Meta-programming

Question

I have just started my journey as a C++ developer, and I have recently been researching a topic called meta-programming. So far, I have gone through a lot of information on the topic, and have few questions.

1. Usage of Reflection in c++: Is it possible? As per my understanding reflection is a meta-programming technique which has the ability to manipulate a state of a program, and its manipulation can be introspection or introcession.

2. What is the difference between reflective architecture and meta-level architecture? In which category does meta-programming architecture fall in?

3. Meta-programming comes in two levels: meta level and base level. In my understanding, the object program or the system under investigation is the base level, and the meta program that is going to operate over the system under investigation is the meta level. Am I correct?

Also help me by providing some guidelines about working on template metaprogramming in C++.
May you suggest me some workable examples I can look for to get hands-on in this programming technique?

Answer 1

Metaprogramming simply means that you have the means to write code that writes other code (in a broad sense). C++ does this through the template mechanism, which is basically a type-safe macro language. It is perfectly possible to just apply the template expansion step, and the result will be valid template-free C++ code.

Reflection is the ability of a programming language and / or runtime environment to inspect (and often modify) its own language elements at run-time. Typically, reflective languages offer data structures that represent language elements like classes, methods, variables, parameters, etc., as well as routines or classes to inspect, modify, and create them. The modification ability makes reflection a type of meta-programming, since it allows you to write code that generates other code.

Other examples of metaprogramming include code generators, self-modifying binaries (though these aren't as popular anymore today as they were two decades ago), C macros; one could argue that currying and similar functional-programming techniques are also a kind of metaprogramming.

Answer 2

1. No. It isn't. C++ programs are compiled to machine code and all information about classes etc. are lost (unless using C++/CLI for the .NET environment, but that's not pure C++).

2-3. I don't know - wait for another answer that helps with that :-)

Meta programming using C++ templates is very powerful, but also complex. The template expansion requires functional style of programming rather than imperative programming that C++ uses. If you don't have previous experience with functional programming I would recommend you to make detour and learn som basic lisp, haskell or scheme.