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Re: simple session question [message #175733 is a reply to message #175731] Sat, 22 October 2011 22:43 Go to previous messageGo to previous message
Tim Streater is currently offline  Tim Streater
Messages: 328
Registered: September 2010
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In article <j7ve6a$svk$1(at)news(dot)albasani(dot)net>,
The Natural Philosopher <tnp(at)invalid(dot)invalid> wrote:

> Richard Damon wrote:
>> On 10/22/11 9:40 AM, Thomas Mlynarczyk wrote:
>>> Jerry Stuckle schrieb:
>>> [$foo = $foo++;]
>>>> > Is that mentioned somewhere in the manual?
>>>> Yes, see how operator precedence works. It is well defined.
>>>
>>> It is the internal order of execution that confuses me a bit. First, the
>>> expression on the right is evaluated, yielding the "not-yet-incremented"
>>> value. Then, two things must happen: Incrementing $foo and assigning
>>> $foo the value from the first step. The result depends on the order of
>>> these two steps. Clearly, the increment should happen before the next
>>> read access to $foo. But whether or not it happens before the next write
>>> access (assigning $foo the value from the first step) is neither
>>> intuitively clear (and optimizers might handle this one way or the
>>> other) nor explicitly stated in
>>> <http://de3.php.net/manual/en/language.operators.increment.php>. There
>>> is only a user comment saying: "The exact moment when post-increment and
>>> post-decrement happen is _just immediately after the variable is
>>> evaluated_ (not "after the line is processed" or something like that)".
>>> If this is meant to be documented behaviour, they should mention it as
>>> such in the manual.
>>>
>>> Greetings,
>>> Thomas
>>>
>>
>> I suspect that there is a difference between the execution model of
>> C/C++ and PHP here, do in part to the fact that C/C++ is (normally) a
>> compiled language with the goal of allowing the compiler to generate as
>> efficient of code as possible, while PHP is designed as a interpreted
>> language.
>>
>> in C, x = x++; is undefined behavior, as the timing of when the =
>> operator is executed and the writing back of the value of x++ from the
>> ++ operator is not specified, depending on when the compiler can most
>> efficiently implement it is ok. The code could be converted into the
>> equivalent of either.
>>
>
> I dont think it is. The value of x is assigned before the increment
> operator is applied: that's defined.
>
>
>> temp = x; /* save original value of x */
>> x = x+1 /* perform increment */
>> x = temp; /* perform the = */
>>
>> or
>> x = x; /* perform the = */
>> x = x+1; /* perform the ++ */
>>
>> PHP doesn't seem to reserve for itself this ability, and there seems to
>> be some comments (which you refer to) asserting that x++ will ALWAY be
>> the equivalent of
>>
>> temp = x;
>> x = x+1;
>> ... do what ever with temp
>>
>
> BUT that doesnt cover the $bar=($foo++);
>
> That should be
> INC [foo]
> MOV [bar].[foo];
>
> NOT the other way around.
>
> But in php it is.

Mmmm. I'm not sure about that. In JavaScript (also interpreted) I do a
lot of:

i = 0;
first = results[i++];
second = results[i++];
etc

and first, second, get the zeroth and oneth elements of results,
respectively. So the ++ is done *after* the assignment.

--
Tim

"That excessive bail ought not to be required, nor excessive fines imposed,
nor cruel and unusual punishments inflicted" -- Bill of Rights 1689

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