Using the `in` keyword causes “E1012 Constant expr

问题:

I've come across some rather unusual behaviour in a bit of Delphi code. When using the in keyword to check if an item is in a constant array, I get the following compilation error:

E1012 Constant expression violates subrange bounds

The constants are defined as follows:

type TSomeEnum = (seFoo = 1000,
                  seBar = 2000,
                  seBoo = 3000,
                  seFar = 4000,
                  seFooBar = 5000,
                  seBooFar = 6000,
                  seLow = 1000,
                  seHigh = 6000,
                  seCount = 6);

The line that is failing is the following:

if someObj.someProperty in [seFoo, seFar, seFooBar] then
...

Whilst I understand the reasoning behind the error showing in another question posted here, where bounds checking on integer arrays wasn't done at compile time when using a variable, it seems odd that I'm getting the same problem with a constant array which is most certainly within bounds.

For now, I've replaced the line with a (much larger) statement comprising of or clauses. However, this is clearly not ideal. Can anyone shed any light on why I'm getting this problem?

回答1:

Documentation about Sets says :

The base type can have no more than 256 possible values, and their ordinalities must fall between 0 and 255.

So even if you can have enums of any value, the if xx in [a,b,c] statement will fail here, since a set cannot hold a value larger than 255.

Use a case statement instead:

case xx of
  a,b,c : // Make something 

end;