$INCLUDEONCE: Difference between revisions

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;* Example: Show how the command prevents included code to be injected multiple times.
;* Example: Show how the command prevents included code to be injected multiple times.
:* First save the include files in your qb64pe folder, then take the main program.
:* First save the include files in your qb64pe folder, then take the main program.
<center>{{Text|'''Save as "once.bm"'''|black}}</center>
{{CodeStart}}
{{Text|<nowiki>'included by test.bas and incl.bm</nowiki>|#919191}}
{{Cm|$INCLUDEONCE}}
{{Cl|PRINT}}
{{Cl|PRINT}} {{Text|<nowiki>"This prints from file once.bm, and should appear only once on screen."</nowiki>|#FFB100}}
{{CodeEnd}}


<center>{{Text|'''Save as "incl.bm"'''|black}}</center>
<center>{{Text|'''Save as "incl.bm"'''|black}}</center>
Line 39: Line 49:


{{Text|<nowiki>'</nowiki>|#919191}}{{Cm|$INCLUDE}}: {{Text|<nowiki>'once.bm'</nowiki>|#919191}}
{{Text|<nowiki>'</nowiki>|#919191}}{{Cm|$INCLUDE}}: {{Text|<nowiki>'once.bm'</nowiki>|#919191}}
{{CodeEnd}}
<center>{{Text|'''Save as "once.bm"'''|black}}</center>
{{CodeStart}}
{{Text|<nowiki>'included by test.bas and incl.bm</nowiki>|#919191}}
{{Cm|$INCLUDEONCE}}
{{Cl|PRINT}}
{{Cl|PRINT}} {{Text|<nowiki>"This prints from file once.bm, and should appear only once on screen."</nowiki>|#FFB100}}
{{CodeEnd}}
{{CodeEnd}}


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  of the $INCLUDEONCE metacommand.
  of the $INCLUDEONCE metacommand.
{{PreEnd}}
{{PreEnd}}
{{Small|Examples by RhoSigma}}





Revision as of 14:40, 19 February 2024

The $INCLUDEONCE metacommand, when placed in include files, prevents that the file's contents is injected multiple times into a program, even if the file is included multiple times directly or indirectly through other include files.


Syntax

$INCLUDEONCE


Description

  • As QB64 metacommand it does not require a comment ' or REM before it.
  • It can be placed everywhere in an include file, but must be the only thing in the line, hence without additional whitespace or comments.
    • Even if placed in the middle or the end of the file, it always designates the entire file contents.
  • If placed in the main program, $INCLUDEONCE does nothing and is simply ignored without error.
  • $INCLUDEONCE will not work if placed inside pre-compiler $IF..$ELSE...$END IF blocks.


Availability


Examples

Example
  • Show how the command prevents included code to be injected multiple times.
  • First save the include files in your qb64pe folder, then take the main program.
Save as "once.bm"
'included by test.bas and incl.bm

$INCLUDEONCE

PRINT
PRINT "This prints from file once.bm, and should appear only once on screen."
Save as "incl.bm"
'included 2 times by test.bas

PRINT
PRINT "This prints from file incl.bm, it should appear 2 times on screen."

'$INCLUDE: 'once.bm'
Save as "test.bas"
'this is a test for $INCLUDEONCE behavior

PRINT "This prints from the test.bas main program."

'$INCLUDE: 'incl.bm'
'$INCLUDE: 'once.bm'
'$INCLUDE: 'incl.bm'

END
If it works, the output looks like this...
This prints from the test.bas main program.

This prints from file incl.bm, it should appear 2 times on screen.

This prints from file once.bm, and should appear only once on screen.

This prints from file incl.bm, it should appear 2 times on screen.
Explanation
 Even as the file once.bm is included 3 times into the test.bas program
 (2 times indirectly through incl.bm and 1 time directly), the contained
 PRINT statements are injected only once into the program due to the use
 of the $INCLUDEONCE metacommand.
Examples by RhoSigma


See also



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