Difference between revisions of "Goto"
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Jump to navigationJump to search (“real-world” example; added discouragement to use that statement) |
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{$I+} | {$I+} | ||
ExitNoErr: | ExitNoErr: | ||
| − | end;</syntaxhighlight>According to the value of {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<syntaxhighlight enclose="none" lang="pascal">returnNilIfGrowHeapFails</syntaxhighlight>}} {{Doc|package=RTL|unit=system|identifier=getmem|text=<syntaxhighlight enclose="none" lang="pascal">getMem</syntaxhighlight>}} possibly may return [[Nil|<syntaxhighlight enclose="none" lang="pascal">nil</syntaxhighlight>]]. | + | end;</syntaxhighlight> |
| − | Instead of placing ''everything'' in “success”-branch, a <syntaxhighlight enclose="none" lang="pascal">goto</syntaxhighlight> | + | According to the value of {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<syntaxhighlight enclose="none" lang="pascal">returnNilIfGrowHeapFails</syntaxhighlight>}} {{Doc|package=RTL|unit=system|identifier=getmem|text=<syntaxhighlight enclose="none" lang="pascal">getMem</syntaxhighlight>}} possibly may return [[Nil|<syntaxhighlight enclose="none" lang="pascal">nil</syntaxhighlight>]]. |
| + | Instead of placing ''everything'' in a “success”-branch, a couple <syntaxhighlight enclose="none" lang="pascal">goto</syntaxhighlight> instructions were chosen. | ||
[[category:Pascal]] | [[category:Pascal]] | ||
[[Category:Control Structures]] | [[Category:Control Structures]] | ||
Revision as of 19:48, 7 March 2018
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goto is an unconditional jump to a previously declared label (either before or after the goto command).
Usage of goto in high-level programming languages such as Pascal is highly discredited, since control structures of all sorts are available.
The last situation a goto is agreed with bad grace is a significant system error, where a “graceful exit” is better than causing a system breakdown.
As an example, here an excerpt from FPC's code base rtl/inc/extres.inc:
324procedure InitResources;
328label ExitErrMem, ExitErrFile, ExitNoErr;
329begin
339 ResHeader:=GetMem(sizeof(TExtHeader));
340 if ResHeader=nil then goto ExitErrFile;
366 goto ExitNoErr;
367
368 ExitErrMem:
369 FreeMem(ResHeader);
370 ResHeader:=nil;
371 ExitErrFile:
372 {$I-}
373 Close(fd);
374 {$I+}
375 ExitNoErr:
376end;
According to the value of returnNilIfGrowHeapFails getMem possibly may return nil.
Instead of placing everything in a “success”-branch, a couple goto instructions were chosen.
