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Portable and non-portable R6 classes

Source: vignettes/Portable.Rmd
Portable.Rmd

One limitation to R’s reference classes is that class inheritance across package namespaces is limited. R6 avoids this problem when the portable option is enabled.

The problem

Here is an example of the cross-package inheritance problem with reference classes: Suppose you have ClassA in pkgA, and ClassB in pkgB, which inherits from ClassA. ClassA has a method foo which calls a non-exported function fun in pkgA.

If ClassB inherits foo, it will try to call fun – but since ClassB objects are created in pkgB namespace (which is an environment) instead of the pkgA namespace, it won’t be able to find fun.

Something similar happens with R6 when the portable=FALSE option is used. For example:

library(R6)
# Simulate packages by creating environments
pkgA <- new.env()
pkgB <- new.env()

# Create a function in pkgA but not pkgB
pkgA$fun <- function() 10

ClassA <- R6Class("ClassA",
  portable = FALSE,
  public = list(
    foo = function() fun()
  ),
  parent_env = pkgA
)

# ClassB inherits from ClassA
ClassB <- R6Class("ClassB",
  portable = FALSE,
  inherit = ClassA,
  parent_env = pkgB
)

When we create an instance of ClassA, it works as expected:

a <- ClassA$new()
a$foo()
#> [1] 10

But with ClassB, it can’t find the foo function:

b <- ClassB$new()
b$foo()
#> Error in b$foo() : could not find function "fun"

Portable R6

R6 supports inheritance across different packages, with the default portable=TRUE option. In this example, we’ll again simulate different packages by creating separate parent environments for the classes.

pkgA <- new.env()
pkgB <- new.env()

pkgA$fun <- function() {
  "This function `fun` in pkgA"
}

ClassA <- R6Class("ClassA",
  portable = TRUE,  # The default
  public = list(
    foo = function() fun()
  ),
  parent_env = pkgA
)

ClassB <- R6Class("ClassB",
  portable = TRUE,
  inherit = ClassA,
  parent_env = pkgB
)


a <- ClassA$new()
a$foo()
#> [1] "This function `fun` in pkgA"

b <- ClassB$new()
b$foo()
#> [1] "This function `fun` in pkgA"

When a method is inherited from a superclass, that method also gets that class’s environment. In other words, method “runs in” the superclass’s environment. This makes it possible for inheritance to work across packages.

When a method is defined in the subclass, that method gets the subclass’s environment. For example, here ClassC is a subclass of ClassA, and defines its own foo method which overrides the foo method from ClassA. It happens that the method looks the same as ClassA’s – it just calls fun. But this time it finds pkgC$fun instead of pkgA$fun. This is in contrast to ClassB, which inherited the foo method and environment from ClassA.

pkgC <- new.env()
pkgC$fun <- function() {
  "This function `fun` in pkgC"
}

ClassC <- R6Class("ClassC",
  portable = TRUE,
  inherit = ClassA,
  public = list(
    foo = function() fun()
  ),
  parent_env = pkgC
)

cc <- ClassC$new()
# This method is defined in ClassC, so finds pkgC$fun
cc$foo()
#> [1] "This function `fun` in pkgC"

Using self

One important difference between non-portable and portable classes is that with non-portable classes, it’s possible to access members with just the name of the member, and with portable classes, member access always requires using self$ or private$. This is a consequence of the inheritance implementation.

Here’s an example of a non-portable class with two methods: sety, which sets the private field y using the <<- operator, and getxy, which returns a vector with the values of fields x and y:

NP <- R6Class("NP",
  portable = FALSE,
  public = list(
    x = 1,
    getxy = function() c(x, y),
    sety = function(value) y <<- value
  ),
  private = list(
    y = NA
  )
)

np <- NP$new()

np$sety(20)
np$getxy()
#> [1]  1 20

If we attempt the same with a portable class, it results in an error:

P <- R6Class("P",
  portable = TRUE,
  public = list(
    x = 1,
    getxy = function() c(x, y),
    sety = function(value) y <<- value
  ),
  private = list(
    y = NA
  )
)

p <- P$new()

# No error, but instead of setting private$y, this sets y in the global
# environment! This is because of the semantics of <<-.
p$sety(20)
y
#> [1] 20

p$getxy()
#> Error in p$getxy() : object 'y' not found

To make this work with a portable class, we need to use self$x and private$y:

P2 <- R6Class("P2",
  portable = TRUE,
  public = list(
    x = 1,
    getxy = function() c(self$x, private$y),
    sety = function(value) private$y <- value
  ),
  private = list(
    y = NA
  )
)

p2 <- P2$new()
p2$sety(20)
p2$getxy()
#> [1]  1 20

There is a small performance penalty for using self$x as opposed to x. In most cases, this is negligible, but it can be noticeable in some situations where there are tens of thousands or more accesses per second. For more information, see vignette("Performance").

Potential pitfalls with cross-package inheritance

Inheritance happens when an object is instantiated with MyClass$new(). At that time, members from the superclass get copied to the new object. This means that when you instantiate R6 object, it will essentially save some pieces of the superclass in the object.

Because of the way that packages are built in R, R6’s inheritance behavior could potentially lead to surprising, hard-to-diagnose problems when packages change versions.

Suppose you have two packages, pkgA, containing ClassA, and pkgB, containing ClassB, and there is code in pkgB that instantiates ClassB in an object, objB, at build time. This is in contrast to instantiating ClassB at run-time, by calling a function. All the code in the package is run when a binary package is built, and the resulting objects are saved in the package. (Generally, if the object can be accessed with pkgB:::objB, this means it was created at build time.)

When objB is created at package build time, pieces from the superclass, pkgA::ClassA, are saved inside it. This is fine in and of itself. But imagine that pkgB was built and installed against pkgA 1.0, and then you upgrade to pkgA 2.0 without subsequently building and installing pkgB. Then pkgB::objB will contain some code from pkgA::ClassA 1.0, but the version of pkgA::ClassA that’s installed will be 2.0. This can cause problems if objB inherited code which uses parts of pkgA that have changed – but the problems may not be entirely obvious.

This scenario is entirely possible when installing packages from CRAN. It is very common for a package to be upgraded without upgrading all of its downstream dependencies. As far as I know, R does not have any mechanism to force downstream dependencies to be rebuilt when a package is upgraded on a user’s computer.

If this problem happens, the remedy is to rebuild pkgB against pkgA 2.0. I don’t know if CRAN rebuilds all downstream dependencies when a package is updated. If it doesn’t, then it’s possible for CRAN to have incompatible binary builds of pkgA and pkgB, and users would then have to install pkgB from source, with install.packages("pkgB", type = "source").

To avoid this problem entirely, objects of ClassB must not be instantiated at build time. You can either (A) instantiate them only in functions, or (B) instantiate them at package load time, by adding an .onLoad function to your package. For example:

ClassB <- R6Class("ClassB",
  inherit = pkgA::ClassA,
  public = list(x = 1)
)

# We'll fill this at load time
objB <- NULL

.onLoad <- function(libname, pkgname) {
  # The namespace is locked after loading; we can still modify objB at this time.
  objB <<- ClassB$new()
}

You might be wondering why ClassB (the class, not the instance of the class objB) doesn’t save a copy of pkgA::ClassA inside of it when the package is built. This is because, for the inherit argument, R6Class saves the unevaluated expression (pkgA::ClassA), and evaluates it when $new() is called.

Wrap-up

In summary:

  • Portable classes allow inheritance across different packages.
  • Portable classes always require the use of self or private to access members. This can incur a small performance penalty, since using self$x is slower than just x.