Is an object referencing another?Source:
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.
# Reassigning an uncopyable object such as an environment creates a # reference: env <- env() ref <- env is_reference(ref, env) #>  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) #>  TRUE # Once you modify on of them, the copy is triggered in the # background and the objects cease to reference each other: vec[] <- 100 is_reference(copy, vec) #>  FALSE