Re: best way of exporting classes/class design

Ulrich Eckhardt <>
Wed, 5 Sep 2007 10:46:45 CST
Two notes up front:
1. Your posting seems to have been linebroken twice, with different numbers
of columns.
2. You are not using your keyboard's shift key.

Both combine to a posting where it is relatively hard to figure out where a
sentence or paragraph begins, making it overall harder to understand. I
reformatted the text partially.

Mark wrote:

i have some classes that i need to export into text files for transmission
between some legacy systems that i was hoping to get some advice on.

Just for your info: when talking about exporting classes, typically the
topic revolves around the problem of exporting class interfaces from
libraries. What you are describing below is exporting data to a file.

the way it works at the moment is a list of changes made to tables is
kept in another table in a database. the table they're stored in just
contains the name of table where the change was made and the primary
key. there are a set of classes that map directly to tables in the
database e.g. PRODUCT => Product, COMPANY => Company. periodically a
class called UpdateFile is instantiated which iterates through the list
of changes in the database, instantiates the class for which the change
was made and inserts the string returned from the classes updateRecord()
function into an ofstream which is eventually written to disk.

in the past there has only been 1 export file format but now another
needs to be added in xml. i have been tasked with this and rather than
adding to the confusion i would to improve the design if i can, which
is where the problem lies.

first of all, does it make sense to move updateRecord() and any other
functions used to generate an export record into it's own class (e.g.
ProductUpdateRecord, CompanyUpdateRecord) even if it means that class
only has the 1 function?

I don't see why you have to use a class at all. Sometimes, a plain function
would work just as well. Further, I would suggest that you split the code
to iterate the database or changes in the database, i.e. write custom
iterators for that. Take a look at Boost, it has some template classes that
help creating iterators.

is there any way of templatizing it so that in UpdateFile i can do

for (each change made) {
UpdateRecord<Product> whatever;
ofstream << whatever.text(); //or whatever.xmltext()

without leaving updateRecord() in the class its generating the export
record for (in which case there wouldn't be any use for templates

In a second stage, you provide functions to dump the content of a record to
a stream using either file format. You can then use the two parts roughly
like this:

   for_each( database_iterator(db_handle),

Note: an alternative way which might be easier to implement (because the
changes are less intrusive) is to provide a callback class:

   struct record_callback {
     virtual void handle( record& r) = 0;

and provide a way to iterate the database as part of the database itself:

   class db {
     // will call cb.handle() for each record
     void iterate_records( record_callback const& cb);


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