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/