Re: Copy On Write and Assign By Reference perform different on PHP5 and PHP7

Sara Golemon wrote on 01/04/2016 02:52:

On Thu, Mar 31, 2016 at 9:47 AM, Huqiu Liao <liaohuqiu@gmail.com> wrote:

I have a question about Assign By Reference and I posted on StackOverflow, I'd like to know the reason behind it, and I did not get any this kind of answer, can anyone give me some clues.

Are you asking out of curiosity? Or because you think a new bug has been introduced in PHP7? If the latter, I'd respond by pointing out that the behavior of using multiple operations with side-effects (like pre/post inc/dec) is defined as undefined. That is, while the PHP reference implementation may give you a particular result, you should not rely on that result.

I just dug into the spec out of curiosity, and I have a nit-pick: there is relevant language under "Section 10: Expressions" [1]:

Unless stated explicitly in this specification, the order in which the operands in an expression are evaluated relative to each other is unspecified. [...] (For example,[...] in the full expression $j = $i + $i++, whether the value of $i is the old or new $i, is unspecified.)

But arguably this doesn't go far enough to justify the PHP 5 engine's behaviour in this case, because it only mentions the *order* of evaluation being unspecified, not the actual semantics. The definition of the pre-increment operator [2] states:

The result is the value of the operand after it has been incremented.

That is, the operator does not return the operand itself, but its value at a particular time. However, there is no order of execution which allows the value of the variable to be 4 on both sides of the addition; to get this requires the actual behaviour of the ++ operator to be re-defined as "return a reference to the incremented operand". (You could make an argument for the two increments happening "simultaneously" and giving a result of 6, but to get 8, they have to both happen simultaneously *and* see each other's results.) I'm not seriously suggesting that the PHP 5 behaviour is problematic, but unless there are additional caveats somewhere else, it does seem to violate the letter of the current spec. [1] https://github.com/php/php-langspec/blob/master/spec/10-expressions.md [2] https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#prefix-increment-and-decrement-operators Regards, -- Rowan Collins [IMSoP]