This program below crashes because the value 12 is not in the enum. I was curious though, how does it know this? Does it have to do a linear search through the enum to find the value? I know "succ" fails at compile time because the enum has assignments but how this works at runtime is another question.

type
  Fruit = (apple = 10,
           banana,
           pear = 20,
           citrus);
var
  f: Fruit;
  i: Integer;
begin
  i := 12;
  f := Fruit(i);
  writeln(f);
end;

Regards,
Ryan Joseph

writeln() converts the ordinal value to the value name and if the entry is not found, the 107 error is raised.

Ondrej

To find out, compile FPC with debug info and step into the writeln() call with F7 (multiple times until you get into fpc_shortstr_enum_intern()):

#0 FPC_SHORTSTR_ENUM_INTERN(4284424, 4284488, POINTER(nil), POINTER($00326000), , 12) at sstrings.inc:575
#1 fpc_write_text_enum(POINTER($00416008), POINTER($00416048), 0, TEXT (), 12) at text.inc:1272
#2 :16

Ondrej

oh, it's writeln which is doing the runtime check, thanks. Doing math on enums is probably not a smart idea though. I wonder why a subscript like [] was never added to enums. It would be nice to do:

f := Fruit[banana.index + 1];

instead of dealing with raw values. A runtime check for creating enums was ordinals would be smart also, like:

f := Fruit.Create(10);

instead of blindly cast ordinals to enums which may be invalid.

Regards,
Ryan Joseph