26: Inheritance. When To Use Private.

This is an advanced C++ topic so don’t get worried if it’s difficult right now. You’ll rarely need to use private inheritance. And protected inheritance is probably even more rare. Some of my programming books just say that this whole topic is “beyond the scope of this book”. I’ll do my best to explain this concept because that’s what this podcast is all about. You get to learn from my experience. If you’re using C#, Java, or some other language that doesn’t support private inheritance, then you’ll have to write your code differently. But even then, understanding this will help you. Once you understand private inheritance, then protected inheritance really gives you the same ability that extends to other derived classes. If you’re not familiar with this topic or research some online forums, you’ll likely wonder why you just can’t use composition. So instead of inheriting from some class that you don’t want to make that relationship public, you instead declare a member variable in your class of the other base class. This is called composition and will be the topic of another episode. Effectively, private inheritance and protected inheritance give you a new type of relationship. Instead of the public IS-A relationship, you can think of this new relationship as an IS-IMPLEMENTED-AS relationship. Inheriting gives you three main benefits that you don’t get with composition: 1. You can access public and protected members in the other class. You’re limited to just the public members with composition. 2. You can override methods in the other class. This is still inheritance even if it is private or protected. 3. You can take advantage of a special compiler optimization that only applies to inheritance. It’s called the empty base class optimization. You don’t get this benefit unless it really is a base class. Listen to the full episode or read the full transcript below. Transcript Even if you’re using C# or Java where the concepts of private inheritance and protected inheritance don’t exist, you can still benefit from understanding the general idea. You’ll just have to write your code differently in those languages. You’ll be a better programmer if you know about this. Many senior C# developers that I know would have no clue about this topic. I went online to search for this topic and on the C# forums, a common reply is, “Why would you want such a thing? Just use composition instead.” Well, after listening to this episode, you’ll know why and I hope will have a better appreciation for C++. We’ll discuss composition in a future episode. The ability to inherit privately or protected becomes an even richer ability when combined with multiple inheritance. We’re not going to get into multiple inheritance today. That will be the subject of the next episode. So stay tuned, you won’t want to miss that one. I’ve already described public inheritance in episode 24 and this episode will build on that one. In that episode, I described an example that had a book class which was the base class for a physicalBook class and an audioBook class. But what if the book class was written to take advantage of yet another class called a document. Here’s where you need to ask yourself, is a book a document? Even though they share some things in common, it doesn’t seem right to me that all books can be considered to be documents. So a direct is-a relationship with public inheritance is probably not a good idea. The best solution would probably be to refactor the common behavior and data into a new class and then make both the book and document classes derive from this new class. That would make book and document related but as siblings that share the same base class instead of one deriving from the other. Maybe we could call this new class something simple like, text. But what i