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Re: Dereferencing
by tinita (Parson) on Mar 30, 2004 at 14:09 UTC
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$$var is just a short form for ${$var}. the $var inside the braces is a reference; in this case a reference to a scalar.
you dereference a variable by putting ${ } around it. it $var were a reference to an array, you'd dereference it by
putting an @{ } around it, and %{ } for a hash.
please read more about it in perldata, perlref etc...
if the $var is not a reference but just a simple string, then
we're dealing with symbolic references here, which you should avoid... | [reply] |
Re: Dereferencing
by Limbic~Region (Chancellor) on Mar 30, 2004 at 14:08 UTC
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Actually, $RESPONSE is the reference, not $$RESPONSE. As for
the quick reference, that just explains how you use references, it assumes you already know what a reference is.
Abigail
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Re: Dereferencing
by dreadpiratepeter (Priest) on Mar 30, 2004 at 14:20 UTC
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While you should read the above mentioned docs, here is a very simple explanation.
Somewhere before this line, $RESPONSE is being set to point to another variable. Something like:
$x = 3;
$RESPONSE=\$x;
After this, $RESPONSE contains the memory address of the $x variable.
If you print $RESPONSE you will get a strange string. Something like "SCALAR(0xec94c)". Which says that $RESPONSE is a reference to a scalar (at memory address 0xec94c).
To get to the value that $RESPONSE is pointing to you need to dereference it. Putting the extra $ before the variable, is a way of dereferencing the variable (there are others, see the docs).
Thus if you print $$RESPONSE, you will get 3.
You can also assign to the dereferenced variable. Doing so would change the value stored in $x.
In your case, whatever $RESPONSE is pointing to will have its value changed to 0
Hope that helps,
-pete
"Worry is like a rocking chair. It gives you something to do, but it doesn't get you anywhere."
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Re: Dereferencing
by monkey_boy (Priest) on Mar 30, 2004 at 14:22 UTC
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simple way to find out what it *WAS*:
use Data::Dumper;
print Dumper($RESPONSE);
should all become clear!
I should really do something about this apathy ... but i just cant be bothered
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Re: Dereferencing
by MCS (Monk) on Mar 30, 2004 at 14:33 UTC
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$RESPONSE is a reference and it is basically a memory address. Now it's great to pass that around instead of copying the actual value which is one of the reasons people use it. Now, you can't just say something like $RESPONSE = 0 because that would assign the memory address of $RESPONSE to 0... not the actual value. So in order to dereference it (or in other words to use the actual value) you put a $ in the front. This basically says, I don't want the address, instead I want the value at that address. In your case you are assigning the value of 0 to it instead of getting a value. It works both ways.
Hope that helped. The docs are pretty good too, I just thought I would give a brief description.
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$RESPONSE is a reference and it is basically a memory address
In interest of full-disclosure, it is not really a memory address, but basically like a memory address. I'm saying this for new-user benefit since MCS probably knows this. A reference is an entry in a symbol-table, if you like...not quite like C/C++ pointers. In Java, well, objects just act as references, so the line is blurred in a rather confusing way if that's where you are coming from.
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Re: Dereferencing
by Fletch (Bishop) on Mar 30, 2004 at 14:09 UTC
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perldoc perlreftut, perldoc perlref.
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[d/l]
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Re: Dereferencing
by dawn (Novice) on Mar 30, 2004 at 16:49 UTC
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I found this tutorial on Perl Monks much easier to understand than the perldocs. | [reply] |
Re: Dereferencing
by pbeckingham (Parson) on Mar 30, 2004 at 14:28 UTC
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You may already understand this well - it depends on what other languages you know well, as there are analogous features in many other languages. What do you know well?
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