# How does \`Base.ImmutableDict\` work internally?
**URL:** https://discourse.julialang.org/t/how-does-base-immutabledict-work-internally/74117
**Category:** General Usage
**Tags:** dictionaries
**Created:** [January 6, 2022, 2:12am UTC](https://discourse.julialang.org/t/how-does-base-immutabledict-work-internally/74117 "2022-01-06T02:12:41Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@wmanning-cra](https://discourse.julialang.org/u/wmanning-cra)
#### Post date: [January 6, 2022, 2:12am UTC](https://discourse.julialang.org/t/how-does-base-immutabledict-work-internally/74117/1 "2022-01-06T02:12:41Z")
I just discovered `Base.ImmutableDict`, and it appears to be a very strange type. I get the core concept of a linked list of key-value pairs. But the reference to the next node is of type `Base.ImmutableDict{K, V}`. How can an immutable struct contain another instance of itself?
I tried defining a similar struct in the REPL, and to my shock, it was allowed. But I still can’t figure out how to construct an instance of it. The `sizeof()` my struct implies that the self-referential field is 8 bytes, which might mean that it’s stored as a pointer.
Can self-referential immutable structs actually be utilized in normal Julia code, or is there some compiler magic involved with this type?
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### Author:  [@dilumaluthge](https://discourse.julialang.org/u/dilumaluthge)
#### Post date: [January 6, 2022, 2:20am UTC](https://discourse.julialang.org/t/how-does-base-immutabledict-work-internally/74117/2 "2022-01-06T02:20:16Z")
[https://docs.julialang.org/en/v1/manual/constructors/#Incomplete-Initialization](https://docs.julialang.org/en/v1/manual/constructors/#Incomplete-Initialization)