A type that can be initialized by string interpolation with a string literal that includes expressions.

Use string interpolation to include one or more expressions in a string literal, wrapped in a set of parentheses and prefixed by a backslash. For example:

let price = 2
let number = 3
let message = "One cookie: $\(price), \(number) cookies: $\(price * number)."
print(message)
// Prints "One cookie: $2, 3 cookies: $6."

Conforming to the ExpressibleByStringInterpolation Protocol

To use string interpolation to initialize instances of your custom type, implement the required initializers for ExpressibleByStringInterpolation conformance. String interpolation is a multiple-step initialization process. When you use string interpolation, the following steps occur:

1. The string literal is broken into pieces. Each segment of the string literal before, between, and after any included expressions, along with the individual expressions themselves, are passed to the init(stringInterpolationSegment:) initializer.
2. The results of those calls are passed to the init(stringInterpolation:) initializer in the order in which they appear in the string literal.

In other words, initializing the message constant in the example above using string interpolation is equivalent to the following code:

let message = String(stringInterpolation:
      String(stringInterpolationSegment: "One cookie: $"),
      String(stringInterpolationSegment: price),
      String(stringInterpolationSegment: ", "),
      String(stringInterpolationSegment: number),
      String(stringInterpolationSegment: " cookies: $"),
      String(stringInterpolationSegment: price * number),
      String(stringInterpolationSegment: "."))

Deprecated: it will be replaced or redesigned in Swift 4.0. Instead of conforming to 'ExpressibleByStringInterpolation', consider adding an 'init(_:String)'.

let s = "\(5) x \(2) = \(5 * 2)"
print(s)
// Prints "5 x 2 = 10"

let s = "\(5) x \(2) = \(5 * 2)"
print(s)
// Prints "5 x 2 = 10"