Sara Golemon wrote on 01/04/2016 02:52:
On Thu, Mar 31, 2016 at 9:47 AM, Huqiu Liao <liaohu...@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]
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