How to design a set of file readers and writers for different format

Question

Digging into a legacy project (C++) that needs to be extended, I realized that there are about 40 reader/writer/parser classes. They are used to read and write various type of data (different objects) in different files format (binary, hdf5, xml, text, ...) ; one type of object is typically bound to one or two file formats. The classes have for most of them just no knowledge of the others. Interfaces and inheritance were apparently unknown to the writer, as well as design patterns.

It seems to me an horrendous mess. On the other hand I am not exactly sure how to handle this situation. I will at least extract interfaces. I would also like to see if I can have common code in some parent classes, for example what is specific to a hdf5 reader/writer. I also thought that the abstract factory pattern could help but the object I get out of the readers are completely different.

How would you handle this situation ? how would you design the classes ? what design pattern would you use if any ? Would you keep the reading and writing parts splitted ?

Answer 1

The Abstract Factory pattern isn't the right track. You usually only need interfaces if you anticipate multiple implementations for a given file type and want both to operate the same way.

Question: can one class be written to multiple file types? As in, object 'a' (of type Class A) potentially needs to be written to either/both XML or text formats?

If that is true, you need to decouple the classes from the readers/writers. Take a look at this question: What design pattern should I use for import/export?