Question

I’m not too sure how to explain this, but I will try.

I have a object A which has a rownr and partition nr. B, C inherits from A and adds a few other variables (and get/setters for them)

I then have a function which takes a variable that is derived from A (B, C… etc) that will create an record in a database/table with the same columns as the variables the object has.

For example:

class A {
   int paritionKey;
   int rowKey;

   set/get for them both
}

class B : A {
   string color;

   ...
}

One table will then be called “B” and have 3 columns, partitionKey, rowKey and color.

Is there any way of not hard coding this? Or would the best way be to create a toString method in the classes that returns a part of the xml request body that will be used to construct the new row in the table? (using REST)

Answer 1

It sounds like you are asking if there is a way to do automated marshalling of C++ objects into a database. The short answer is no, there is no built-in way in the C++ language to do this. Your toString() method isn’t a bad approach, although it does require you to write toString() (and likely at some point also fromString()) methods for each of your classes… whether that is too much work or not would depend on how many such classes you need to support.

Alternatively you might also take a look at Qt’s property system — if you don’t mind subclassing your data objects from QObject, you can decorate your class definitions with Q_PROPERTY declarations, along with getter methods for each property, and then you can write generic code that uses Qt’s QMetaObject class to iterate over all declared properties of any given QObject in a generic fashion. This works because Qt’s moc preprocessor (which you will be running anyway if you are using Qt) will parse the Q_PROPERTY macros and it can auto-generate a lot of the necessary glue code for you. You’ll still have to write the last step (converting the QObject’s data to XML or SQL commands by iterating over myObject->metaObject()->property(int) and calling myObject->property(propName) for each property) yourself, but at least you can do that in a generic fashion, without having to write a separate marshalling routine for each class.