Design pattern for multiple output formats

Question

I have a class structure which represents (internally) the data I wish to output to a file.

Some of the member variables are private to the data class so that it can manage itself and stop things going awry.

I then want this data to be output into a number of file formats. I could do something like

savefile_formatA(DataClass* pDataClass, ofstream& fout);
savefile_formatB(DataClass* pDataClass, ofstream& fout);

except that the functions need to then see the private member variables of DataClass. I could of course just make savefile_formatXYZ() friend functions but then I would need to add a friend declaration for every different format.

Is there a standard design pattern for solving this kind of thing? How would you solve this problem?

Thanks!

Answer 1

Depending upon the complexity of your data class you may wish to use a Visitor pattern. If you have some kind of nested data structure then Visitor may well be what you need.

If formatting is something relatively simple, for example you are producing variations on something such as a comma separated list then you can take an approach like this.

Your formatter objects all implement an interface such as (pseudo code)

 IFormatter ( start(); addInt(name, value), addString(name, value) .... end() );

then the data class has a method

  public void formatMyself( IFormatter formatter ) {

        formatter.start()
        formatter.addString("aField", myA);
        formatter.addInteger("bfield", myB);
        formatter.end();          
  }

This makes the class being formatted responsible for the choice of data to be formatted, and the formatter responsible for the details of the format.