When will a vector of structs be contiguous in memory?

General Usage Performance
2

Maybe you can try Pair{V, Union{Nothing,Vector{V}}} or a similar named tuple instead of a tuple

julia> Base.allocatedinline(Tuple{Int,Union{Vector{Int},Nothing}})
false

julia> Base.allocatedinline(Pair{Int,Union{Vector{Int},Nothing}})
true

julia> Base.allocatedinline(@NamedTuple{first::Int,second::Union{Vector{Int},Nothing}})
true

julia> sizeof(Pair{Int,Union{Vector{Int},Nothing}})
16

Using a null pointer would mean using an undef field. However, you don’t get inlined allocation with this approach

julia> struct P{K,V}
           a::K
           b::V
           P{K,V}(a, b) where {K,V} = new{K,V}(a,b)
           P{K,V}(a) where {K,V} = new{K,V}(a)
       end

julia> P{Int,Vector{Int}}(1)
P{Int64, Vector{Int64}}(1, #undef)

julia> Base.allocatedinline(P{Int,Vector{Int}})
false
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