There are typically two situations where two symbols may refer to the same object.

  • R objects usually have copy-on-write semantics. This is an optimisation that ensures that objects are only copied if needed. When you copy a vector, no memory is actually copied until you modify either the original object or the copy is modified.

    Note that the copy-on-write optimisation is an implementation detail that is not guaranteed by the specification of the R language.

  • Assigning an uncopyable object (like an environment) creates a reference. These objects are never copied even if you modify one of the references.

is_reference(x, y)


x, y

R objects.


# Reassigning an uncopyable object such as an environment creates a # reference: env <- env() ref <- env is_reference(ref, env)
#> [1] TRUE
# Due to copy-on-write optimisation, a copied vector can # temporarily reference the original vector: vec <- 1:10 copy <- vec is_reference(copy, vec)
#> [1] TRUE
# Once you modify on of them, the copy is triggered in the # background and the objects cease to reference each other: vec[[1]] <- 100 is_reference(copy, vec)
#> [1] FALSE