Question

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How does an underscore in front of a variable in a cocoa objective-c class work?

So i have found out that you have to use underscore when synthesizing properties and that doesn’t make a single bit of sense to me.

So, let’s start.
In our .h file we write this line:

@property (nonatomic) double speed;

In our .m file we do this:

@synthesize speed = _speed;

Why? As far as I know, property makes an instance variable and creates setters and getters for it.
But what the hell does line

@synthesize speed = _speed 

do?
As common sense tells me, we assign value in _speed to speed. Okay.
Where did we declare _speed? Why doesn’t compiler give us an error? What is it supposed to mean? Why such an obfuscated code?

My questions here:

What happens if I do just

@synthesize speed;

without _speed, will I get error or some bugs?
What is the reason after this syntax? What were they thinking when making it? Where does _speed come from? What is it? Is it a pointer or a real value? What is going on?

Answer 1

Well, _speed is the instance variable used by the property. If you really want to declare it fully, you need to write:

@interface Foo
{
    double _speed;
}

@property double speed;

@end

@implementation Foo

@synthetize speed = _speed;

@end

Now Xcode 4.4 is unleashed, most of that stuff is unnecessary and should be omitted for sanity.

Basically, all you have to do is:

@interface Foo

@property double speed;

@end

No instance variable, no @synthetize, and everything still works as before. You can either use self.speed or self->_speed for direct access.