>The syntax is "#define pseudovariable expression" - so the expression is not an optional clause. The expression may evaluate to an empty string:
>
>#define out ""
>
>would be proper syntax, and the pseudovariable "out" would evaluate to empty string. In Martin's case, I think the preprocessor should report something like "missing expression" instead of compiling OK and having mysterious output later.

I am not sure how relevant the following is for the discussion, but note that whatever you #define need not be a valid expression. It is just text which is replaced before the actual compilation phase. In theory, this might be used to your advantage, sometimes - although it might also make the program quite confusing.

Here is an example:

#define mydef "this is

clear
? mydef a text"

The output shows This is a text