Difference between accessing property methods and class fields (Objective-C)

Question

Assume that I have this piece of code:

@interface Foo : NSObject {
    Bar *bar;
}

@property (retain, nonatomic) Bar *bar;

@end

When using this field/property, is there any difference between lines:

[self.bar doStuff];

and

[bar doStuff];

?

When doing assignment, property method will perform correct retaining, but what about the read access to the property, as described above? Is there any difference?

Answer 1

A synthesized (or correctly hand-written) nonatomic accessor will be functionally equivalent to

- (Bar *)bar
{
    return bar;
}

so there is no functional difference between your two examples.

However, outside of -dealloc or your initializers, consistently accessing the property via its accessor is a good idea.

Answer 2

Using the accessor self.bar is translated into a method call: [self bar]. The period syntax is just for looks. Accessing the member variable directly doesn't involve an extra function call, and is therefore slightly faster. It really only matters if you're accessing it within a loop, or in some process where that difference will add up. (On the iPhone) The setters created for properties also have some extra overhead for doing key value coding. A KVO notification is sent when you call "setBar:" or say "self.bar =" , so calling it over and over will result in a flood of notifications.

Jim is right, though - there's no functional difference between a nonatomic @property and a direct use of the variable in your code. Unless you're really concerned with the speed, using the property is probably your best bet.

Answer 3

There is a big difference. [self.bar doStuff] is equivalent to [[self bar] doStuff]

[bar doStuff] is equivalent to [self->bar doStuff]

The former uses the accessor method, the latter just accesses the instance variable bar directly.

If you use the @synthesize directive on your bar property, the compiler will generate two methods for you:

- (void)setBar:(Bar*)b;
- (Bar*)bar;

Also note, that the compiler generated setter method is retaining your Bar instance as you told it in the @property declaration.