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Re: simple session question [message #175731 is a reply to message #175727] Sat, 22 October 2011 21:56 Go to previous messageGo to previous message
The Natural Philosoph is currently offline  The Natural Philosoph
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Registered: September 2010
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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.

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