# ExpressibleByStringInterpolation A type that can be initialized by string interpolation with a string literal that includes expressions. ``` protocol ExpressibleByStringInterpolation : ExpressibleByStringLiteral ``` ## Overview 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." ``` # Extending the Default Interpolation Behavior Add new interpolation behavior to existing types by extending `DefaultStringInterpolation`, the type that implements interpolation for types like `String` and `Substring`, to add an overload of `appendInterpolation(_:)` with their new behavior. For more information, see the `DefaultStringInterpolation` and `StringInterpolationProtocol` documentation. # Creating a Type That Supports the Default String Interpolation To create a new type that supports string literals and interpolation, but that doesn’t need any custom behavior, conform the type to `ExpressibleByStringInterpolation` and implement the `init(stringLiteral: String)` initializer declared by the `ExpressibleByStringLiteral` protocol. Swift will automatically use `DefaultStringInterpolation` as the interpolation type and provide an implementation for `init(stringInterpolation:)` that passes the interpolated literal’s contents to `init(stringLiteral:)`, so you don’t need to implement anything specific to this protocol. # Creating a Type That Supports Custom String Interpolation If you want a conforming type to differentiate between literal and interpolated segments, restrict the types that can be interpolated, support different interpolators from the ones on `String`, or avoid constructing a `String` containing the data, the type must specify a custom `StringInterpolation` associated type. This type must conform to `StringInterpolationProtocol` and have a matching `StringLiteralType`. For more information, see the `StringInterpolationProtocol` documentation. --- 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)