Composition: Building with Parts | DevelClan

Building complex objects in Ruby with composition and modules as roles, and knowing when each one beats inheritance for your design.When an object needs behavior that already exists elsewhere, most people’s default answer is inheritance: create a superclass and hang the classes that share code from it. It works, but inheritance solves a very specific problem (the specialization of a type) and gets expensive as soon as you push it beyond its intended use. Ruby offers two other tools for the cases where inheritance doesn’t fit: What happens when a role with real implementation is shared by classes that have no relationship with each other? That’s what modules are for. How do you build a complex object out of simpler ones, instead of growing a deeper and deeper hierarchy? That’s what composition is for.