Is an object copyable?Source:
When an object is modified, R generally copies it (sometimes
lazily) to enforce value semantics.
However, some internal types are uncopyable. If you try to copy
them, either with
<- or by argument passing, you actually create
references to the original object rather than actual
copies. Modifying these references can thus have far reaching side
# Let's add attributes with structure() to uncopyable types. Since # they are not copied, the attributes are changed in place: env <- env() structure(env, foo = "bar") #> <environment: 0x55765b1673e0> #> attr(,"foo") #>  "bar" env #> <environment: 0x55765b1673e0> #> attr(,"foo") #>  "bar" # These objects that can only be changed with side effect are not # copyable: is_copyable(env) #>  FALSE structure(base::list, foo = "bar") #> function (...) .Primitive("list") #> attr(,"foo") #>  "bar" str(base::list) #> function (...) #> - attr(*, "foo")= chr "bar"