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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.