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6.10.4 Dynamic Strings

Streams provide a powerful abstraction for a number of data structures. Concepts like current position, writing the next position, and changing the way you view a data structure when convenient combine to let you write compact, powerful code. The last example is taken from the actual Smalltalk source code—it shows a general method for making an object print itself onto a string.

        printString [
            | stream |
            stream := WriteStream on: (String new).
            self printOn: stream.
            ^stream contents

This method, residing in Object, is inherited by every class in Smalltalk. The first line creates a WriteStream which stores on a String whose length is currently 0 (String new simply creates an empty string. It then invokes the current object with printOn:. As the object prints itself to “stream”, the String grows to accommodate new characters. When the object is done printing, the method simply returns the underlying string.

As we've written code, the assumption has been that printOn: would go to the terminal. But replacing a stream to a file like /dev/tty with a stream to a data structure (String new) works just as well. The last line tells the Stream to return its underlying collection, which will be the string which has had all the printing added to it. The result is that the printString message returns an object of the String class whose contents are the printed representation of the very object receiving the message.