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You can send the output of the standard library's print(_:to:) and dump(_:to:) functions to an instance of a type that conforms to the TextOutputStream protocol instead of to standard output. Swift's String type conforms to TextOutputStream already, so you can capture the output from print(_:to:) and dump(_:to:) in a string instead of logging it to standard output.

var s = ""
for n in 1...5 {
    print(n, terminator: "", to: &s)
}
// s == "12345"

Conforming to the TextOutputStream Protocol

To make your custom type conform to the TextOutputStream protocol, implement the required write(_:) method. Functions that use a TextOutputStream target may call write(_:) multiple times per writing operation.

As an example, here's an implementation of an output stream that converts any input to its plain ASCII representation before sending it to standard output.

struct ASCIILogger: TextOutputStream {
    mutating func write(_ string: String) {
        let ascii = string.unicodeScalars.lazy.map { scalar in
            scalar == "\n"
              ? "\n"
              : scalar.escaped(asASCII: true)
        }
        print(ascii.joined(separator: ""), terminator: "")
    }
}

The ASCIILogger type's write(_:) method processes its string input by escaping each Unicode scalar, with the exception of "\n" line returns. By sending the output of the print(_:to:) function to an instance of ASCIILogger, you invoke its write(_:) method.

let s = "Hearts ♡ and Diamonds ♢"
print(s)
// Prints "Hearts ♡ and Diamonds ♢"

var asciiLogger = ASCIILogger()
print(s, to: &asciiLogger)
// Prints "Hearts \u{2661} and Diamonds \u{2662}"