On Sat, Oct 3, 2015 at 6:53 PM, Carlo Lucibello
<[email protected]> wrote:
>
>
> Il giorno sab 3 ott 2015 alle ore 19:56 Yichao Yu <[email protected]> ha
> scritto:
>>
>> On Sat, Oct 3, 2015 at 1:36 PM, Carlo Lucibello
>> <[email protected]> wrote:
>> > Ok, thanks, we are getting closer to what I need, but still...
>> >
>> > julia> a=1
>> > 1
>> > julia> b=Ref(a)
>> > Base.RefValue{Int64}(1)
>> > julia> b[]+=3
>> > 4
>> > julia> a
>> > 1
>> >
>> > while I was expecting a==4
>>
>> This is not allowed. All the assignments has to be explicit and you
>> should either just use `b[]` to replace `a` or update `a` with `a =
>> b[]`.
>
> The pattern I showed in the first post it's not uncommon, and easily
> achieved in C/C++ through pointers/references. Achieving a similar
> functionality in julia, by any way I can think of, would be tedious,
> inelegant and slower, so I think this is a serious lack of the language.I
I don't see why it's "tedious, inelegant and slower"
tedious:
There are way more difference between julia and c++
inelegant:
The Ref type is pretty much like a c pointer or a c++ Ref. I don't
see why `b[]` is so much different from `*b` when you have a pointer
in c
slower:
Quite the opposite. If we change the simple meaning of assignment,
it will immediately kill the performance of any code with any kinds of
type instability
> hope there will be some way in the future to accomadate this use case
Just use a Ref type anywhere you want a c pointer or a variable that
you want to take reference of.
>>
>> >
>> > Il giorno sab 3 ott 2015 alle ore 19:16 Yichao Yu <[email protected]> ha
>> > scritto:
>> >>
>> >> On Sat, Oct 3, 2015 at 12:48 PM, Carlo Lucibello
>> >> <[email protected]> wrote:
>> >> > Those don't seem to be viable solutions. In fact
>> >> >
>> >> > julia> a=2
>> >> > 2
>> >> >
>> >> > julia> b=Ref(a)
>> >> > Base.RefValue{Int64}(2)
>> >> >
>> >> > julia> b += 3
>> >> > ERROR: MethodError: `+` has no method matching
>> >> > +(::Base.RefValue{Int64},
>> >> > ::Int64)
>> >>
>> >> b[] += 3
>> >>
>> >> >
>> >> > and also how do you define a 0-dimensional array?
>> >> >
>> >> > a=Array{Float64,0}()
>> >> > ERROR: MethodError: `convert` has no method matching
>> >> > convert(::Type{Array{Float64,0}})
>> >> > This may have arisen from a call to the constructor
>> >> > Array{Float64,0}(...),
>> >> > since type constructors fall back to convert methods.
>> >> >
>> >> >
>> >> >
>> >> > Il giorno sabato 3 ottobre 2015 18:07:51 UTC+2, Jameson ha scritto:
>> >> >>
>> >> >> the `Ref` type or the single-valued `Array{T,0}` provides this
>> >> >> ability
>> >> >> for
>> >> >> Julia.
>> >> >>
>> >> >> On Fri, Oct 2, 2015 at 5:10 PM Carlo Lucibello
>> >> >> <[email protected]>
>> >> >> wrote:
>> >> >>>
>> >> >>> Hi julians,
>> >> >>> I'd like to emulate the following behaviour from C++
>> >> >>>
>> >> >>> class A{
>> >> >>> public:
>> >> >>> double x;
>> >> >>> }
>> >> >>>
>> >> >>> class B{
>> >> >>> public:
>> >> >>> double& x;
>> >> >>> }
>> >> >>> A a(1);
>> >> >>> B b(a.x);
>> >> >>> a.x=2;
>> >> >>> assert(b.x == 2);
>> >> >>>
>> >> >>> Whilie it would be easy to obtain this behaviour with `composite`
>> >> >>> types,
>> >> >>> since they are passed by referece, for elementary types, such as
>> >> >>> Float64,
>> >> >>> thi is not possible since they have a sort of by vlue semantic
>> >> >>> (yes, I
>> >> >>> know
>> >> >>> it's not exactly like that, but still...)
>> >> >>>
>> >> >>> Greetings,
>> >> >>> Carlo
