# Allocations when assigning Static Array in mutable struct

**URL:** <https://discourse.julialang.org/t/allocations-when-assigning-static-array-in-mutable-struct/77508>\
**Category:** New to Julia\
**Tags:** memory-allocation, staticarrays, allocations\
**Created:** [March 7, 2022, 1:51pm UTC](https://discourse.julialang.org/t/allocations-when-assigning-static-array-in-mutable-struct/77508 "2022-03-07T13:51:39Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ncxst](https://avatars.discourse-cdn.com/v4/letter/n/e47774/32.png) [@ncxst](https://discourse.julialang.org/u/ncxst)\
**Post date:** [March 7, 2022, 1:51pm UTC](https://discourse.julialang.org/t/allocations-when-assigning-static-array-in-mutable-struct/77508/1 "2022-03-07T13:51:39Z")

</div>

Hi,  
I encountered some allocations in my code that I can pin down to the following foobar example.  
When assigning a static array to a variable in a struct i get allocations, which I don’t get when the static array gets allocated to a variable outside of a struct.

```julia
julia> mutable struct Foo
           m ::SMatrix{3,2,Float64,6}
           v ::SVector{3,Float64}
       end

julia> bar = Foo([1.,2,3,4,5,6], [1.,2,3])
Foo([1.0 4.0; 2.0 5.0; 3.0 6.0], [1.0, 2.0, 3.0])

julia> @benchmark bar.m = f()
BenchmarkTools.Trial: 10000 samples with 973 evaluations.
 Range (min … max): 66.960 ns … 3.199 μs ┊ GC (min … max): 0.00% … 96.51%
 Time (median): 73.921 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 83.220 ns ± 132.248 ns ┊ GC (mean ± σ): 8.16% ± 5.00%

    ▁█▆▆▄▂
  ▃▅██████▅▃▃▃▃▂▂▂▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▁▂▂▁▂▂▁▁▂▂▂▁▁▁▂▁▁▁▂▂▂▂▂▂▂▂▂▂▂ ▃
  67 ns Histogram: frequency by time 145 ns <

 Memory estimate: 128 bytes, allocs estimate: 2.

julia> @benchmark m = f()
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max): 0.025 ns … 0.265 ns ┊ GC (min … max): 0.00% … 0.00%
 Time (median): 0.027 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 0.027 ns ± 0.003 ns ┊ GC (mean ± σ): 0.00% ± 0.00%

             ▂ █ ▁
  ▂▁▁▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▁▁▄▁▁▁▁▁▁▁▁▁▁▂ ▂
  0.025 ns Histogram: frequency by time 0.03 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia> g() = SVector(1.,2.,3.)
g (generic function with 1 method)
julia> @benchmark bar.v = g()
BenchmarkTools.Trial: 10000 samples with 963 evaluations.
 Range (min … max): 60.506 ns … 2.470 μs ┊ GC (min … max): 0.00% … 95.78%
 Time (median): 67.485 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 73.114 ns ± 81.647 ns ┊ GC (mean ± σ): 3.95% ± 3.46%

  ▆▇▅▃▇█▇█▆▇▇▆▆▅▄▃▂▂▁▁▁▁▁ ▂▁▁ ▁▃▂▁▂▂ ▁ ▃
  █████████████████████████▇▇█████▇▆▆▁▃▆███████▇██▇▆▆▇█▇▅▆▇▇▇ █
  60.5 ns Histogram: log(frequency) by time 115 ns <

 Memory estimate: 64 bytes, allocs estimate: 2.
julia> @benchmark v = g()
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max): 0.024 ns … 0.049 ns ┊ GC (min … max): 0.00% … 0.00%
 Time (median): 0.027 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 0.027 ns ± 0.001 ns ┊ GC (mean ± σ): 0.00% ± 0.00%

                    ▅ █
  ▂▁▁▁▁▁▁▁▁▄▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▁█▁▁▁▁▁▁▁▁▃▁▁▁▁▁▁▁▁▂ ▂
  0.024 ns Histogram: frequency by time 0.03 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

```

Can you explain me where the allocation come from and how I can get rid off them?

Thank you,  
Nico

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<div class="post-metadata">

**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:** [March 7, 2022, 2:01pm UTC](https://discourse.julialang.org/t/allocations-when-assigning-static-array-in-mutable-struct/77508/2 "2022-03-07T14:01:14Z")

</div>

Probably these are benchmarking artifacts because you are not interpolating the variables:

```julia
julia> mutable struct Foo
           m ::SMatrix{3,2,Float64,6}
           v ::SVector{3,Float64}
       end

julia> foo = Foo(rand(SMatrix{3,2,Float64,6}),rand(SVector{3,Float64}))
Foo([0.8724518857438199 0.3632403606484935; 0.8892847298840123 0.12143759712364588; 0.7082712449691183 0.7665415510363147], [0.1758060685606172, 0.7266211355083249, 0.33830992545256433])

julia> @btime foo.v = x # wrong
  47.284 ns (2 allocations: 64 bytes)
3-element SVector{3, Float64} with indices SOneTo(3):
 0.982401257786785
 0.9164914648537852
 0.5761051073188066

julia> @btime $(foo.v) = $x # correct
  1.640 ns (0 allocations: 0 bytes)
3-element SVector{3, Float64} with indices SOneTo(3):
 0.982401257786785
 0.9164914648537852
 0.5761051073188066

julia> f() = @SVector [1., 2., 3.];

julia> @btime $(foo.v) = f()
  0.022 ns (0 allocations: 0 bytes)
3-element SVector{3, Float64} with indices SOneTo(3):
 1.0
 2.0
 3.0

```