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
effects.
Examples
# 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: 0x562ec15b5dc0>
#> attr(,"foo")
#> [1] "bar"
env
#> <environment: 0x562ec15b5dc0>
#> attr(,"foo")
#> [1] "bar"
# These objects that can only be changed with side effect are not
# copyable:
is_copyable(env)
#> [1] FALSE
structure(base::list, foo = "bar")
#> function (...) .Primitive("list")
#> attr(,"foo")
#> [1] "bar"
str(base::list)
#> function (...)
#> - attr(*, "foo")= chr "bar"