Q43252: Must DECLARE a FUNCTION Invoked from an External Library

Article: Q43252
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890405-127 B_BasicCom | mspl13_basic
Last Modified: 15-DEC-1989

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.

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

1. 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.

2. 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.

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