On Thu, 2006-07-27 at 14:48, Adam Zey wrote:
> KermodeBear wrote:
> > Robert Cummings wrote:
> >
> > Then how come when I do a foreach on an array (without modifying
> > anything within the foreach), it still makes a copy of the array that
> > consumes memory? I think it's dangerous to generalize that it's always
> > best to let PHP make copies of things. In the foreach situation, the
> > preferred solution when memory is a problem is to either use a
> > reference, or have foreach iterate over the keys of the array.
> >
> > Regards, Adam.
> >
> > -----
> >
> > PHP doesn't seem to make a real copy of data until one of the copies is
> > modified, making it necessary to create a new set of data. So, it is pretty
> > smart about that.
> >
> > Here is a small CLI script that seems to support this:
> > <?php
> > $a_array = array();
> > for($i = 0; $i < 10000; $i++) {
> > $a_array[] = time(); // Just an arbitrary piece of data
> > }
> > echo 'Memory Usage: ' . memory_get_usage() . "\n";
> > echo "Making a copy of the array.\n";
> > $a_copy = $a_array;
> > echo 'Memory Usage: ' . memory_get_usage() . "\n";
> > echo "Modifying the copy:\n";
> > $a_copy[] = time();
> > echo 'Memory Usage: ' . memory_get_usage() . "\n";
> > ?>
> >
> > On my machine, this displays:
> > Memory Usage: 640280
> > Making a copy of the array.
> > Memory Usage: 640440
> > Modifying the copy:
> > Memory Usage: 1106056
> >
> > -K. Bear
>
> I note that your example doesn't use a foreach loop, which is what my
> post was about.
>
> Some testing shows my original premise was wrong, but it's still
> possible to get PHP to copy the array when it doesn't need to. In this
> case, using a reference can save memory even when a reference isn't
> needed. If you want to modify the original array in the foreach, then it
> is better to step over the array yourself and save memory, or pass the
> foreach a reference to the array to stop it from doing a copy.
>
> I'm just trying to say, I don't think it's always a good idea to rely on
> the language deciding when to copy. Sometimes it's a good idea to use a
> reference to stop the copy if memory is a concern.
PHP creates a real copy when one of the original arrays changes. This is
the principle of copy-on-write. If you are getting a copy then one of
the arrays changed, so now you really have to choose between a copy and
reference knowing that a reference will affect all arrays to which you
made a reference or a copy which will only affect the copy to which the
change was made. So this returns to my original assertion that you
should only use references when a reference is necessary.
Cheers,
Rob.
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