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.
The reason why I ask for an intrinsic instead of something like "if
(Value >= Low(TEnumType)) and (Value <= High(TEnumType))" is because
such code may be optimised out due to it logically being impossible for
Value to be less than Low(TEnumType), for example, so it's harder than
you think to check if the variable is in range. Additionally, by having
a generic intrinsic, it can generate more optimised code (e.g. the
equivalent of "if (Value - Low(TEnumType)) < (High(TEnumType) -
Low(TEnumType))", taking advantage of unsigned overflow if Value is less
than the lower bound) and also to check for invalid values that fall
within the maximum range (e.g. if you have something like "type
TEnumType = (etZero, etOne, etSpecial = $FF);" and your Value holds the
equivalent of $FE).
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.
Gareth aka. Kit
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