What is the difference between a zero-dimensional array holding the value x, which you construct using fill(x), and a Ref(x)? In both cases you would access the underlying data via [], which can be set or read. And it seems that both store a pointer to the actual data.

One is an array — which means that it supports a bit more arithmetic and such like: julia> fill(1) + fill(2) 3 Ref is about as barebones of a 0-dimensional container as you can get. But just like all 0-dimensional containers, it’ll work nicely with broadcast.

[image] e3c6: What is the difference between a zero-dimensional array holding the value x , which you construct using fill(x) , and a Ref(x) ? julia> typeof(fill('a')) Array{Char,0} julia> typeof(Ref('a')) Base.RefValue{Char} [image] e3c6: In both cases you would access the underlying d…

I typically use them to have a simple (quick and dirty) modifiable field in a struct or for a pass-by-reference argument into a function. I presume there is no real performance difference between using one or the other in such a case?

Ref is an abstract type so can have performance overhead if stored in a struct. julia> struct S s::Ref{Int} end julia> b = S(1) S(Base.RefValue{Int64}(1)) julia> f(b) = b.s[] f (generic function with 1 method) julia> @code_warntype f(b) Variables #self#::Core.Compiler.Const(f…

Array is always a pretty fat jl_array from the runtime, always heap allocated, etc. In contrast, RefValue is almost the same as a mutable struct with a single field plus convenience access functions. This means that the optimizer can see through Ref, and often stack allocate it or refuse to allocat…

I guess one can use struct S s::Base.RefValue{Int} end S(x::Int) = S(Ref(x)) instead. Then julia> @code_warntype f(b) Body::Int64 1 ─ %1 = (Base.getfield)(b, :s)::Base.RefValue{Int64} │ %2 = (Base.getfield)(%1, :x)::Int64 └── return %2

[image] e3c6: I guess one can use […] yep, that’s one level of pointer indirection less than Array. However, mutable struct S s::Int end is still one less level of pointer indirection (and one less allocation).

[image] mbauman: Ref is about as barebones of a 0-dimensional container as you can get. But just like all 0-dimensional containers, it’ll work nicely with broadcast. I think that Refwas hijacked incidentally for this purpose, possibly following this suggestion . Cf …

Just the other day a colleague asked me how to make something behave as a scalar under broadcasting. I showed him Ref and he was suprised that he couldn’t find anything about it in the docs.

Ref doesn’t really have anything to do with broadcasting. It’s just a convenient, short-named zero-dimensional container that doesn’t have the overhead of the Array header. As long as Ref is a zero-dimensional container, I don’t see any conflating of semantics. Now, that view isn’t entirely histo…

I meant that Ref is now the de facto solution for escaping broadcasting. You are of course right, and indeed any 0-dimensional container would do, so in this sense Ref isn’t special, but in practice Ref has assumed this role. It would be somewhat cleaner if we had something like Scalar proposed in…

Ref vs zero-dimensional arrays?

General Usage

nsajko April 12, 2025, 11:42am 13

Made a package to supplant non-FFI uses of Ref:

Topic		Replies	Views
[ANN]: ZeroDimensionalArrays.jl: zero-dimensional arrays/references/boxes Package Announcements package , announcement , array , mutable-structure , ref	24	1166	October 15, 2025
What is Ref? New to Julia broadcasting	26	12823	January 19, 2022
Understanding type Ref{T} New to Julia	9	1171	December 17, 2023
What does Ref on one hand and "aliasing a vector" on the other mean? New to Julia	7	928	April 28, 2021
How to make a struct that contains a reference to other memory? General Usage memory-allocation	12	347	July 20, 2024

Ref vs zero-dimensional arrays?

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