# DefaultStringInterpolation Represents a string literal with interpolations while it’s being built up. ``` @frozen struct DefaultStringInterpolation ``` ## Overview You don’t need to create an instance of this type directly. It’s used by the compiler when you create a string using string interpolation. Instead, use string interpolation to create a new string by including values, literals, variables, or expressions enclosed in parentheses, prefixed by a backslash (`\(`…`)`). ``` let price = 2 let number = 3 let message = """ If one cookie costs \(price) dollars, \ \(number) cookies cost \(price * number) dollars. """ print(message) // Prints "If one cookie costs 2 dollars, 3 cookies cost 6 dollars." ``` When implementing an `ExpressibleByStringInterpolation` conformance, set the `StringInterpolation` associated type to `DefaultStringInterpolation` to get the same interpolation behavior as Swift’s built-in `String` type and construct a `String` with the results. If you don’t want the default behavior or don’t want to construct a `String`, use a custom type conforming to `StringInterpolationProtocol` instead. # Extending default string interpolation behavior Code outside the standard library can extend string interpolation on `String` and many other common types by extending `DefaultStringInterpolation` and adding an `appendInterpolation(...)` method. For example: ``` extension DefaultStringInterpolation { fileprivate mutating func appendInterpolation( escaped value: String, asASCII forceASCII: Bool = false) { for char in value.unicodeScalars { appendInterpolation(char.escaped(asASCII: forceASCII)) } } } print("Escaped string: \(escaped: string)") ``` See `StringInterpolationProtocol` for details on `appendInterpolation` methods. `DefaultStringInterpolation` extensions should add only `mutating` members and should not copy `self` or capture it in an escaping closure. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)