bearophile can say "i told you so" (re uint->int implicit conv)

[H. S. Teoh]

On Thu, Mar 28, 2013 at 09:03:07PM +0100, Adam D. Ruppe wrote:
> I was working on a project earlier today that stores IP addresses in
> a database as a uint. For some reason though, some addresses were
> coming out as 0.0.0.0, despite the fact that if(ip == 0) return; in
> the only place it actually saves them (which was my first attempted
> quick fix for the bug).
> 
> Turns out the problem was this:
> 
> if (arg == typeid(uint)) {
> 	int e = va_arg!uint(_argptr);
> 	a = to!string(e);
> }
> 
> 
> See, I copy/pasted it from the int check, but didn't update the type
> on the left hand side. So it correctly pulled a uint out of the
> varargs, but then assigned it to an int, which the compiler accepted
> silently, so to!string() printed -blah instead of bigblah... which
> then got truncated by the database, resulting in zero being stored.

IMO, the compiler should insert bounds checks in non-release mode when
implicitly converting between signed and unsigned.

Also, I don't like repeating types, precisely for this reason; if that
second line had been written:

	auto e = va_arg!uint(_argptr);

then this bug wouldn't have happened. But once you repeat 'uint' twice,
there's the risk that you'll forget to update both instances when
changing/copying the code. DRY is a good principle to live by when it
comes to coding.


> I've since changed it to be "auto e = ..." and it all works
> correctly now.

Yep! :)


> Anyway I thought I'd share this just because one of the many times
> bearophile has talked about this as a potentially buggy situation, I
> was like "bah humbug"... and now I've actually been there!
> 
> I still don't think I'm for changing the language though just
> because of potential annoyances in other places unsigned works (such
> as array.length) but at least I've actually felt the other side of
> the argument in real world code now.

Maybe it's time to introduce cast(signed) or cast(unsigned) to the
language, as bearophile suggests?


T

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