# Type stability and OffsetArray components of a struct

**URL:** <https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313>\
**Category:** General Usage\
**Tags:** question, struct, code\_warntype, offsetarrays\
**Created:** [August 3, 2025, 5:20am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313 "2025-08-03T05:20:01Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![billmclean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/billmclean/32/3790_2.png) [@billmclean](https://discourse.julialang.org/u/billmclean)\
**Post date:** [August 3, 2025, 5:20am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/1 "2025-08-03T05:20:01Z")

</div>

I found that `@code_warntype` indicated a type instability in a finite difference code. The same issue arises in the following simple example.

```julia-auto
using OffsetArrays

abstract type AbstractGrid end

struct Grid <: AbstractGrid
    x::Vector{Float64}
end 

function Grid(L, N)
    x = range(0, L, N+1)
    return Grid(x)
end 

struct OGrid <: AbstractGrid
    x::OffsetVector{Float64}
end 

function OGrid(L, N)
    x_ = range(0, L, N+1)
    x = OffsetVector(x_, 0:N)
    return OGrid(x)
end 

function average(f::Function, grid::AbstractGrid)
    x = grid.x
    s = 0.0
    for k in eachindex(x)
        s+= f(x[k])
    end 
    return s / length(x)
end

```

If I define

```julia-auto
grid = Grid(2.0, 10)
ogrid = OGrid(2.0, 10)
f(x::Float64)::Float64 = exp(-x) * cos(x)

```

then `average(f, grid)` and `average(f, ogrid)` both return `0.3123517502648151`, as expected. However,

```julia-auto
@code_warntype average(f, grid)

```

shows no type instabilities, whereas

```julia-auto
@code_warntype average(f, ogrid)

```

flags `x`, `s` and `k` in red. Is there a way to avoid this problem when using an `OffsetArray` in a `struct`?

---

<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [August 3, 2025, 5:30am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/2 "2025-08-03T05:30:26Z")

</div>

> [@billmclean](#):
>
> `x::OffsetVector{Float64}`

You need a parameter for the parent array.

```julia-auto
struct OffsetArray{T,N,AA<:AbstractArray{T,N}} <: AbstractArray{T,N}
    parent::AA
    offsets::NTuple{N,Int}
...
end

const OffsetVector{T,AA<:AbstractVector{T}} = OffsetArray{T,1,AA}

```

---

<div class="post-metadata">

**Author:** ![billmclean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/billmclean/32/3790_2.png) [@billmclean](https://discourse.julialang.org/u/billmclean)\
**Post date:** [August 3, 2025, 6:21am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/3 "2025-08-03T06:21:14Z")

</div>

Thanks for the tip. Yes, replacing

```julia-auto
struct OGrid <: AbstractGrid
    x::OffsetVector{Float64}
end 

```

to

```julia-auto
struct OGrid <: AbstractGrid
    x::OffsetVector{Float64, Vector{Float64}}
end 

```

fixed the problem.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [August 4, 2025, 8:46am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/4 "2025-08-04T08:46:24Z")

</div>

I don’t know how well your MWE approximates your actual use case, but I would just point out that by forcing `Grid` and `OGrid` to use `Vector{Float64}` you are converting ranges to fully allocated arrays, which seems a bit wasteful, given that Julia provides the very nice, lightweight range datatypes.

Unless you mostly use non-uniformly spaced grids, isn’t it better to use ranges?

---

<div class="post-metadata">

**Author:** ![billmclean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/billmclean/32/3790_2.png) [@billmclean](https://discourse.julialang.org/u/billmclean)\
**Post date:** [August 5, 2025, 10:54pm UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/5 "2025-08-05T22:54:18Z")

</div>

Yes, for uniform grids it makes sense to save on storage using ranges.

```julia-auto
struct OGrid <: AbstractGrid
    x::OVector{Float64, StepRangeLen{Float64, TwicePrecision{Float64},
               TwicePrecision{Float64}, Int64}}
end

```

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [August 6, 2025, 2:03am UTC](https://discourse.julialang.org/t/type-stability-and-offsetarray-components-of-a-struct/131313/6 "2025-08-06T02:03:33Z")

</div>

if you need to put that into some other data structure and is annoyed by having to type out type parameter, you can try [FHist.jl/src/polybinedges.jl at 26b3c1c9cdd162e4906581316e07e525f29ac328 · Moelf/FHist.jl · GitHub](https://github.com/Moelf/FHist.jl/blob/26b3c1c9cdd162e4906581316e07e525f29ac328/src/polybinedges.jl#L9)