[fpc-devel] [Suggestion] Enumeration range-check intrinsic

[Michael Van Canneyt]

On Tue, 2 Jul 2019, J. Gareth Moreton wrote:

> Hi everyone,
>
> So there have been a lot of issues going back and forth regarding case blocks 
> when dealing with enumerations and what happens if the enum falls outside the 
> valid range of values for some reason, because the internal domain check is 
> sometimes omitted for performance reasons, since the value /shouldn't/ be 
> invalid.  Thus, if it is invalid and a jump table is used, an access 
> violation is raised.
>
> Ultimately, the range check will likely not be included to safeguard against 
> this, but there are still situations where an enumeration will take on an 
> out-of-range value, like if it reads it from an external data file that is 
> corrupted or of an incompatible version for the program.  
> So I would like to 
> suggest a new intrinsic called InRange() or IsDomainValid() or something to 
> that effect that takes an ordinal-type parameter and returns True if it 
> contains a valid value for the enumeration (always True if it's a Byte or 
> Word etc) and False if it is outside the valid domain, hence fulfilling the 
> range check if it's something you really need.


I thought this was already discussed and accepted as desirable ?
If memory serves well, the only discussion was about form...

>
> What are your thoughts? Would this be a decent compromise to fix the case 
> range-check issue that keeps cropping up occasionally? I know it's causing 
> some people to choose not migrate away from Delphi, and that is never really 
> a good thing.  It should be easy enough to document as well, putting in an 
> explicit warning for case to say that if the enumeration is out of range, it 
> may cause a crash, and to prevent it, use the intrinsic to check beforehand.

Why would this stop anyone from migrating away from Delphi ?

Delphi has the exact same problem: an enumerated may be out of range.
I had it on multiple occasions when database data was invalid.

It may not have the case jump table problem, but other than that it is not better
than FPC.

Michael.