Reference versus copy

General Usage
1

I create a simple MWE to demonstrate some kind of reference problem which is leading to unwanted behavior when creating matrices with function elements. Here is the code:

function test()
    zfcts = Any[t -> 0., t -> 0.]
    fcts = Any[t -> sin(t), t -> cos(t)]
    g = Any[]
    push!(g, zfcts)
    push!(g, zfcts)
    g[1][1] = fcts[1]  # sin
    g[2][1] = fcts[2]  # cos
    x = 2.1
    @show g[1][1](x)
    @show (g[2][1](x))
    @show (g[1] === g[2])  # Should  not be true!!!  <<<<<<
    @show (g[1][1] === g[2][1])  # true
    @show (g[1][2] === g[2][2])  # true
    @show (g[1][1] === g[1][2])  # true
    @show (fcts[1] === fcts[2])  # false (as it should be)
end

test()

I expect

g[1][1] = sin(2.1)
g[2][[1] = cos(2.1)

However, the results are always the same, and equal to fcts[2], the second function define in the fcts list. I thought that push! did a copy.

Here is the code output:

> test()
((g[1])[1])(x) = -0.5048461045998576
((g[2])[1])(x) = -0.5048461045998576
g[1] === g[2] = true
(g[1])[1] === (g[2])[1] = true
(g[1])[2] === (g[2])[2] = true
(g[1])[1] === (g[1])[2] = false
fcts[1] === fcts[2] = false
false

As usual, any insight is appreciated. I feel as if I should understand this coming from a C++ background, but I clearly am missing something important about functions and push!. Thanks.

2

Your title implies you know exactly what’s going on here?

g is a vector containing two references to the same vector. When you change the first element of that vector, it changes, no matter which reference you use to access it.

3

My title indicates that I “should” know what is going on.
I just solve the problem (thanks for the push):

push!(g, copy(zfcts))
push!(g, copy(zfcts))

The rest of the code is the same.

4

Where did you read that push! copies? I can’t find that in the documentation. I would try it, but it appears you already have.

Does push!(g, copy(zfcts)) work?

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5

Yes, simply copying the list of functions inside the push! did the trick. Regarding operation of the push, I just assumed. I do not check every little detail in the documentation. I guess it does do a copy: what was copied was the pointer to the beginning of the list of functions (using C++ lingo).

6

Passing arguments in Julia (as for most languages) works just like assignment (=): it binds a
new name to the same object, it does not copy the object. This not a little detail — it is fundamental to how the language works.

(Of course, a function like push! could internally call copy on its argument before storing it in a data structure, but this is the exception rather than the rule.)

(Nor is Julia’s behavior particularly unusual among languages here.)

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