Reference to Objects Very Slow


#1

My actual example is conceptually like this:
A Family has an Array of Children.
Families send their Children to Schools, so each School will have an Array of Children.
I do not want to copy the Children, so the School has an Array of Ref{Child}. Seems to work.
But accessing the Children via these Ref values seems glacial. I must be doing something wrong!!?? Here is a code example

type Foo
  i::Int64
end

function as_array(foos::Array{Foo,1})
  for f in foos
    f.i += 1
  end
end

function with_refs(foos::Array{Ref{Foo},1})
  for f in foos
    f.x.i += 1                   # is this the right way to dereference a Ref? Seems to work...
  end
end

foos = Array{Foo,1}()            # couple of empty arrays
ref_foos = Array{Ref{Foo},1}()

for i in 1:100000                # fill the arrays
  push!(foos, Foo(0))
  push!(ref_foos, Ref(foos[end]))
end

as_array(foos); @time as_array(foos)

with_refs(ref_foos); @time with_refs(ref_foos)

for i in 1:5
  @printf(STDERR, "i = %d foo.i %d %d\n", i, foos[i].i, ref_foos[i].x.i)     # seems to be working
end

When I run this, I get

0.000405 seconds (130 allocations: 7.719 KB)
0.013922 seconds (4 allocations: 160 bytes)

of course my actual task is much longer running, but doing things via a reference is > 30x slower.

Was not able to find help anything like this. In the C++ world, having pointers to objects is quite natural, but how should one go about this in Julia?

Any advice most welcome…

thanks

Ian


#2

What are you trying to do? Foo is already a mutable type so there’s no need to put it in a Ref.

Ref is an abstract type so you’ll get terrible performance when using it as the type of a slot.

No. There’s no way to dereference a Ref in a generic way. For Base.RefValue, use ref[].


#3

I get it, thank you Yuyichao

I can create an Array of Foos, then put those Foo’s in any number of other Arrays, and it is the same Foo item. I had assumed that adding the Foo objects to another Array would make a copy of each item - like what would happen in C++.

Julia is not C++ - too many languages to keep straight!

sorry…

Thank you for helping me, timing looks much better now - code is much simpler too!

Ian