A weak reference is a special R object which makes it possible to keep a reference to an object without preventing garbage collection of that object. It can also be used to keep data about an object without preventing GC of the object, similar to WeakMaps in JavaScript.
Objects in R are considered reachable if they can be accessed by following
a chain of references, starting from a root node; root nodes are
specially-designated R objects, and include the global environment and base
environment. As long as the key is reachable, the value will not be garbage
collected. This is true even if the weak reference object becomes
unreachable. The key effectively prevents the weak reference and its value
from being collected, according to the following chain of ownership:
weakref <- key -> value.
When the key becomes unreachable, the key and value in the weak reference
object are replaced by NULL, and the finalizer is scheduled to execute.
Arguments
- key
The key for the weak reference. Must be a reference object – that is, an environment or external pointer.
- value
The value for the weak reference. This can be
NULL, if you want to use the weak reference like a weak pointer.- finalizer
A function that is run after the key becomes unreachable.
- on_quit
Should the finalizer be run when R exits?
Examples
e <- env()
# Create a weak reference to e
w <- new_weakref(e, finalizer = function(e) message("finalized"))
# Get the key object from the weak reference
identical(wref_key(w), e)
#> [1] TRUE
# When the regular reference (the `e` binding) is removed and a GC occurs,
# the weak reference will not keep the object alive.
rm(e)
gc()
#> used (Mb) gc trigger (Mb) max used (Mb)
#> Ncells 1226339 65.5 2266418 121.1 2266418 121.1
#> Vcells 2235258 17.1 8388608 64.0 5255940 40.1
identical(wref_key(w), NULL)
#> [1] TRUE
# A weak reference with a key and value. The value contains data about the
# key.
k <- env()
v <- list(1, 2, 3)
w <- new_weakref(k, v)
identical(wref_key(w), k)
#> [1] TRUE
identical(wref_value(w), v)
#> [1] TRUE
# When v is removed, the weak ref keeps it alive because k is still reachable.
rm(v)
gc()
#> used (Mb) gc trigger (Mb) max used (Mb)
#> Ncells 1226373 65.5 2266418 121.1 2266418 121.1
#> Vcells 2235134 17.1 8388608 64.0 5255940 40.1
identical(wref_value(w), list(1, 2, 3))
#> [1] TRUE
# When k is removed, the weak ref does not keep k or v alive.
rm(k)
gc()
#> used (Mb) gc trigger (Mb) max used (Mb)
#> Ncells 1226395 65.5 2266418 121.1 2266418 121.1
#> Vcells 2235172 17.1 8388608 64.0 5255940 40.1
identical(wref_key(w), NULL)
#> [1] TRUE
identical(wref_value(w), NULL)
#> [1] TRUE