how to define opCast outside class

[Timothee Cour]

yes, that was precisely the case I was concerned about: two types for
which I don't have control over. That's a  common enough case.
My current workaround is to use a
----
T1 to2(T1,T2)(T2 a)
----
which is ugly as it only works for *that* conversion and doesn't play
well with the rest of phobos (eg: won't work recursively, if I want to
convert a vector of A to a vector of B).

I could also reimplement a whole std.conv.to (say, to2) that also
checks for out of class opCast, but I don't see what's so bad about
having it in std.conv.to.

>> This was discussed many times, even recently enhancement request was created in bugzilla

Do you have any link? Thanks!



On Sat, Mar 30, 2013 at 12:21 AM, Jonathan M Davis <jmdavisProg at gmx.com> wrote:
> On Friday, March 29, 2013 23:57:56 Timothee Cour wrote:
>> version(good) works.
>> version(bad) does not.
>> Can we make std.conv.to work for out-of-class opCast?
>> Use case: same reason as UFCS, where we don't wanna define all methods
>> inside class def, leading to possible extensions later.
>> Currently this IS a problem when class A is in a library over which we
>> have no control and a user defined class B wants to cast from A.
>>
>> ----
>> import std.conv;
>> struct B{ int x;}
>> struct A{
>>     int x;
>>     version(good)
>>     auto opCast(T)() {return B(x);}
>> }
>> version(bad)
>> auto opCast(T)(A a) {return B(a.x);}
>> void main(){    auto b=to!B(A.init); }
>> ----
>
> If that's what you want to do, create a constructor on B which takes an A. As
> long as you're in control of one of the types, you can either declare the
> appropriate opCast or the appropriate constructor (which one depending on
> which direction you're trying to do the conversion in and which one you have
> control over). The only time that you're out of luck is when you don't have
> control over the API of either type.
>
> - Jonathan M Davis