# Initialise a dict in a mutually circular type
**URL:** https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476
**Category:** New to Julia
**Created:** [June 15, 2020, 10:32pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476 "2020-06-15T22:32:03Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@nacnudus](https://discourse.julialang.org/u/nacnudus)
#### Post date: [June 15, 2020, 10:32pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/1 "2020-06-15T22:32:03Z")
I need to define two mutually circular types. I have read this [issue](https://github.com/JuliaLang/julia/issues/269), which suggests using a forward declaration. That works. However, I also need to contain the types in `Dict`s, which must be initialised.
```nohighlight
abstract type AbstractB end
mutable struct A
b::AbstractB
A() = new()
end
mutable struct B <: AbstractB
a::A
adict::Dict{String, A}
B() = new()
end
a = A()
b = B()
# This works
a.b = b
b.a = a
# But this gives an error, because the dictionary wasn't initialised.
b.adict["1"] = a
# ERROR: UndefRefError: access to undefined reference
```
This time, I use an inner constructor to initialise the `Dict` to an empty dictionary. It seems to recursively call the circular types, even though the dictionary is empty.
```nohighlight
# NEW SESSION =========================
# This time, initialize the dictionary with an empty dictionary.
abstract type AbstractB end
mutable struct A
b::AbstractB
stringdict::Dict{String, AbstractB}
A() = new(B(), Dict{String, B}())
end
mutable struct B <: AbstractB
a::A
adict::Dict{String, A}
B() = new(A(), Dict{String, A}())
end
a = A() # StackOverflowError. Why, given that the dict has zero entries?
```
Is there a way around this? I’d really like to use the types, because these structures will be used a lot.
---
### Author:  [@anon94023334](https://discourse.julialang.org/u/anon94023334)
#### Post date: [June 15, 2020, 10:36pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/2 "2020-06-15T22:36:58Z")
> [@nacnudus](#):
>
> `Why, given that the dict has zero entries?`
Because your constructor for `A()` calls `B()`, which is the inner constructor for `B`, which calls `A()`, the inner constructor for which calls `B()`,…
---
### Author:  [@nacnudus](https://discourse.julialang.org/u/nacnudus)
#### Post date: [June 15, 2020, 10:40pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/3 "2020-06-15T22:40:46Z")
Thanks. I’d like to not initialise `a` or `b`, but that doesn’t seem to be possible. I tried an inner constructor, but it had the original `UndefRefError` problem :
```nohighlight
function B()
self = new()
self.adict = Dict{String, A}()
self
end
```
---
### Author:  [@anon94023334](https://discourse.julialang.org/u/anon94023334)
#### Post date: [June 15, 2020, 10:42pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/4 "2020-06-15T22:42:13Z")
One possible workaround is to make `B.a` a union of `A, Nothing` and have the default constructor set `a` to `nothing`.
Also, [this might help](https://docs.julialang.org/en/v1/manual/constructors/#Incomplete-Initialization-1).
---
### Author:  [@nacnudus](https://discourse.julialang.org/u/nacnudus)
#### Post date: [June 15, 2020, 10:54pm UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/5 "2020-06-15T22:54:30Z")
That works. Also I just realised you’re the author of LightGraphs, which inspired me to try Julia. Thank you on both counts!
I had read that link before, and it lost me at “As with incomplete objects returned from constructors, if `complete_me` or any of its callees try to access the `data` field of the `Lazy` object before it has been initialized, an error will be thrown immediately.” It seems to recommend delegating completion to a function that isn’t allowed to access the incomplete data field.
---
### Author:  [@anon94023334](https://discourse.julialang.org/u/anon94023334)
#### Post date: [June 16, 2020, 5:40am UTC](https://discourse.julialang.org/t/initialise-a-dict-in-a-mutually-circular-type/41476/6 "2020-06-16T05:40:52Z")
> [@nacnudus](#):
>
> Also I just realised you’re the author of LightGraphs, which inspired me to try Julia.
Thanks very much for this. I hope it’s working for you.
> [@nacnudus](#):
>
> I had read that link before, and it lost me at “As with incomplete objects returned from constructors, if `complete_me` or any of its callees try to access the `data` field of the `Lazy` object before it has been initialized, an error will be thrown immediately.”
I wouldn’t worry too much about that part. Basically, it just means this:
```julia
julia> mutable struct A
x::String
A() = new()
end
julia> z = A()
A(#undef)
julia> z.x
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getproperty(::A, ::Symbol) at ./Base.jl:33
julia> z.x = "hello, world!"
"hello, world!"
julia> z
A("hello, world!")
```
(Note that if `x` is an `Integer` or some other primitive (?) type, it will receive an invalid, as opposed to undefined, value:
```julia
julia> mutable struct A
x::Int
A() = new()
end
julia> z = A()
A(140069649368400)
```
)