help cast

Jonathan M Davis newsgroup.d at jmdavisprog.com
Sun Mar 18 20:07:28 UTC 2018


On Sunday, March 18, 2018 14:56:04 Steven Schveighoffer via Digitalmars-d 
wrote:
> On 3/18/18 2:24 PM, Jonathan M Davis wrote:
> > On Sunday, March 18, 2018 13:10:28 Steven Schveighoffer via
> > Digitalmars-d
> >
> > wrote:
> >> On 3/18/18 4:34 AM, sdvcn wrote:
> >>> dchar v11=dchar.max;
> >>>
> >>>       auto vp11 = [v11];
> >>>
> >>>       auto v2 = cast(ubyte[]) (vp11);   //v2.length=4
> >>>       auto v22 = cast(ubyte[])( [v11]); //v2.length=1
> >>
> >> This seems like a bug to me.
> >>
> >> It appears that v22 has truncated v11 to a byte and made only a single
> >> byte array out of it.
> >
> > Except that that's precisely how you usually get an array any integral
> > type smaller than an integer. e.g.
> >
> > auto arr = cast(ubyte[])([1, 2, 3, 4]);
> >
> > In this case, you could do
> >
> > ubyte[] arr = [1, 2, 3, 4];
> >
> > instead, but if you're not dealing with an initializaton or assignment
> > like this (e.g. you're passing the array to a functon), then the cast
> > is the way you do it. Normally, you do it with integer literals, and I
> > could see an argument that it shouldn't allow it without VRP being used
> > to make it work, but it _is_ a cast, and casts are a bit of a blunt
> > instrument.
> >
> > So, I really don't think that it's a bug.
>
> It's quite possible that you aren't understanding what is happening:
>
> ubyte[] arr = cast(ubyte[])[555];
> writeln(arr); // [43]
>
> Why is this not a bug? I didn't cast the 555 to a ubyte, so it should
> either complain that it can't do it, or give me an array of 4 bytes.
>
> I guess it could be explained as the same thing as:
>
> ubyte[] arr = [cast(ubyte)555];
>
> But this is surprisingly weird behavior.

That's exactly what it's doing, and when you have multiple elements in the
literal, it quickly gets a lot more pleasant than casting each element
individually. e.g.

cast(ubyte[])[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

vs

[cast(ubyte)0, cast(ubyte)1, cast(ubyte)2, cast(ubyte)3, cast(ubyte)4,
 cast(ubyte)5, cast(ubyte)6, cast(ubyte)7, cast(ubyte)8, cast(ubyte)9,
 cast(ubyte)10]

I use this trick all the time when creating arrays of integral types smaller
than int, precisely because casting each element is a royal pain and way
harder to read.

- Jonathan M Davis



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