BYVAL - QB64 Wiki

BYVAL

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The BYVAL statement is used to pass a numerical parameter's value with procedures made in other programming languages.


Syntax:

SUB Procedure_Name (BYVAL variable1, BYVAL variable2)


Description:

  • QB64 can only use BYVAL in DECLARE LIBRARY procedures that add DLL or Operating System API functions.
  • Supported with DECLARE LIBRARY SUB and FUNCTION procedure declarations when passing numerical values only.
  • Passing numerical values BYVAL assures that the original numerical variable value is not changed by another procedure.
  • Use parenthesis around program SUB or FUNCTION parameters passed BY VALUE. Ex: CALL Procedure ((x&), (y&))


Historical:

  • Qbasic versions below 7 do not use BYVAL unless the SUB program referred to is from a different programming language.
  • PDS versions can use BYVAL as it is intended in any SUB or FUNCTION parameters.
  • BYVAL can also be used with ABSOLUTE in Qbasic only.


Errors:

  • Do not use BYVAL before STRING or TYPE variables or in regular prograam SUB or FUNCTION parameters!
  • Many Qbasic keyword variable names CAN be used with a STRING suffix($) ONLY! You CANNOT use them without the suffix, use a numerical suffix or use DIM, REDIM, _DEFINE, BYVAL or TYPE variable AS statements!


Example 1: BYVAL is used to preserve the values sent to an external procedure so they remain the same after they are used:

DECLARE LIBRARY SUB MouseMove ALIAS SDL_WarpMouse (BYVAL xoffset&, BYVAL yoffset&) END DECLARE

Note: Since the DLL library was already used by QB64 SDL(up to v.954), it did not require a DLL name. GL would!


Example 2: Passing parameters "by value" using brackets when calling a SUB or FUNCTION in Qbasic or QB64.

CALL MySUB (a%, (b%), (c%)) 'CALL SUB b and c stay 0 after sub MySUB a%, b%, (c%) 'call SUB again without CALL only c stays 0 after sub PRINT "After procedures: "; a%, b%, c% SUB MySUB (a%, b%, c%) a% = a% + 1: b% = b% + 1: c% = c% + 1 PRINT "Inside procedure: "; a%, b%, c% END SUB

Inside procedure: 1 1 1 Inside procedure: 2 1 1 After procedures: 2 1 0

Explanation: Both SUB calls pass just the values of b% and c% to the procedure. The first variable, a%, is passed by reference (the default) so the value was changed by the SUB procedure. Brackets can only be used in the CALL or function reference!


See also:



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