# Immutables with reference-fields: Why boxed?

**URL:** <https://discourse.julialang.org/t/immutables-with-reference-fields-why-boxed/7706>\
**Category:** Internals & Design\
**Created:** [December 12, 2017, 12:44pm UTC](https://discourse.julialang.org/t/immutables-with-reference-fields-why-boxed/7706 "2017-12-12T12:44:22Z")\
**Posts on this page:** 1\
**Showing post:** 7

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**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [December 12, 2017, 5:15pm UTC](https://discourse.julialang.org/t/immutables-with-reference-fields-why-boxed/7706/7 "2017-12-12T17:15:13Z")

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> Probably fixed by [fix :typeinfo with views (closes #25038) by rfourquet · Pull Request #25040 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/25040)

That’s fast, wow.

Another very annoying fact is that all the new allocation optimizations appear to work only when inlined. That is (sorry for the silly example):

```julia
@inline foo_in(n) = (n, Vector{Int}(n))
@noinline foo_ni(n) = (n, Vector{Int}(n))

function ft_in(n)
       s= 0
       for i= 1:n
        jj,v = foo_in(i)
       s+=sum(v)
       end
       s
end

function ft_ni(n)
       s= 0
       for i= 1:n
        jj,v = foo_ni(i)
       s+=sum(v)
       end
       s
end

@time ft_in(1000)
  0.001948 seconds (1.00 k allocations: 3.962 MiB)

@time ft_ni(1000)
  0.002083 seconds (2.00 k allocations: 3.992 MiB)

```

This means that multiple return values, some of which are not bitstype, are only performant when inlined.

This means that one needs to decide on API design whether to @inline, or make the API look like old-style C by demanding horrible `Ref{Int}` inputs for writing the output (where the caller is hopefully re-using the `Ref{Int}`). And also hope that the caller is re-using the Ref{Int} often enough to not cause additional cache-misses (because it can’t point to the stack, as in C).

I mean, we have a really nice ABI for returning bitstype-structs, please use it for immutables containing ref-fields as well!

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_[View the full topic](https://discourse.julialang.org/t/immutables-with-reference-fields-why-boxed/7706)._