Programming in Objective-C, Sixth Edition

More on Properties, Synthesized Accessors, and Instance Variables

The widely-used convention is to use an underscore (_) as the leading character for an instance variable name. So any references you see in the template code generated by Xcode to variables starting with a _ are referencing the instance variables directly by name.

When you see an @synthesize directive that looks like this

@synthesize window = _window;

it says to synthesize the getter and setter for the property named window and to associate that property with an instance variable called _window (which does not have to be explicitly declared). This helps to distinguish the use of the instance variable from the property and to encourage you to set and retrieve the value of the instance ...