$IFDEF and $IFNDEF Compiler Directives

The $IFDEF directive provides conditional compilation.

Format

$IFDEF name

...statements...

$ELSE

...statements...

$ENDIF

The $ELSE and associated statements are optional. Statements conditioned by the $IFDEF directive are ignored if name has not been defined by a previous $DEFINE directive. Statements under the $ELSE directive are ignored if name has been defined by a previous $DEFINE directive.

The $IFNDEF directive provides the inverse action of $IFDEF, compiling the following statements if name has not been defined.

Multiple token names may specified in a single directive with an intervening AND or OR operator (or their symbolic equivalents, & and !). The expression is evaluated left to right and brackets are not permitted.

$IFDEF and $IFNDEF statements may be nested to any depth though nesting more than two deep can make program maintenance very difficult.

Portable Software

One use of conditional compilation is to allow a single source stream to be compiled in different multivalue environments rather than having to maintain separate source streams. When compiling on QM, the name QM is always defined thus a construct such as

$IFDEF QM

SETLEFT 'DATE' FROM INV.F

LOOP

SELECTRIGHT 'DATE' FROM INV.F SETTING DT

UNTIL STATUS()

READLIST IDS THEN GOSUB PROCESS

REPEAT

$ENDIF

might be used scan an alternate key index on QM. The same program might contain an equivalent for UniVerse

$IFDEF UV

BSCAN DT, IDS FROM INV.F USING 'DATE' RESET THEN

LOOP

GOSUB PROCESS

BSCAN DT, IDS FROM INV.F USING 'DATE' ELSE EXIT

REPEAT

END

$ENDIF

though UniVerse does not define the UV token automatically. This would need to be done in the source code, probably as part of a platform specific include record.

Some environments such as D3 do not support conditional compilation. There is a skeleton pre-processor program in the BP file of the QMSYS account that can be compiled on D3, etc to resolve conditional compilation constructs.