Performance of structs with incompletely parameterized fields

General Usage Performance
2

In general, “it depends” - you could incur a lot of overhead (particularly in a hot loop), and you’d be making life harder for the compiler, but the flexibility may be worth it if runtime isn’t critical. The only way to answer the question of performance penalty for your particular application is through benchmarks, but just to give you a rough idea, compare:

julia> abstract type GenericContainer end

julia> struct MyGenericArrayStruct{T} <: GenericContainer
          val::Array{T}
       end

julia> struct MyGenericVectorStruct{T} <: GenericContainer
          val::Array{T,1}
       end

julia> total(g::GenericContainer) = sum(g.val)
total (generic function with 1 method)

julia> using BenchmarkTools

julia> mgas = [MyGenericArrayStruct(rand(3)) for i = 1:10^6];

julia> mgvs = [MyGenericVectorStruct(rand(3)) for i = 1:10^6];

julia> @btime mapreduce(total, +, $mgas)
  63.524 ms (1999999 allocations: 30.52 MiB)
1.5007591209103481e6

julia> @btime mapreduce(total, +, $mgvs)
  6.469 ms (0 allocations: 0 bytes)
1.499610642683319e6
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