Why is Julia allocating when accessing these structs?

[mikmoore]

@dellagd

struct AllocationsStruct
    arr::Array{SubStruct}
end

Array{T} is not a concrete type. Array{T,1} (or equivalently Vector{T}) is. You are missing this type parameter, so the field is abstractly typed.

In your second, parametric version you almost get there except you write

@dellagd

return AllocationsStruct{Array{SubStruct{Float64}}}(subStructArr)

If you’d instead written return AllocationsStruct(subStructArr), the type would have been correctly inferred.

But the preceding comment about type stability after s = 0 also holds.