objective-c - 这段代码中的保留计数是多少？

Question

摘自《在 Mac 上学习 Objective-C（第二版）》一书。为什么retain count在以下代码块中是 2 而不是 1：-

- (void) setEngine: (Engine *) newEngine
{
   [engine release];
   engine = [newEngine retain];
   // More BAD CODE: do not steal. Fixed version below.
}  // setEngine

some_function 
{
   Engine *engine = [Engine new]; // count: 1

   Car *car1 = [Car new];
   Car *car2 = [Car new];

   [car1 setEngine: engine];   // count: 2 But why? 1-1=0 (due to [engine release]  
                              // in -(void) SetEngine.    
                              // and after engine=[newEngine retain] retain count is 1.

   [engine release]; // count 1 
   [car2 setEngine: [car1 engine]]; // oops!

   return 0;
}

Answer 1

首先，请考虑切换到ARC。在普通编程中确实没有理由需要使用 MRC。

其次，你把你的范围弄混了。请记住，这段代码中确实有五个Engine*指针，但只有一个Engine对象。

内some_function：

1. engine, 局部变量
2. car1.engine
3. car2.engine

在setEngine:方法内：

1. engine, 伊瓦尔
2. newEngine, 参数

这是代码中发生的事情：

1. engine = [Engine new];保留引擎，保留计数 = 1
2. [car1 setEngine:engine];
   • [engine release];释放汽车的引擎 ivar，它可能设置为nil，并且什么都不做。保留计数 = 1
   • engine = [newEngine retain];保留引擎，保留计数 = 2
3. [engine release];释放引擎，保留计数 = 1
4. car2 setEngine:engine];
   • [engine release];释放汽车的引擎 ivar，它可能设置为nil，并且什么都不做。保留计数 = 1
   • engine = [newEngine retain];保留引擎，保留计数 = 2

现在，请帮自己一个忙，让编译器为您处理引用计数。

Answer 2

请检查我的评论

- (void) setEngine: (Engine *) newEngine
{
   // setEngine: is method defined inside Car.
   // so engine reference is retained by Car
   // and it should be released when Car is released 
   // ie. in car dealloc method
   [engine release];
   engine = [newEngine retain];
   // More BAD CODE: do not steal. Fixed version below.
}  // setEngine

- (void)dealloc{
  [engine release]; // Retaining count is reducing
  [super dealloc];
}    


some_function 
{
   Engine *engine = [Engine new]; // count: 1

   Car *car1 = [Car new]; // +1 for car1
   Car *car2 = [Car new]; // +1 for car2

   [car1 setEngine: engine];   // count: 2 But why? 1-1=0 (due to [engine release]  
                              // in -(void) SetEngine.    
                              // and after engine=[newEngine retain] retain count is 1.

   [engine release]; // count 1 
   [car2 setEngine: [car1 engine]]; // count 2 - Here Car2 retaining same 
                                    //engine retained by Car1. So retain 
                                    //count of engine increases.

   [car1 release];// count 1
   [car2 release];// count 0


   return 0;
}