# Use of Base.@propagate\_inbounds in OffsetArrays.jl

**URL:** <https://discourse.julialang.org/t/use-of-base-propagate-inbounds-in-offsetarrays-jl/41313>\
**Category:** General Usage\
**Tags:** question\
**Created:** [June 13, 2020, 4:03am UTC](https://discourse.julialang.org/t/use-of-base-propagate-inbounds-in-offsetarrays-jl/41313 "2020-06-13T04:03:41Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [June 13, 2020, 4:03am UTC](https://discourse.julialang.org/t/use-of-base-propagate-inbounds-in-offsetarrays-jl/41313/1 "2020-06-13T04:03:41Z")

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I am trying to understand how `Base.@propagate_inbounds` works by studying its use in `OffsetArrays.jl`. Currently `setindex!` is implemented as

```julia
@propagate_inbounds function Base.setindex!(A::OffsetArray{T,N}, val, I::Vararg{Int,N}) where {T,N}
    @boundscheck checkbounds(A, I...)
    J = @inbounds map(parentindex, axes(A), I)
    @inbounds parent(A)[J...] = val
    val
end

```

However the use of `@propagate_inbounds` here seems unnecessary to me, as the underlying parent array is indexed with `@inbounds` anyway. Am I correct in this, and would it be the same if the `@propagate_inbounds` macro is removed from the function definition?

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<div class="post-metadata">

**Author:** ![fredrikekre](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fredrikekre/32/1688_2.png) [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)\
**Post date:** [June 13, 2020, 8:24am UTC](https://discourse.julialang.org/t/use-of-base-propagate-inbounds-in-offsetarrays-jl/41313/2 "2020-06-13T08:24:11Z")

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It disables the `@boundscheck` line if the caller used `@inbounds`:

```julia
julia> using OffsetArrays

julia> o = OffsetVector([1, 2, 3], -1);

julia> function f(o)
           @inbounds o[-1] = 1
           return o
       end
f (generic function with 1 method)

julia> function g(o)
           o[-1] = 1
           return o
       end
g (generic function with 1 method)

julia> f(o)
3-element OffsetArray(::Array{Int64,1}, 0:2) with eltype Int64 with indices 0:2:
 1
 2
 3

julia> g(o)
ERROR: BoundsError: attempt to access 3-element OffsetArray(::Array{Int64,1}, 0:2) with eltype Int64 with indices 0:2 at index [-1]
Stacktrace:
 [1] throw_boundserror(::OffsetArray{Int64,1,Array{Int64,1}}, ::Tuple{Int64}) at ./abstractarray.jl:537
 [2] checkbounds at ./abstractarray.jl:502 [inlined]
 [3] setindex! at /home/fredrik/.julia/packages/OffsetArrays/JNUm0/src/OffsetArrays.jl:164 [inlined]
 [4] g(::OffsetArray{Int64,1,Array{Int64,1}}) at ./REPL[4]:2
 [5] top-level scope at REPL[6]:1

```

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<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [June 13, 2020, 10:50am UTC](https://discourse.julialang.org/t/use-of-base-propagate-inbounds-in-offsetarrays-jl/41313/3 "2020-06-13T10:50:46Z")

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Isn’t this what `@inbounds` is supposed to do anyway if the function gets inlined? I understood `@propagate_inbounds` to push the `@inbounds` down one layer, but in this case the inner function is called with bounds checking disabled anyway, so propagating the `@inbounds` should have no effect.

In other words, I understand if there’s an `@inline` here, but the `@propagate_inbounds` seems unnecessary. Is there something that I’m missing?