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

[J. Gareth Moreton]

That is exactly how it should behave, because the left-side value may 
not be valid, and the is operator is meant to help confirm that.

In the meantime, I've updated your patch to work with the current trunk: 
https://bugs.freepascal.org/view.php?id=33603#c117040

Gareth aka. Kit


On 02/07/2019 21:31, Ondrej Pokorny wrote:
> On 02.07.2019 21:48, J. Gareth Moreton wrote:
>> I should probably test this myself, but if you, say, read a value 
>> from a file directly into a variable of type TEnumType, will your 
>> patch return False for "if (Value is TEnumType) then" if Value, 
>> despite being TEnumType, contains an invalid value?  No problem if it 
>> isn't, just that you would have to read an Integer-type first then 
>> typecast it.
>
> Yes:
>
> var
>   Value: TEnumType;
> begin
>   Value := TEnumType(-1);
>   IsValid := Value is TEnumType; // IsValid gains false
>
> The compiler may not do any optimizations here (like return always 
> true if the left side value is the enum type at the right side). This 
> should be clearly stated and documented if the feature is added to FPC.
>
> This is similar to the object-is operator that gets evaluated as well 
> even if the type of the left-side value is the type at right side:
>
> var
>   Value: TPersistent;
> begin
>   Value := TPersistent(TObject.Create);
>   IsValid := Value is TPersistent; // IsValid gains false
>
> Ondrej
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
>
>

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus