Must DECLARE a FUNCTION Invoked from an External Library (43252)

SUMMARY

To use a FUNCTION that is in a library (.LIB) or Quick Library (.QLB), you must have a DECLARE statement at the top of each module that uses the FUNCTION. This is documented in the "Microsoft QuickBasic 4.0: Basic Language Reference" manual for Versions 4.00 and 4.00b, Page 139, and the QuickBasic 4.50 on-line QB Advisor by choosing <Help> <Index> DECLARE Statement (Basic Procedures) <Details> and in the "Microsoft Basic 7.0: Programmer's Guide" on Page 53.

The documentation states that you must use DECLARE if you invoke a FUNCTION that is defined in another module. Library files (.LIB and .QLB) are "other modules," as are SUBprogram or FUNCTION modules that start from a separate .BAS or MS-DOS file.

This information applies to Microsoft QuickBasic Versions 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft Basic Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft Basic PDS Version 7.00 for MS-DOS and MS OS/2.

MORE INFORMATION

There are two reasons that DECLAREs are necessary for FUNCTIONs and not for SUBprograms.

The only way to identify a SUBprogram without a DECLARE is with a CALL. You cannot use CALL with a FUNCTION because a FUNCTION returns a value and must be used in an assignment statement.
Without the DECLARE, the compiler cannot know if you are referring to a variable, array, or FUNCTION.

Consider the following Basic statements:

   'Is this a FUNCTION call or assignment of a variable?
   VAR1# = Func1#
   ' Is this a FUNCTION call or is this assigning
   ' an array element to a variable?
   VAR1# = Func1#(VAR2,VAR3)

The DECLARE FUNCTION Func1#(<variables>) statement tells the compiler that you are using a FUNCTION when you use the name Func1# and that this is not a variable or an array.