Hi there,

I've been away for a while from the D world, so I guess I missed some of the new things.

Regarding opCast, the documentation says:

"This only happens, however, for instances of structs. Class references are converted to bool by checking to see if the class reference is null or not."

Yet this works:

class B
{
    bool v;

    this(bool v)
    {
        this.v = v;
    }

    T opCast(T)()
    {
        return v;
    }
}

unittest
{
    B bfalse = new B(false);
    B btrue = new B(true);
    assert(cast(bool) bfalse == false);
    assert(cast(bool) btrue == true);
}

Is that a new feature? Can I rely on it? Is it documented somewhere?

Thanks! :-)

Regards,
Luís

On 03/12/2013 01:08 PM, "Luís Marques" <luismarques@gmail.com>" wrote:
> Hi there,
>
> I've been away for a while from the D world, so I guess I missed some of
> the new things.
>
> Regarding opCast, the documentation says:
>
> "This only happens, however, for instances of structs. Class references
> are converted to bool by checking to see if the class reference is null
> or not."
>
> Yet this works:
>
> class B
> {
> bool v;
>
> this(bool v)
> {
> this.v = v;
> }
>
> T opCast(T)()
> {
> return v;
> }
> }
>
> unittest
> {
> B bfalse = new B(false);
> B btrue = new B(true);
> assert(cast(bool) bfalse == false);
> assert(cast(bool) btrue == true);
> }
>
> Is that a new feature? Can I rely on it? Is it documented somewhere?
>
> Thanks! :-)
>
> Regards,
> Luís

I don't know what the intended behavior but there is a distinction between automatic vs. implicit. These pass as well:

    assert(bfalse);
    assert(btrue);

So, apparently implicit conversion considers the class variable and explicit conversion considers the class object. And this produces a compilation error:

    B bnull;
    assert(cast(bool)bnull);

Error: null dereference in function _D6deneme19__unittestL123991_1FZv

Ali

On Tuesday, 12 March 2013 at 20:17:33 UTC, Ali Çehreli wrote:
> On 03/12/2013 01:08 PM, "Luís Marques" <luismarques@gmail.com>" wrote:
> > Hi there,
> >
> > I've been away for a while from the D world, so I guess I
> missed some of
> > the new things.
> >
> > Regarding opCast, the documentation says:
> >
> > "This only happens, however, for instances of structs. Class
> references
> > are converted to bool by checking to see if the class
> reference is null
> > or not."
> >
> > Yet this works:
> >
> > class B
> > {
> > bool v;
> >
> > this(bool v)
> > {
> > this.v = v;
> > }
> >
> > T opCast(T)()
> > {
> > return v;
> > }
> > }
> >
> > unittest
> > {
> > B bfalse = new B(false);
> > B btrue = new B(true);
> > assert(cast(bool) bfalse == false);
> > assert(cast(bool) btrue == true);
> > }
> >
> > Is that a new feature? Can I rely on it? Is it documented
> somewhere?
> >
> > Thanks! :-)
> >
> > Regards,
> > Luís
>
> I don't know what the intended behavior but there is a distinction between automatic vs. implicit. These pass as well:
>
>     assert(bfalse);
>     assert(btrue);
>
> So, apparently implicit conversion considers the class variable and explicit conversion considers the class object. And this produces a compilation error:
>
>     B bnull;
>     assert(cast(bool)bnull);
>
> Error: null dereference in function _D6deneme19__unittestL123991_1FZv
>
> Ali

bool toto = bfalse; // Error: cannot implicitly convert expression (bfalse) of type module.B to bool

So it isn't the implicit cast kickin here, but a 3rd behavior. The kind of behavior that makes D so special and create theses edges cases we all love !

On 03/13/2013 12:41 PM, deadalnix wrote:
> On Tuesday, 12 March 2013 at 20:17:33 UTC, Ali Çehreli wrote:
>> ...
>> So, apparently implicit conversion considers the class variable and
>> explicit conversion considers the class object. And this produces a
>> compilation error:
>>
>>     B bnull;
>>     assert(cast(bool)bnull);
>>
>> Error: null dereference in function _D6deneme19__unittestL123991_1FZv
>>
>> Ali
>
> bool toto = bfalse; // Error: cannot implicitly convert expression
> (bfalse) of type module.B to bool
>
> So it isn't the implicit cast kickin here, but a 3rd behavior. The kind
> of behavior that makes D so special and create theses edges cases we all
> love !

It's an unnecessary special case. assert(objRef); checks whether the object reference is not null and then it checks the object invariant. Walter thinks this is useful.