ViewController = _ViewController meaning [duplicate]

Question

Possible Duplicate:
Prefixing property names with an underscore in Objective C

I just started iphone App development and noticed that when you generate a new project, the following code can be seen in AppDelegate.m

@synthesize window = _window;
@synthesize viewController = _viewController;

AND in the AppDelegate.h file it says

@property (strong, nonatomic) UIWindow window;
@property (strong, nonatomic) ViewController controller;

I would just like to know what exactly this means, particularly the synthesizing part. Is it instantiating a local private variable? If so, how is this different from saying @synthesize viewController;

Thanks

Answer 1

The pattern @synthesize foo = bar; allows you to define a property of key foo which gets synthesized in combination with an instance variable of name bar (or _foo if you want), while @synthesize foo; simply synthesizes a property and instance variable with the same name (foo that is).

@property (...) Foo *foo;
@synthesize foo = _foo;

is kind of equivalent to this:

@interface MyClass : NSObject {
    //result of @synthesize...:
    Foo *_foo;
}

//result of @property...:
- (void)setFoo:(Foo *)foo;
//result of @property...:
- (Foo *)foo;

@end

@implementation MyClass

//result of @synthesize...:
- (void)setFoo:(Foo *)foo {
    _foo = foo; //simplified!
}

//result of @synthesize...:
- (Foo *)foo {
    return _foo; //simplified!
}

@end

The synthezised instance variable would the be used via either _foo or self->_foo (of which the former actually is an implicit form), which would involve no accessor method call whatsoever.

While the synthezised property would the be used via self.foo, which would then utilize a call to one of the synthesized accessor methods.

Just think of @synthesize foo; as an implicit @synthesize foo = foo; (note the lack of an _ here, equal names).