为什么这会导致内存泄漏？

Question

I'm new to Objective-C, and I've read some memory management articles but am having trouble.

I hava a class something like this:

-(UIWebView*)getWebView{
    //UIWebView* webview = [UIWebView initWithFrame:self.frame];  edited,the original wrong line
    UIWebView* webview = [[UIWebView alloc] initWithFrame:self.frame]; 
    return webview; 
}
-(void)addToSelf{
    UIWebView* view = [self getWebView];
    [self addSubview:view];
    [view release];  //release here
}

In my thought, objc objects are all like C pointers (it is ?) And thinking like this:

UIWebView* webview = [[UIWebView alloc] initWithFrame:self.frame];  //retain +1
UIWebView* view = [self getWebView];  //just a pointer to the same object ?
[self addSubview:view]; //retain +1
[view release];   //retain -1

Now view's retainCount = 1 . Then this viewController will handle the webview's life cycle.

There must be something wrong with my thought (sure also the code), but I don't know why .

UIWebView* webview = [[[UIWebView alloc] initWithFrame:self.frame] autorelease];

When I remove the last release line, code works fine, why? What's the difference?

--------------edit line---------------

Few minutes ago , there's an answer about it but it's disappeared , now I rewrite my thought:

The answer says:

When returning object from method , i must use autorelease to tell compiler I have end with it, and then using addSubview , finish (no need releasing).

I know this is right , but why it's right ?

In most of codes:

-(void)someMethod{
    UIWebView* webview = [[UIWebView alloc] initWithFrame:self.frame]; 
    [self addSubview:webview];
    [webview release];
}

this works fine, but when separate the code to two methods like the top ,it's not.

Why i must use autorelease when returning object?

Answer 1

It looks as if you don't alloc the UIWebView:

UIWebView* webview = [[UIWebView alloc] initWithFrame:self.frame];

That means you probably don't have a valid UIWebView, and a retain count that is undefined, since the code for UIWebView is actually using some unknown piece of memory. If this works, you are lucky.

Answer 2

Your general idea of how retains and releases balance each other is correct. When you remove that last retain call, you are leaking. This is a defect that won't be noticeable unless you're observing memory use. (Leaks are important on any computer, but have a greater impact on handheld devices.)

In practice, if a method creates ( alloc ) and returns an object, autorelease it. The caller can then just use that object, and not worry about its lifetime.

Answer 3

*#1: Method that 'alloc's should have the responsibility to call the release-statements on the object.

#2: When you create a separate method to alloc an object for you, you cannot have 'release' statement, since that will release the object even before call site method uses it. Hence, instead of calling release we call autorelease and thus obeying the principal of #1, but still allowing call-site to use the object. As call-site didn't do any alloc by itself, it also does not have to worry about any releases. ***