Swift Instance Variable Underscore Prefix?

Question

Did Swift drop the underscore-prefix convention for instance variables, e.g., _window? If so, why?

Answer 1

Apple still uses _ in its Xcode boilerplate code as a convention for a non-public variable. You'll see a pattern like such:

class Foo {
  var _bar : Bar? = nil
  var  bar : Bar {
    if _bar == nil {
      /* compute some stuff */
      _bar = Bar (/* ... */)
    }
    return _bar!
  }
}

where access to the property is meant to be through the bar computed property. You find this in Apple CoreData templates.

Answer 2

In Objective-C when you declare a property @synthesize would create the getter and setters for you automatically since clang 3.2. So, the default @synthesize for a property "foo" would look like this:

@synthesize foo = _foo

because of that _foo would then be the iVar. In other words you could have done the @synthesize yourself and called the iVar whatever you liked:

@synthesize foo = myiVarFoo

so in this case there is no "_"

So now in Swift from the documentation:

Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly.

So from the documentation it's clear that swift does not have a corresponding instance variable thus no need to have the "_" around anymore.

Answer 3

The underscore prefix was meant not to confuse the instance variable _foo with its getter foo.

In swift there's not such distinction between ivars and getters. Instead you only have properties, making the _ convention non necessary.