[fpc-pascal] Boolean expression short-circuit

Michalis Kamburelis michalis at camelot.homedns.org
Tue Nov 22 15:57:55 CET 2005

Kornel Kisielewicz wrote:
> Ok, I've been arguing over that with a friend of mine for a couple of 
> e-mail's now... I need your help!
> Let me show an example first:
> type TFoo = class end;
> var  Foo : TFoo;
> The question is, wether this is valid:
> if (Foo <> nil) and (Foo.inheritsFrom(TFoo)) then do_something;
> I would have no questions about that (first the Foo is checked, so 
> inheritsFrom is called ONLY if Foo <> nil) if it weren't for the 
> statement in the manual:
> <quote>
> Remark: The order in which expressions of the same precedence are 
> evaluated is not guaranteed to be leftto-right. In general, no 
> assumptions on which expression is evaluated first should be made in 
> such a case. The compiler will decide which expression to evaluate first 
> based on optimization rules.
> </quote>

This concerns +, -, * and such operators. Compiler can rearrange your 
arithmetical statements to make them optimized. Both Delphi and FPC.

This *doesn't* concern boolean operators, like "and", "or" -- these are 
guaranteed to be calculated left-to-right. That's because we like them 
to be predictable, just like your example "if (Foo <> nil) and (Foo..." 

> Can I make the assumption that boolean expressions are GUARANTEED to be 
> evaluated left-to-right?



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