On Monday, 17 February 2014 at 04:40:52 UTC, Jonathan M Davis
wrote:
On Sunday, February 16, 2014 21:35:01 Hannes Steffenhagen wrote:
isImplicitlyConvertible!(int,ulong) is true. Maybe this is just
me, but I get the impression that this is quite nuts. Why is an
implicit conversion from a signed to an unsigned type possible?
The other way round would be at least somewhat understandable
if
there's a static check that the values actually fit.
signed to unsigned conversions (and vice versa) are implicit as
are
conversions from smaller integral types to larger integral
types. Converting
from smaller integral types to larger really doesn't cause any
problems, but
the signed to unsigned (or vice versa) can cause issues - one
of the biggest
of those being the comparison of signed and unsigned values,
and IIRC, there
was some discussion on making that a warning or error. However,
while there
are occasional problems from the conversion being implicit, if
it weren't
implicit, you'd be forced to cast a lot more when the signed
and unsigned
types interact, which would lead to messier code and could
actually increase
the number of bugs, because if you got in the habit of casting
everywhere to
get the signed to unsigned conversions to work, you'd risk
accidentally doing
stuff like casting a ulong to int and losing data, since the
compiler would
assume that you knew what you were doing with the cast.
So, it's a tradeoff, and neither making the signed to unsigned
(or vice versa)
conversions explicit nor implicit would be without problems.
Walter went with
it being implicit, which matches what C does. However, unlike
C, conversions
that actually lose data (e.g. long -> int) do require casts so
that it's
easier to catch those problems. But no data is actually lost
with a sign
conversions, as casting it back to what it was will result in
the same value
(unlike with converting to a smaller integral value).
Of slightly bigger concern IMHO is that bool and the character
types are all
treated as integral types, which is at times useful but also
risks some
entertaining bugs. But again, it's a matter of tradeoffs. If
they required
casts when interacting with integral types, then a lot more
casting would be
required, risking a different set of bugs. There really isn't a
right answer
as to whether the conversions should be implicit or explicit.
It just comes
down to the tradeoffs that you prefer.
- Jonathan M Davis
I've been bitten by signed / unsigned comparisons before, and I'm
sure others have been as well. On the other hand, I can't recall
any bugs that were due to an explicit cast. I can see how
explicit casts might cause unexpected bugs (if the original type
changes but is still a valid cast), but in my personal
experience, explicit casts are safer than implicit casts.
Walter decided to adopt C-style switches for D, to simplify
translating code. However, implicit fall-through is notorious for
causing bugs in C. So as a tradeoff, D still allows fall-through
but only by explicitly writing "goto case;".
We could speculate all day, but ultimately it comes down to
experience of what works and what doesn't. If something is
generally safe in practice, then perhaps we're better with
leaving it alone. But if something is a known nuisance for
causing bugs, then find a better solution.
