问题
I'm comparing a SmallInt variable with the result of the Ord function.
Example:
var
MySmallInt : SmallInt;
begin
MySmallInt := 5;
if(MySmallInt > Ord('C'))
then ShowMessage('True')
else ShowMessage('False');
end
After doing this, the following warning message is shown (W1023):
W1023 Comparing signed and unsigned types - widened both operands
Delphi's hint on the Ord function says that it should return a SmallInt and that's why I can't understand what causes the warning message. (I've looked for Ord function in the System unit but I didn't find it).
Further Informations:
- I'm testing under Delphi XE7.
- Under Delphi 2007, the same code doesn't give me any warning.
回答1:
As David said, Ord() is a so called "compiler magic" (or, as they call it now, "intrinsic" or "pseudo-") function, i.e. not a real function that is called, but just something that uses a function syntax, but is recognized by the compiler as a special construct and turned into code directly. The same is true for e.g. Chr(), Writeln(), etc. They can usually have different and/or multiple types of parameters or return values and sometimes even have additional syntax elements.
The documentation says, about Ord(X):
The result is the ordinal position of X; its type is the smallest standard integer type that can hold all values of X's type.
In Delphi XE7, 'C' is a WideChar, and the return value of Ord('C') will be a 16 bit unsigned type (Word). Smallint is signed type. That is why you get the warning, because you are comparing a signed and an unsigned type of the same size, so the values must be widened to the next larger type (Integer).
In Delphi 2007, 'C' is not a WideChar, it is an AnsiChar, so the result of Ord('C') is a Byte. There is no need for widening to the next larger type, since Smallint can contain all values of Byte, so both can be promoted to Smallint.
I agree that the info hint in the editor is deceptive. Ord() does not always return a Smallint, it returns the minimum type that is needed to hold all values of the argument.
回答2:
Ord() is an intrinsic function that yields an unsigned type. Hence the warning. In your case, you are passing it a WideChar, and so the matching integral type is Word.
来源：https://stackoverflow.com/questions/44288051/warning-on-comparing-a-smallint-with-result-of-ord-function