How to automatically register a class on creation

Question

I was wondering whether a design pattern or idiom exists to automatically register a class type. Or simpler, can I force a method to get called on a class by simply extendin

Answer 1

@AMCoder: This is not the answer you want. The answer you want, reflection (e.g., what_am_I()) doesn't really exist in C++.

C++ has in a rather limited form via the RTTI. Using RTTI in the base class to determine the "true" type of the object being constructed won't work. Calling typeid(*this) in a base class will instead give you the typeinfo for the class being constructed.

You can make your class Animal have a non-default constructor only. This works fine for classes that derive directly from Animal, but what about classes that derive from a derived class? This approach either precludes further inheritance or requires class builders to create multiple constructors, one of which is for use by derived classes.

You can use Luchian's CRTP solution, but this too has problems with inheritance, and it also precludes you from having a collection of pointers to Animal objects. Add that non-template base class back into the mix so you can have a collection of Animals and you have the original problem all over again. Your documentation will have to say to only use the template to make a class that derives from Animal. What happens if someone doesn't do that?

The easiest solution is the one you don't like: Require that every constructor of a class that derives from Animal must call register_animal(). Say so in your documentation. Show the users of your code some examples. Put a comment in front of that example code, // Every constructor must call register_animal(). People who use your code are going to use cut and paste anyhow, so have some cut-and-paste ready solutions on hand.

Worrying about what happens if people don't read your documentation is a case of premature optimization. Requiring that every class call register_animal() in their constructors is a simple requirement. Everyone can understand it and everyone can easily implement it. You've got much bigger troubles on your hands with your users than a failed registration if your users can't even follow this simple instruction.