undef initializer with custom array type?

[MilesCranmer]

What is the right way to initialize an empty StructArray with a custom array type without multiple allocations?

I tried this but no luck:

julia> StructArray{ComplexF64, 1, @NamedTuple{re::Vector{Float64},im::Vector{Float64}}}(undef, 5)
ERROR: MethodError: no method matching (StructVector{ComplexF64, @NamedTuple{re::Vector{Float64}, im::Vector{Float64}}})(::UndefInitializer, ::Int64)
The type `StructVector{ComplexF64, @NamedTuple{re::Vector{Float64}, im::Vector{Float64}}}` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  (StructArray{T, N, C})(::Any) where {T, N, C<:Union{Tuple, NamedTuple}}
   @ StructArrays ~/.julia/packages/StructArrays/n5wxA/src/structarray.jl:16

The current example in the docs implies you would have to initialize each array manually:

julia> using StructArrays, StaticArrays

julia> x = StructArray([SVector{2}(1,2) for i = 1:5])
5-element StructArray(::Vector{Tuple{Int64, Int64}}) with eltype SVector{2, Int64}:
 [1, 2]
 [1, 2]
 [1, 2]
 [1, 2]
 [1, 2]

but ideally I wouldn't need to make these extra allocations.

I'm interested in using this in particular in combination with FixedSizeArrays.jl. @giordano @nsajko any ideas?

[aplavin]

Not sure I understand, can you show what's the desired output?
You can do StructArray(a=YourArray(...), b=YourArray(...)), and those arrays would be used as-is.

[MilesCranmer]

Maybe I have a misunderstanding of StructArrays.jl. For StructArray{ComplexF64}(undef, 5) does it create two allocations or just one (i.e., representing them both in a single Memory object)?

[MilesCranmer]

Answering your question: I'm trying to create a single StructArray, backed by FixedSizeArrays, which all live on a single Memory. I'd like to do this with a single StructArray{ComplexF64,1, ... FixedSizeArrays{ComplexF64}}(undef, 5) or something.

I mean. Maybe I should be swapping the order of these and wrapping StructArray with FixedSizeArray?

[aplavin]

I don't know how to create arrays/fixedsizearrays on the same memory, but assuming you know how to do that – everything else should be straightforward:

re, im = ... create two arrays on the same memory ...
StructArray{ComplexF64}(; re, im)

[MilesCranmer]

Ok, thanks. I think I just misinterpreted the implementation here, I had thought it was doing memory tricks under the hood