Is an object referencing another?

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.

  • is_reference
library(rlang) # NOT RUN { # Reassigning an uncopyable object such as an environment creates a # reference: env <- env() ref <- env is_reference(ref, env) # Due to copy-on-write optimisation, a copied vector can # temporarily reference the original vector: vec <- 1:10 copy <- vec is_reference(copy, vec) # 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) # }
Documentation reproduced from package rlang, version 0.2.0, License: GPL-3

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