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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