objc_storeStrong

• 定义

  void objc_storeStrong(id *object, id value);
  

• 说明
  • Precondition: object is a valid pointer to a __strong object which is adequately aligned for a pointer. value is null or a pointer to a valid object.
  • Performs the complete sequence for assigning to a __strong object of non-block type [*]. Equivalent to the following code:

• 代码实现

  void objc_storeStrong(id *object, id value) {
    id oldValue = *object;
    value = [value retain];
    *object = value;
    [oldValue release];
  }
  

• This does not imply that a __strong object of block type is an invalid argument to this function. Rather it implies that an objc_retain and not an objc_retainBlock operation will be emitted if the argument is a block.

相关理解：

• 在Objective-C中，对象的引用关系由引用修饰符来决定，如__strong、__weak、__autorelease等等，编译器会根据不同的修饰符生成不同逻辑的代码来管理内存。
  • 在MRC时代Retain修饰符将会使被引用的对象引用计数+1
  • 在ARC中__strong修饰符作为其替代者

在正向开发写代码，在给__strong变量赋值时

obj = otherObj;

内部其实会调用对应的runtime的函数：

// 会变成如下函数调用
objc_storeStrong(&obj, otherObj);