我正在看自动布局库中的一些代码。其中，UIView采用了一个协议：

extension UIView: Constrainable {}
extension UILayoutGuide: Constrainable {
    // LayoutGuide doesn't have baseline anchors, so just use the bottom anchor
    public var firstBaselineAnchor: NSLayoutYAxisAnchor {
        return bottomAnchor
    }
    public var lastBaselineAnchor: NSLayoutYAxisAnchor {
        return bottomAnchor
    }
}

public protocol Constrainable {
    var topAnchor:      NSLayoutYAxisAnchor { get }
    var bottomAnchor:   NSLayoutYAxisAnchor { get }
    var leftAnchor:     NSLayoutXAxisAnchor { get }
    var rightAnchor:    NSLayoutXAxisAnchor { get }
    var leadingAnchor:  NSLayoutXAxisAnchor { get }
    var trailingAnchor: NSLayoutXAxisAnchor { get }

    var centerXAnchor:  NSLayoutXAxisAnchor { get }
    var centerYAnchor:  NSLayoutYAxisAnchor { get }

    var widthAnchor:    NSLayoutDimension { get }
    var heightAnchor:   NSLayoutDimension { get }

    var firstBaselineAnchor : NSLayoutYAxisAnchor { get }
    var lastBaselineAnchor  : NSLayoutYAxisAnchor { get }
}

一个空虚的{ get }成就是什么？