# Question about allocations and generators

**URL:** <https://discourse.julialang.org/t/question-about-allocations-and-generators/62693>\
**Category:** Performance\
**Tags:** question, memory-allocation, generator\
**Created:** [June 10, 2021, 6:39pm UTC](https://discourse.julialang.org/t/question-about-allocations-and-generators/62693 "2021-06-10T18:39:01Z")\
**Posts on this page:** 1\
**Showing post:** 3

<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:** [June 11, 2021, 7:51pm UTC](https://discourse.julialang.org/t/question-about-allocations-and-generators/62693/3 "2021-06-11T19:51:08Z")

</div>

Note that all allocations come from not knowing the size of the output at compile time. If the size of the static vector to be created is known at compile time, the function does not allocate. This can be shown here, for example, in this case, where the size of the output is provided as a parameter of the type, even if the actual vector that the type contains is not static:

```julia
julia> struct Vec{N}
         v::Vector{Int}
       end

julia> g4(v::Vec{N}) where N = SVector{N,Int}(v.v[i] for i in 1:N)
g4 (generic function with 1 method)

julia> @btime g4(v) setup=(v=Vec{3}(rand(Int,3)))
  2.189 ns (0 allocations: 0 bytes)

```

(which means, more generally, that creating static vectors generally does not allocate if you can provide to the function where the allocations occur the size of the vectors as one parameter of the input types).

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