Meta Programming, whats it good for?

Question

So Meta Programming -- the idea that you can modify classes/objects at runtime, injecting new methods and properties. I know its good for framework development; been working with Grails, and that framework adds a bunch of methods to your classes at runtime. You have a name property on a User object, and bamm, you get a findByName method injected at runtime.

1. Has my description completely described the concept?
2. What else is it good for (specific examples) other than framework development?

Answer 1

To me, meta-programming is "a program that writes programs".

Meta-programming is especially good for reuse, because it supports generalization: you can define a family of concepts that belong to a particular pattern. Then, through variability you can apply that concept in similar, but different scenarios.

The simplest example is Java's getters and setters as mentioned by @Sjoerd:

Both getter and setter follow a well-defined pattern: A getter returns a class member, and a setter sets a class member's value. Usually you build what it's called a template to allow application and reuse of that particular pattern. How a template works depends on the meta-programming/code generation approach being used.

If you want a getter or setter to behave in a slightly different way, you may add some parameters to your template. This is variability. For instance, if you want to add additional processing code when getting/setting, you may add a block of code as a variability parameter. Mixing custom code and generated code can be tricky. ABSE is currently the only MDSD approach that I know that natively supports custom code directly as a template parameter.