ios伙伴分配之间有什么区别？

Question

method1:

NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:
                         [NSURLRequest requestWithURL:
                          [NSURL URLWithString:appRecord.imageURLString]] delegate:self];
self.imageConnection = conn;
[conn release];

method2:

self.imageConnection = [[NSURLConnection alloc] initWithRequest:
                        [NSURLRequest requestWithURL:
                         [NSURL URLWithString:appRecord.imageURLString]] delegate:self];

Answer 1

Answer: it depends. I'm assuming that your self.imageConnection property is set to retain.

If you have Automatic Reference Counting (ARC) enabled, there isn't really a difference (that you need to know about), because the compiler will take care of your memory management for you...of course, if you did have ARC enabled you could only use the second example you give, because you can't call release .

In a non-ARC environment there is quite a big difference: the first method you posted releases its memory correctly, and the second one leaks it.

To explain, if you declare a property as retain it will, as the name suggests, retain it. When you create your NSURLConnection you alloc/init it, which retains it once. You then assign it to self.imageConnection - doing so increments the retain count again.

In your first example you've used a temporary variable, which means once you've assigned the property you can call release to 'normalise' the retain count. But in your second example you have no temporary variable - you've lost the reference, and by assigning it to a property you've retained it twice.

On solution to this is to add an autorelease statement when you assign the property, like this:

self.imageConnection = [[NSURLConnection alloc] initWithRequest: [NSURLRequest 
                                                 requestWithURL: [NSURL URLWithString:appRecord.imageURLString]] delegate:self] 
                                                 autorelease];

... or you could just do what you show in your first example, and use a temporary variable.

This doesn't hold true for all properties - when you declare you property you also declare whether you'd like it to be retained ('retain'), just kept as a pointer ('assign'), or copied ('copy'). An assigned property wouldn't have this problem.