## 14.5 Why R6?

R6 is very similar to a built-in OO system called reference classes, or RC for short. I prefer R6 to RC because:

• R6 is much simpler. Both R6 and RC are built on top of environments, but while R6 uses S3, RC uses S4. This means to fully understand RC, you need to understand how the more complicated S4 works.

• R6 has comprehensive online documentation at https://r6.r-lib.org.

• R6 has a simpler mechanism for cross-package subclassing, which just works without you having to think about it. For RC, read the details in the “External Methods; Inter-Package Superclasses” section of ?setRefClass.

• RC mingles variables and fields in the same stack of environments so that you get (field) and set (field <<- value) fields like regular values. R6 puts fields in a separate environment so you get (self$field) and set (self$field <- value) with a prefix. The R6 approach is more verbose but I like it because it is more explicit.

• R6 is much faster than RC. Generally, the speed of method dispatch is not important outside of microbenchmarks. However, RC is quite slow, and switching from RC to R6 led to a substantial performance improvement in the shiny package. For more details, see vignette("Performance", "R6").

• RC is tied to R. That means if any bugs are fixed, you can only take advantage of the fixes by requiring a newer version of R. This makes it difficult for packages (like those in the tidyverse) that need to work across many R versions.

• Finally, because the ideas that underlie R6 and RC are similar, it will only require a small amount of additional effort to learn RC if you need to.