Andrzej goes into detail how variants can be seen as a type of polymorphism in his recent blog post.

Another polymorphism

by Andrzej Krzemieński

From the article:

In this post we will try to see by a practical example what Boost.Variant is for. You can sometimes see examples that use type variant<int, double, string>, but to me they are artificial: I never needed to use something that is either a double or int; but I still consider this library useful. Even if you are already familiar with Boost.Variant an its concepts of “never-empty guarantee” and “static visitor”, I made sure there is still something you can get from reading this post.