Common Base Type Let’s say you have the need to store a variety of related things. In C/C++, Java, etc., you define a common type and things are related by being dervied from that type. For example, if you have a list of tools, each member is derived from some common class like Tool. The collection is instantiated for Tools. You dervive a specific tool, like a Scredriver or Hammer from Tool and put it in the collection.