可可中的内存管理

Question

I am trying to understand the memory management concepts in Cocoa. I am stuck at the following portion:::

//This is wrong code. But i want to understand why it is wrong!  
- (void) setEngine: (Engine *) newEngine  
{  
      [engine release];  
      engine = [newEngine retain];  
}  

When newEngine and engine are same, then how does cocoa decides which one to be treated as an instance variable and which one as a parameter?

For example:

Engine *engine = [Engine new]; // count: 1  
Car *car1 = [Car new];  
Car *car2 = [Car new]; 

[car1 setEngine: engine]; // count: 2 -----How is the retain count 2 here? 
[engine release]; // count 1 

[car2 setEngine: [car1 engine]]; 

Please help as i want to learn this concept thoroughly so as to maintain efficiency in my programs. I am sure a lot many are stuck at this as me and my friends are.. Thanks in advance... :)

Answer 1

Firstly, you want to make sure your setter is checking to see if the objects are different:

- (void)setEngine:(Engine *)newEngine {
    if ( engine != newEngine ) {
        [engine release];
        [newEngine retain];
        engine = newEngine;
    }
}

Here's what happens inside this method:

1. Check to see that the engine that has been passed in is different to the one we already have. If it is the same, nothing needs to be done.
2. Release the old engine. If we were the last object to own it, its retain count will be zero and it will be deallocated. If the old engine was nil , then the release message will simply be ignored.
3. Retain the new engine. If it is released somewhere else, it will not be deallocated because we have taken ownership of it. If the new engine is nil , then the retain message will simply be ignored.
4. Set the engine instance variable to newEngine .

Secondly, the reason that the retain count is two after you call setEngine: is because it started as 1 when it was created. Then you assigned it to car1, which retained it, bringing the retain count to 2. The retain count is two because your outer code owns it and car1 owns it. You should pass it to car1 and car2 in the same way ( [car2 setEngine:car] ) which would bring the retain count to 3. Once you have passed it along to car1 and car2, you can release it it your outer code - this tells Obcjective-C that your outer code no longer owns it, the ownership lies only with car1 and car2.

Here's a full example:

// create an engine, we will own it for now
Engine *engine = [[Engine alloc] init];

/* engine retain count: 1, owned by this context */

Car *car1 = [[Car alloc] init];
Car *car2 = [[Car alloc] init];

// pass engine along to cars
[car1 setEngine: engine];
[car2 setEngine: engine];

/* engine retain count: 3, owned by car1, car2 and this context */

// release engine, we no longer need it
[engine release];

/* engine retain count: 2, owned by car1 and car2 */