 关键字：
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">[</span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> stringWithFormat</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">:...];</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFBridgingRetain</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="kwd" style="border: 0px; margin: 0px; padding: 0px;">string</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">);</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">...</span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="border: 0px; margin: 0px; padding: 0px;">CFRelease</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">cfString</span><span class="pun" style="border: 0px; margin: 0px; padding: 0px;">);</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="com" style="border: 0px; margin: 0px; padding: 0px;">// 由于Core Foundation不在ARC管理范围内，所以需要主动release。</span></li></ol>
所有权被转移的同时，被转换变量将失去对象的所有权。当Core Foundation对象类型向Objective-C对象类型转换的时候，会经常用到 __bridge_transfer 关键字。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">CFStringCreate</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">...();</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">__bridge_transfer </span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*)</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">cfString</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">;</span></li><li class="L2" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span></li><li class="L3" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="com" style="color:#8800;border: 0px; margin: 0px; padding: 0px;">// CFRelease(cfString); 因为已经用 __bridge_transfer 转移了对象的所有权，所以不需要调用 release</span></li></ol>
同样，我们可以使用 CFBridgingRelease() 来代替 __bridge_transfer 关键字。
<ol class="linenums" style="border: 0px; margin: 0px; padding: 0px 0px 0px 26px;"><li value="1" class="L0" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal;"><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">CFStringRef</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> cfString </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">CFStringCreate</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">...();</span></li><li class="L1" style="border: 0px; margin: 0px 0px 0px 20px; padding: 0px; list-style-type: decimal; background-color: rgb(238, 238, 238);"><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">NSString</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">*</span><span class="kwd" style="color:#0088;border: 0px; margin: 0px; padding: 0px;">string</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">=</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;"> </span><span class="typ" style="color:#66066;border: 0px; margin: 0px; padding: 0px;">CFBridgingRelease</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">(</span><span class="pln" style="border: 0px; margin: 0px; padding: 0px;">cfString</span><span class="pun" style="color:#66660;border: 0px; margin: 0px; padding: 0px;">);</span></li></ol>
由上面的学习我们了解到 ARC 中类型转换的用法，那么我们实际使用中按照怎样的原则或者方法来区分使用呢，下面我总结了几点关键要素。
1. 声明 id obj 的时候，其实是缺省的申明了一个 __strong 修饰的变量，所以编译器自动地加入了 retain 的处理，所以说 __bridge_transfer 关键字只为我们做了 release 处理。
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原文地址：http://blog.csdn.net/super_man_ww/article/details/51217412
