Object Reflection

Question

Does anyone have any references for building a full Object/Class reflection system in C++ ?

Ive seen some crazy macro / template solutions however ive never found a system which solves everything to a level im comfortable with.

Thanks!

Answer 1

I'm not entirely sure that I understood you intention, however the Qt framework contains a powerful meta object system that lets you do most operation expected from a reflection a system: Getting the class name as string, checking if a object is a instance of a given type, listing and invoking methods, etc.

Answer 2

Using templates and macros to automatically, or semi-automatically, define everything is pretty much the only option in C++. C++ has very weak reflection/introspection abilities. However, if what you want to do is mainly serialization and storage, this has already been implemented in the Boost Serialization libraries. You can do this by either implementing a serializer method on the class, or have an external function if you don't want to modify the class.

This doesn't seem to be what you were asking though. I'm guessing you want something like automatic serialization which requires no extra effort on the part of the class implementer. They have this in Python, and Java, and many other languages, but not C++. In order to get what you want, you would need to implement your own object system like, perhaps, the meta-object system that IgKh mentioned in his answer.

If you want to do that, I'd suggest looking at how JavaScript implements objects. JavaScript uses a prototype based object system, which is reasonably simple, yet fairly powerful. I recommend this because it seems to me like it would be easier to implement if you had to do it yourself. If you are in the mood for reading a VERY long-winded explanation on the benefits and elegance of prototypes, you can find an essay on the subject at Steve Yegge's blog. He is a very experienced programmer, so I give his opinions some credence, but I have never done this myself so I can only point to what others have said.

If you wanted to remain with the more C++ style of classes and instances instead of the less familiar prototypes, look at how Python objects and serialization work. Python also use a "properties" approach to implementing its objects, but the properties are used to implement classes and inheritance instead of a prototype based system, so it may be a little more familiar.

Sorry that I don't have a simpler answer to your question! But hopefully this will help.

Answer 3

I've used ROOT's Reflex library with good results. Rather than using crazy macro / template solutions like you described, it processes your C++ header files at build time to create reflection dictionaries then operates off of those.