WeakValueDicts

julia> using WeakValueDicts

julia> mutable struct MyStruct
           value::Any
       end

julia> wvd = WeakValueDict()
WeakValueDict{Any,Any} with 0 entries

# Hold an explicit reference to an instance of MyStruct so it wont be GC'd
julia> foo = MyStruct("foo")
MyStruct("foo")

julia> wvd["foo"] = foo
MyStruct("foo")

julia> wvd["foo"]
MyStruct("foo")

# Clear the reference and run the garbage collector
julia> foo = nothing

julia> Base.GC.gc(true)

# We no longer have a foo entry in the dict because it was finalized (GC'd)
julia> wvd["foo"]
ERROR: KeyError: key "foo" not found

Limitations

Note: All values in WeakValueDicts must be mutable due to how Julia implements finalizers. Mutable datatypes include String, Array, and mutable structs but excludes types such as Int and immutable structs (i.e., structs without the mutable keyword).