I've got a struct and it would be very convenient if I could specify what happens when I write `if(value)` - is this possible?

On 09/10/2016 04:10 PM, pineapple wrote:
> I've got a struct and it would be very convenient if I could specify
> what happens when I write `if(value)` - is this possible?

`if (value)` implies a cast to bool. Define opCast!bool and it gets called:

----
struct S
{
    bool opCast(T : bool)() { return true; }
}
void main()
{
    S value;
    import std.stdio: writeln;
    if (value) writeln("yup");
}
----

On Saturday, 10 September 2016 at 14:24:23 UTC, ag0aep6g wrote:
> On 09/10/2016 04:10 PM, pineapple wrote:
>> I've got a struct and it would be very convenient if I could specify
>> what happens when I write `if(value)` - is this possible?
>
> `if (value)` implies a cast to bool. Define opCast!bool and it gets called:
>
> ----
> struct S
> {
>     bool opCast(T : bool)() { return true; }
> }
> void main()
> {
>     S value;
>     import std.stdio: writeln;
>     if (value) writeln("yup");
> }
> ----

Huh, I could've sworn that some time ago I tried that and it didn't work. Was this a recent addition to the language?

On 09/10/2016 04:29 PM, pineapple wrote:
> Was this a recent addition to the language?

I don't think so.

On Saturday, September 10, 2016 14:29:33 pineapple via Digitalmars-d-learn wrote:
> On Saturday, 10 September 2016 at 14:24:23 UTC, ag0aep6g wrote:
> > On 09/10/2016 04:10 PM, pineapple wrote:
> >> I've got a struct and it would be very convenient if I could
> >> specify
> >> what happens when I write `if(value)` - is this possible?
> >
> > `if (value)` implies a cast to bool. Define opCast!bool and it
> > gets called:
> >
> > ----
> > struct S
> > {
> >
> >     bool opCast(T : bool)() { return true; }
> >
> > }
> > void main()
> > {
> >
> >     S value;
> >     import std.stdio: writeln;
> >     if (value) writeln("yup");
> >
> > }
> > ----
>
> Huh, I could've sworn that some time ago I tried that and it didn't work. Was this a recent addition to the language?

No. That's how it's been for years now - certainly well before TDPL was released - and I suspect that it was that way in D1 (though I've never really used D1, so I'm not sure).

- Jonathan M Davis

On 9/10/16 6:47 PM, Jonathan M Davis via Digitalmars-d-learn wrote:
>
> No. That's how it's been for years now - certainly well before TDPL was
> released - and I suspect that it was that way in D1 (though I've never
> really used D1, so I'm not sure).

It's not that way in D1. I think it was added around the time operator overloads were changed to templates.

Combined with changelog search and this hint, I think this is correct:

https://github.com/dlang/dlang.org/commit/b60bf3881ccad5e09cfb6e773c3927637ecca70c#diff-87ae512b433ac9f86b715f03fa17cb0e

Version 2.041 added it.

-Steve