Question

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

I am beginner in ios app development.I looked over many tutorial and i came across following line

.h file contains:

@property (strong) ScaryBugData *data;

.m contains:

@synthesize data = _data; 

But I am not getting meaning of data = _data.
why this is required or what it means.

ref : http://www.raywenderlich.com/1797/how-to-create-a-simple-iphone-app-tutorial-part-1

Answer 1

_data is the name of the instance variable that is automatically created for you.
data is the name of the property that has a getter and setter.

If you don’t specify a custom ivar name it will just default to the name of the property.

In this case you can directly set the ivar using _data = [ScaryBugData data], or you can use the setter using self.data = [ScaryBugData data]. The same goes for getting.

The synthesized setter will make sure the old value is properly released, the new value is properly retained and some more stuff under the hood.