Hi all, 2013/7/21 Sherif Ramadan <theanomaly...@gmail.com>
> The problem is I'm not sure where this type of information should be > documented. It makes sense to put this on the increment/decrement operators > page, but doesn't seem appropriate the operator precedence page. So to make > sure we're all in agreement I'm suggesting the following language to be > added as a note on the increment/decrement operators page at > www.php.net/language.operators.increment with supplement examples > provided above to help users get a better understanding of what we mean > when we say undefined behavior and add an additional warning box not to > rely on this type of behavior. > Since the issue int "++$a + $a++" is not a precedence issue, but a evaluation orders and side effects. Describing it without complete explanation in precedence section makes users confuse. (I'm the one also) I partially agree that documenting the issue in www.php.net/language.operators.increment . It would better to be described fully in a section since it is not a increment/decrement only issue. > "As noted from the examples above the use of multiple increment/decrement > operators in the same expression is considered undefined behavior because > the order of evaluation cannot be guaranteed. Using such evaluations may > lead to unexpected results." > > "Warning: > Relying on this behavior is discouraged as it may be subject to change and > without notice. Results are not typical and are implementation-specific." > > These will fit into www.php.net/language.operators.increment. If PHP is going to inherit C/C++ feature for better compiler implementation and parallelism, we should document fully. (Note: Java/C# seem always evaluate left to right and side effect is immediately visible. Therefore, Java/C# don't have undefined behaviors like C/C++. It's possible to behave like Java/C#, but PHP is going to follow C/C++ way I suppose.) http://en.cppreference.com/w/cpp/language/eval_order 1) Between the previous and next sequence point a scalar object shall have its stored value modified at most once by the evaluation of an expression. i = ++i + i++; // undefined behavior i = i++ + 1; // undefined behavior i = ++i + 1; // undefined behavior (well-defined in C++11) ++ ++i; // undefined behavior (well-defined in C++11) f(++i, ++i); // undefined behavior f(i = -1, i = -1); // undefined behavior 2) Between the previous and next sequence point , the prior value of a scalar object that is modified by the evaluation of the expression, shall be accessed only to determine the value to be stored. cout << i << i++; // undefined behavior a[i] = i++; // undefined bevahior In PHP, it would be $i = ++$i + $i++; $i = $i++ + 1; $i = ++$i + 1; ++ ++$i; f(++$i, ++$i); // This is good for parallelism. e.g. parameter evaluation can be done in parallel. f(complex_func(), extremely_complex_func()); f($i = -1, $i = -1); // Same as above echo $i, $i++; // Same as above $a[$i] = $i++; The best benefit of undefined evaluation order is parallelism (i.e. out of order execution) even if PHP doesn't have it now. C/C++ doesn't execute function parameters in parallel automatically, but PHP doesn't have to follow this. It would be good to have dedicated section for this. Discussion would be needed if PHP is going to support out of order execution for func(f1(), f2(), f3()) or like, which one make undefined, adding more undefined, etc. Regards, -- Yasuo Ohgaki yohg...@ohgaki.net
