# Make mutable struct read-only in local scope **URL:** \ **Category:** General Usage\ **Created:** [November 4, 2024, 5:55pm UTC](https://discourse.julialang.org/t/make-mutable-struct-read-only-in-local-scope/122244 "2024-11-04T17:55:11Z")\ **Posts on this page:** 2\ **Page:** 1

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\ **Post date:** [November 4, 2024, 5:55pm UTC](https://discourse.julialang.org/t/make-mutable-struct-read-only-in-local-scope/122244/1 "2024-11-04T17:55:11Z")

Is there a way to make a mutable-struct read-only in a given scope? (I don’t need that to be strictly impossible, just not user-friendly). Something like having: ```julia struct A x::Vector{Int} end ``` and making it hard to use `setindex` on `x` of an instance of `A`. The use case is having a callback function in an algorithm, and in this callback function the user would have access to variables which he/she should be able to read, but not to modify. (I guess this is mostly the case of all callback functions in optimization algorithms - and we just trust the users to not modify the data). I imagined the possibility of creating a custom array type to wrap `x`, and thus expose ```julia struct A x::ReadOnlyArray{Vector{Int}} end ``` without defining `setindex` for `ReadOnlyArray`. (now that I wrote the name it feels that someone invented this already…). But implementing an array interface seems a bit too much. ---

> [@lmiq](#): > > (now that I wrote the name it feels that someone invented this already…) Yeah, there it is: [GitHub - JuliaArrays/ReadOnlyArrays.jl: A wrapper type around AbstractArray that is read-only](https://github.com/JuliaArrays/ReadOnlyArrays.jl)