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
