On Sunday 10 July 2011 22:15:05 Simen Kjaeraas wrote:
> On Sun, 10 Jul 2011 21:39:54 +0200, Jonathan M Davis <jmdavisp...@gmx.com>
>
> wrote:
> > On Sunday 10 July 2011 20:47:04 David Nadlinger wrote:
> >> On 7/10/11 3:29 PM, Simen Kjaeraas wrote:
> >> > I'm trying to have this sort of code compile:
> >> >
> >> > ////////////////
> >> > struct Foo {
> >> > int data;
> >> > this(int n) {
> >> > data = n;
> >> > }
> >> > }
> >> >
> >> > void bar(Foo f) {}
> >> >
> >> > bar(3);
> >> > ////////////////
> >> >
> >> > Is this even possible?
> >>
> >> If I remember correctly, Walter said that this is a deliberate
> >> limitation the last time I asked (when implementing my units of
> >> measurement library). I think the intention is to avoid confusion
> >> about
> >> what is actually going on behind the scenes, like it is possible in
> >> C++
> >> with implicitly called constructors.
> >
> > Yes. In C++, the compiler will do up to 3 conversions for you, so you
> > can end
> > up calling a very different function than you might have intended to.
> > It's
> > usually not a problem, but it can definitely lead to issues and can
> > effectively
> > be a form of function hijacking. So, D forces you to do the conversion
> > yourself by constructing a temporary of the appropriate type or doing a
> > cast
> > or whatnot. I expect that if there is anything that allows you to get
> > away
> > with it (such as what Bearophile did), it's a bug.
> >
> > The only exceptions to this are things which would allow for an implicit
> > cast
> > (such as alias this). There has been talk from time to time of adding an
> > opImplicitCast, which would allow you to define when this sort of thing
> > coudl
> > happen (at least when you've defined the type being passed in), but it's
> > never
> > been added to the language, and I think that alias this is pretty much
> > supposed to solve that problem, so I don't expect that it ever will.
>
> So then the idea of creating typedef as has been described before (and as
> was part of the argument for removing the feature) as a template is
> impossible.
Well, if you can do it with alias this, then it's possible. If not, then it
probably isn't. But I don't know what opImplicitCast would give you if alias
this doesn't (particularly if alias this is working properly; the current
implementation is buggy enough that whether it works now or not doesn't
necessarily mean much). I really don't know what would be required to get a
TypeDef template working properly though. I don't really see any need for such
a beast personally, so I haven't really looked into it.
Regardless, implicitly casting from one type to another is typically verboten
in D in order to avoid bugs. There are exceptions - particularly with built-in
types - and alias this is intended to give you the ability to have implicit
casting on some level, but it's definitely something that causes hijacking in
C++, so D went for a stricter route which generally disallows it.
- Jonathan M Davis
