Smalltalk and Object Orientation: an Introduction - Free

to reconstruct the data into objects when you read the text file. This would mean that you would have to
invent some way of indicating that certain data was associated with c ertain objects and that certain
objects were related (if you were dealing with a composite object). All of this would be very time
consuming and error prone. It would also be very unlikely that the ASCII data written out by someone
else would be in the right format for your system.
The BOSS is provided by VisualWorks as a way around this. It allows objects to be directly stored
to a file in a compact and encoded form. You do not need to convert the objects into anything special
nor do you need to worry abou t reconstructing the objects when you load them back in. In addition
everyone else who uses the BOSS will be able to read and write to your BOSS files. The action of
writing objects to a BOSS file is referred to by Smalltalkers as BOSSing out (or in) the objects.
13.4.1 Saving to BOSS
It is possible to “BOSS out” a single object, a series of objects or a whole collection of objects. It is also
possible to BOSS out a composite object. If an object references another object, then both the original
object and the objects which it contains are saved to the BOSS file. The BOSS file is an incremental file
so it is possible to add objects to a BOSS file at different times (i.e. in different executions of the same
image). It is also possible to read the BOSS file one objec t at a time (assuming the objects were written
incrementally). This means that you do not have to read the whole file to access the third object etc.
The procedure for creating a BOSS file is relatively straight forward and is outlined below:
1. Create a dat a stream to write the objects to. This is usually a write stream onto a specified
filename.
2. Create the BinaryObjectStorage system (BOSS) object. This is done using the instance creation
method onNew: with the data stream as the parameter.
3. Using the nextPut: message save each object to the BOSS object. Each object you wish to
save is passed as the parameter of the nextPut: message.
4. Once you have finished BOSSing out the object you can close it by sending it the message close.
If you want to save a whole co llection of objects to the BOSS file you can do so by using the
nextPutAll: message (instead of the nextPut: message). Each object will be stored separately,
which allows each object to be retrieved separately at a later date.
If you wish to append an obje ct to an existing BOSS file then you create a read -append data stream
(rather than a write data stream), then create the BOSS file using the onOld: message (rather than the
onNew: message). Finally, you need to move the file pointer to the end of the BOSS file. You can do
that by sending the setToEnd message to the BOSS object.
A simple example of creating and saving to a BOSS file is presented below. The bosFileName is set
to a string representing an appropriate file on the host computer system.
valuesFileStream := bosFileName asFilename writeStream.
bosFileStream := BinaryObjectStorage onNew: valuesFileStream.
bosFileStream nextPutAll: aList.
valuesFileStream close.
This example saves all the elements in the variable aList to the bosFile indicated by the
bosFileName. It then closes the file.
A point to note is that it can be a good idea to wrap the writing of objects to the BOSS file with a
valueNowOrOnUnwindDo: message. This message is used to ensure that the BOSS file you ar
writing to is closed safely even if an abnormal interruption occurs. That is, the file will be closed and
freed up in an appropriate manner. The message is sent to a block in which you define the write
expression. It takes a second block as its argument, which executes if there is a problem or when the
first block terminates. For example, we could have written the write part of the above as:
[bosFileStream nextPutAll: aList] valueNowOrOnUnwindDo:
[valuesFileStream close].
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