Q101025: How FoxPro Passes Parameters to Subroutines

Article: Q101025
Product(s): Microsoft FoxPro
Version(s): MS-DOS:1.02,2.0,2.5,2.5a; WINDOWS:2.5,2.5a,3.0
Operating System(s): 
Keyword(s): kbenv
Last Modified: 05-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
- Microsoft FoxPro for Windows, versions 2.5, 2.5a 
- Microsoft FoxPro for MS-DOS, versions 1.02, 2.0, 2.5, 2.5a 
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SUMMARY
=======

By default, FoxPro passes parameters to procedures (subroutines called with a DO
<subroutine> command) by reference and parameters to functions
[subroutines called with a =<subroutine>() or ?<subroutine>()
command] by value. The information below describes how parameters are passed to
subroutines.

MORE INFORMATION
================

When parameters are passed by value, a copy of the original variable is made in
the routine. The variable declared in the subroutine is a different variable
from the original variable. When control is passed back to the calling program,
the variables declared in the routine are released and the value of the original
variable is not changed.

When parameters are passed by reference, a reference to the original variable is
made in the routine. Any change made to the variable in the subroutine is
reflected in the calling program.

Passing Parameters to User-Defined Functions
--------------------------------------------

By default, parameters are passed by value to user-defined functions. In essence,
any change made to the variables declared as parameters in the subroutine will
not affect the original variables in the calling program. For example:

     * Main program
     CLEAR
     mvar1=1
     mvar2=2
     mvar3=ADDIT(mvar1)   && Passing mvar1 as a parameter: by value
     ?
     ?"Value of memory variables after the routine is executed"
     DISP MEMO LIKE m*
     ?
     ?"The value of mvar1 is unchanged"

     FUNCTION ADDIT
     PARAMETER mpar1
     mpar1=mpar1+1    && Copy of mvar1 is incremented
     mvar3=mpar1+1     && Variable to be returned to the calling program.
     ?"Value of memory variables in the routine"
     DISPLAY MEMORY LIKE m* && mvar1=1 and mpar1=1
     RETURN mvar3      && Value returned to the calling program mvar3

To pass a parameter by reference to a user-defined function, use the at sign (@)
before the variable passed. For example:

     mvar3=ADDIT(@mvar1)

To change the default and pass all the parameters by reference, use SET UDFPARMS
command. For example:

  SET UDFPARMS TO REFERENCE

Passing Parameters to Procedures
--------------------------------

By default, FoxPro passes parameters by reference to procedures. Any changes made
to variables in the subroutine will be made to the original variable.
Additionally, FoxPro hides the original variables passed as parameters in the
subroutine. For example:

     * Main program
     CLEAR
     mvar1=1
     mvar2=2
     DO ADDIT WITH mvar1  && Passing mvar1 as a parameter by reference
     ?
     ?"Value of memory variables after the routine is executed"
     DISPLAY MEMORY LIKE m*
     ?
     ?"The value of mvar1 is modified"

     PROCEDURE  ADDIT
     PARAMETER mpar1
     mpar1=mpar1+1            && Reference to mvar1 is incremented
     ?"Value of memory variables in the routine"
     DISPLAY MEMORY LIKE m*   && mvar1=2 and mpar1= @mvar1

To pass a parameter to a procedure by value, use parentheses around the variable.
For example:

  DO ADDIT WITH (mvar1)

REFERENCES
==========

"Language Reference," version 2.5, page L3-1012

"Using FoxPro Version 2," Slater and Arnott, pages 613-614, Que, 1992

Additional query words: VFoxWin FoxDos FoxWin 2.x array

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Keywords          : kbenv 
Technology        : kbVFPsearch kbAudDeveloper kbFoxproSearch kbZNotKeyword3 kbFoxPro200DOS kbFoxPro250DOS kbFoxPro250aDOS kbFoxPro250 kbFoxPro250a kbVFP300
Version           : MS-DOS:1.02,2.0,2.5,2.5a; WINDOWS:2.5,2.5a,3.0

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