# Mutating a flag in a nonmutating struct

**URL:** <https://discourse.julialang.org/t/mutating-a-flag-in-a-nonmutating-struct/132341>\
**Category:** General Usage\
**Tags:** question, internals\
**Created:** [September 13, 2025, 9:41am UTC](https://discourse.julialang.org/t/mutating-a-flag-in-a-nonmutating-struct/132341 "2025-09-13T09:41:45Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![karei](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/karei/32/214809_2.png) [@karei](https://discourse.julialang.org/u/karei)\
**Post date:** [September 13, 2025, 10:13am UTC](https://discourse.julialang.org/t/mutating-a-flag-in-a-nonmutating-struct/132341/2 "2025-09-13T10:13:33Z")

</div>

**Option A** : Use `const` on array fields inside a mutable struct

```julia
mutable struct StochasticSettings{T}
    remove_scha_forces :: Bool
    clean_start_gradient_centroids :: Bool
    clean_start_gradient_fcs :: Bool
    initialized :: Bool
    const original_force :: Array{T}
    const original_fc_gradient :: Array{T}
end

```

**Option B** : Embed a mutable struct inside an immutable wrapper, and flatten fields with ForwardMethods.jl

```julia
using ForwardMethods

mutable struct StochasticSettings
    remove_scha_forces :: Bool
    clean_start_gradient_centroids :: Bool
    clean_start_gradient_fcs :: Bool
    initialized :: Bool
end

struct StochasticParam{T}
    settings :: StochasticSettings
    original_force :: Array{T}
    original_fc_gradient :: Array{T}
end

@define_interface StochasticParam interface = properties delegated_fields = (settings,)

x = StochasticParam(...)
x.remove_scha_forces = true # OK

```

**Option C** : Use `Base.RefValue`

```julia
struct StochasticSettings{T}
    remove_scha_forces :: Base.RefValue{Bool}
    clean_start_gradient_centroids :: Base.RefValue{Bool}
    clean_start_gradient_fcs :: Base.RefValue{Bool}
    initialized :: Base.RefValue{Bool}
    original_force :: Array{T}
    original_fc_gradient :: Array{T}
end

x = StochasticParam(...)
x.remove_scha_forces[] = true

```

A related discussion on using `Base.RefValue` inside immutable structs:  
[https://discourse.julialang.org/t/ref-t-vs-base-refvalue-t/127886](https://discourse.julialang.org/t/ref-t-vs-base-refvalue-t/127886)

**Option D** : Using Accessors.jl

```julia
using Accessors

struct StochasticSettings{T}
    remove_scha_forces :: Bool
    clean_start_gradient_centroids :: Bool
    clean_start_gradient_fcs :: Bool
    initialized :: Bool
    original_force :: Array{T}
    original_fc_gradient :: Array{T}
end

s = StochasticSettings(...)
s2 = @set s.initialized = true
s2.original_force === s.original_force # true

```

The arrays are not deep-copied.

I would choose Option B. But it’s difficult for me to evaluate the performance of different options.

---

_[View the full topic](https://discourse.julialang.org/t/mutating-a-flag-in-a-nonmutating-struct/132341)._