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

[J. Gareth Moreton]

Okay, I've done some preliminary testing, and "if Value < Low(TMyEnum) 
then" and "if Value > High(TMyEnum) then" work as expected and aren't 
optimised out, even when using -O4 -Cr.  So admittedly there is still a 
way to check to see if the enumeration is within the valid range, at 
least for now.  I'm not sure if the compiler will be changed so this 
would be optimised out (in which case, an intrinsic or "is" would be 
necessary).

I do think one needs to put in a clear warning in the documentation for 
'case' so developers who do fall foul of the slightly counterintuitive 
behaviour have a means to work around it.

This was my test, written in a way so I couldn't be told off for using 
'absolute', pointers or assembly language and is comparable to a 
real-world example of reading an enumeration from a data stream:

----

program EnumTest;

uses
   Classes;

type
   TMyEnum = (meOff = 1, meLowered, meStowed);

const
   ByteSequence: array[0..5] of Byte = (0, 2, 4, 127, 128, 255);

var
   Stream: TMemoryStream;
   EnumVal: TMyEnum;
   X: Integer;

begin
   Stream := TMemoryStream.Create;
   try
     { Fill the memory stream }
     Stream.SetSize(Length(ByteSequence));
     Stream.Write(ByteSequence, Length(ByteSequence));

     Stream.Seek(0, soBeginning);

     { Okay, now read back the values }
     while Stream.Read(EnumVal, 1) > 0 do
     begin
       if EnumVal < Low(TMyEnum) then
         WriteLn('Below valid range')
       else if EnumVal > High(TMyEnum) then
         WriteLn('Above valid range')
       else
         WriteLn(EnumVal);
     end;
   finally
     Stream.Free;
   end;
end.

----

If I make the enumeration non-contiguous and try to call 
WriteLn(EnumVal) with an invalid value that is otherwise still within 
the lower and upper bounds, then I do get an exception, which is to be 
expected - "EInOutError: Unknown Run-Time error : 107" - the only real 
fault with it is that it's listed as an unknown error.  If an intrinsic 
or "is" is accepted as a convenient shortcut for the bounds check, it 
should probably catch values that fall within these gaps.

Gareth aka. Kit


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