Objective-C: why @synthesize foresees an assignment?

I noticed that the @synthesize directive admits an assignment to the property it receives, and I can't understand why.

I mean, I have in the application delegate's implementation file:

@synthesize window=_window;

W开发者_如何学编程hile if I have to synthesize a property into a view controller's .m file I can simply write:

@synthesize messageString;

without assigning it a _messageString.

I can't figure out why I should need the former syntax.

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As mentioned, this is setting the ivar to be separate from the accessor. So, in the window=_window code above, your accessors would look like this.

- (UIWindow *)window
{
     return _window;
}

- (void)setWindow:(UIWindow *)w
{
     [w retain];
     [_window release];
     _window = w;
}

The usual reason for separating the ivar and the property is to prevent collision in a subclass. This way, subclasses can create a variable named window without colliding with the superclasses "window", since it is actually _window.

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It just allows you to expose properties under a different name than the associated instance variables - which is useful because the conventions on property naming are quite well established, but the conventions on instance variables are much less well defined.

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This is not an assignment. It just names the ivar the property is based on.

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The former syntax (@synthesize window=_window) allows you to specify that the instance variable should have a separate name than the property. I believe that this doesn't make any real difference to the implementation, however it's a nice way for you to be able to ensure that your public accessors are separate from the private ivar.