WeakKeyIdDicts

Implements one public type, WeakKeyIdDict. It constructs a hash table where the keys are weak references to objects that may be garbage collected even when referenced in a hash table. Like WeakKeyDict it only supports keys that are mutable objects (all objects satisfying ismutable(obj) == true, such as Strings, Arrays and objects defined with mutable struct). Like IdDict the keys are hashed by objectid.

A dictionary with weak keys is useful when we wish to store information about an object that survives only for the life-time of that object, usually to avoid memory leaks. For instance, if we wish to cache some hard-to-compute value associated with the keys of the dictionary.

_tmp_key = [1]
wkd = WeakKeyIdDict(_tmp_key => 1)
let tmp = [42]
    wkd[tmp] = 2
    @show length(wkd) # 2
end
# at this point there is no strong reference left to the vector [42]
# previously reachable via tmp
GC.gc(true)

@show length(wkd) # 1

Avoid assuming a key exists in a WeakKeyIdDict, especially when allocation occurs. Keys can be removed any time garbage collection occurs. Favor the methods get! and get over getindex.

Credits

Much of the credit for the implementation of WeakKeyIdDict goes to Mauro Werder (@mauro3) for the initial work in the julia repository, and DataStructures. Max Horn (@fingolfin) then updated and debugged a version merged to AbstractAlgebra.

The version here adds a few additional tests for edge-cases, fixing related bugs along the way, using code from some of the IdDict tests from Base julia.