A StringBuilder class for Swift

StringBuilder.swift

/**
    Supports creation of a String from pieces
*/
public class StringBuilder {
    private var stringValue: String
    
    /**
        Construct with initial String contents

        :param: string Initial value; defaults to empty string
    */
    public init(string: String = "") {
        self.stringValue = string
    }
    
    /**
        Return the String object
        
        :return: String
    */
    public func toString() -> String {
        return stringValue
    }
    
    /**
        Return the current length of the String object
    */
    public var length: Int {
        return countElements(stringValue)
    }
    
    /**
        Append a String to the object

        :param: string String
    
        :return: reference to this StringBuilder instance
    */
    public func append(string: String) -> StringBuilder {
        stringValue += string
        return self
    }
    
    /**
        Append a Printable to the object
        
        :param: value a value supporting the Printable protocol
    
        :return: reference to this StringBuilder instance
    */
    public func append<T: Printable>(value: T) -> StringBuilder {
        stringValue += value.description
        return self
    }

    /**
        Append a String and a newline to the object
        
        :param: string String
    
        :return: reference to this StringBuilder instance
    */
    public func appendLine(string: String) -> StringBuilder {
        stringValue += string + "\n"
        return self
    }

    /**
        Append a Printable and a newline to the object
        
        :param: value a value supporting the Printable protocol
    
        :return: reference to this StringBuilder instance
    */
    public func appendLine<T: Printable>(value: T) -> StringBuilder {
        stringValue += value.description + "\n"
        return self
    }
    
    /**
        Reset the object to an empty string

        :return: reference to this StringBuilder instance
    */
    public func clear() -> StringBuilder {
        stringValue = ""
        return self
    }
}

/**
    Append a String to a StringBuilder using operator syntax

    :param: lhs StringBuilder
    :param: rhs String
*/
public func += (lhs: StringBuilder, rhs: String) {
    lhs.append(rhs)
}

/**
    Append a Printable to a StringBuilder using operator syntax

    :param: lhs Printable
    :param: rhs String
*/
public func += <T: Printable>(lhs: StringBuilder, rhs: T) {
    lhs.append(rhs.description)
}

/**
    Create a StringBuilder by concatenating the values of two StringBuilders

    :param: lhs first StringBuilder
    :param: rhs second StringBuilder

    :result StringBuilder
*/
public func +(lhs: StringBuilder, rhs: StringBuilder) -> StringBuilder {
    return StringBuilder(string: lhs.toString() + rhs.toString())
}


// Examples

let sb = StringBuilder()
sb.append(1).append("+").append(1).append("=").append(2)
sb.toString()

let sb2 = StringBuilder()
sb2 += "2+2"
sb2 += "="
sb2 += "4"
sb2.toString()

let sbAdd = sb + sb2
sbAdd.toString()

…That said, I just encountered the first case where I needed a reference object to which I can send append messages.

@GuyKahlon a bit late here, but I discovered that Swift's String class uses an exponential growth strategy that makes appending to a string a constant time operation when averaged over many appends. See here for the source. So this means that there's no need for a StringBuilder class in Swift as using += on a swift String is already doing the performant thing that Java's StringBuilder class is doing.