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

[Martin Frb]

On 13/07/2019 13:47, J. Gareth Moreton wrote:
> Here lies the problem - if you have a enumeration stored in a byte 
> field, say, in a file, and there's a chance it's been corrupted, 
> there's no reliable way to detect that, since even typecasting may not 
> be reliable, and once again I'm implying that enums are unsafe to use, 
> but right now, what else can one do?
Well yes. Reading from a file is like a typecast.

TEnum(SomeInt) implies that you have done the validity check *before* 
this. Because by using the type you declare that the result is valid.

Of course reading from a file/api you can not do the test before....

But if you want the test afterwards, you must change the entire 
definition of the type. Changing it for just one statement (the "is") 
can not work for technical reasons (see my previous post). You do not 
know what other side-effects you may have done in the statement that 
typecasted an unchecked/invalid value. If you do not know the list of 
possible side effects, then you can not test for all of them.

I would guess this would work

{$mode delphi}
{$z1}
type
  tmyFileEnum = (ea, eb, ec, emax = 255);
  tmyenum = (ea, eb, ec);

read(file, AFileEnum);
if AFileEnum is TMyEnum
then AMyEnum := TMyEnum(AFileEnum)
else DoError;