This protocol should be adopted by types that wish to customize their
textual representation. This textual representation is used when
objects are written to an OutputStreamType.

protocol Printable {
    var description: String { get }
}

当DebugPrintable协议仅用于调试时,还有DebugPrintable协议：

This protocol should be adopted by types that wish to customize
their textual representation used for debugging purposes. This
textual representation is used when objects are written to an
OutputStreamType.

protocol DebugPrintable {
    var debugDescription: String { get }
}

Documentation(感谢@MartinR)

注意：在评论中提到的@Antonio和@MartinR,这在操场上(从Xcode6 GM开始)不起作用;这是一个已知的错误.它在编译应用程序中工作.

从Xcode6 GM发行说明：

In Playgrounds,println() ignores the Printable conformance of
user-defined types. (16562388)

截至目前,Swift 2.0 Printable已成为CustomStringConvertible.一切都保持与以前一样,你仍然需要实现

var description: String { get }

但现在它叫CustomStringConvertible.而调试是CustomDebugStringConvertible