The R6 package allows the creation of classes with reference
semantics, similar to R's built-in reference classes. Compared to
reference classes, R6 classes are simpler and lighter-weight, and
they are not built on S4 classes so they do not require the methods
package. These classes allow public and private members, and they
support inheritance, even when the classes are defined in different
packages.

WWW: https://cran.r-project.org/web/packages/R6/