Introducing C++

Chapter 14. Using std::variant and std::visit

You have seen how to vary behavior with classes, starting with an abstract base class to provide an interface. You can then add as many derived classes as you need to provide varying implementations. I took a few chapters to teach you all the building blocks to do this in C++. There are several parts you need, but the approach is extensible: you can continue to add extra derived types as you need them, without changing existing code.

You can use another C++ feature, called the std::variant, to vary behavior too. The std::variant is a class template introduced in C++17. The std::variant holds one of many alternative types, which C programmers may recognize as an approach similar to a union. The types can be completely unrelated. This approach requires a fixed set of types up front, so it isn’t as extensible as OOP. However, std::variant allows you to extend the behavior of the set of types unintrusively. If you want to add functionality to classes, you can add a new method in the base class, which will affect all the derived classes. If you use a std::variant, you can provide new functions without changing the types the std::variant contains. OOP makes it easy to add new types, but hard to add new operations. Variants make it easy to add new operations but hard to add new types, so OOP and variants have trade-offs.

This chapter will show you how to use a std::variant to add potential bonus payments or fines in your trading game: for ...