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>

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

regards, and thank you,

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..." demonstrates.

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

Yes.

Michalis

Michalis Kamburelis wrote

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

Thanks, I guess I lost the argument :]. Anyway, I guess it wouldn't hurt to clarify that just a little bit in the manual :-/

Thanks, I guess I lost the argument :]. Anyway, I guess it wouldn't hurt to clarify that just a little bit in the manual :-/

However, IIRC "complete boolean eval" (which is what delphi calls it) would blow the code up, as even if Foo was nil, the second half would be evaluated regardless.

Maybe that will go some way to winning part of the argument? ;-)

Boolean expression evaluation is explicitly treated in both reference manual and programmers reference.

However, I'll add an explicit remark about the consequence this has for the order of evaluation, namely that the order is guaranteed left-to-right. (although I think this is a logical consequence of the shortcut boolean evaluation principle)

Michael.