Modifying a reference to a literal string

iakobski has asked for the wisdom of the Perl Monks concerning the following question:

Supposing you have

my $ref = \"stuff";
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then I know this is a reference to a literal string, and I would be a bit silly to try

$$ref =~ s/u/i/;
## or even 
$$ref .= " and more stuff";
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But, I have this function that modifies a string, and is designed to modify the actual value:

sub modify_string{
  my $arg = \shift;

  ## $$arg is modified, no need for the details, eg
  $$arg =~ s/u/i/;
  return $$arg;
}
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so now I can say

my $string = "stuff";
modify_string( $string );

## or even
my $copy = modify_string( $string );
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Now, some time later I wrote:

my $new_string = modify_string ( "literal stuff" );
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And to my surprise this works fine! I tested my new code on my ActivePerl 5.6.0 and everything was cool and dandy, so I think, just another of the Perl magic DWIMs.

Then I moved the code to the unix server, also running 5.6.0, it all fell over in a heap. Or I should say, gave the error I expected in the first place: Attempt to modify constant value.

So does anyone (a) know why there is the difference (b) which one is correct and (c) do I need a slapping for writing $arg = \shift; in the first place??

iakobski

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Re: Modifying a reference to a literal string by hdp (Beadle) on May 02, 2001 at 19:53 UTC
I'm not sure why there's a difference -- I don't have a windows machine to test it on, but I would have expected the "attempt to modify readonly value" error everywhere. Keep in mind that you can directly modify the parameters passed to a function by messing with `@_`. You don't need to use `\shift`; I suggest rewriting modify_string as: `sub modify_string { $_[0] =~ s/u/i/; return $_[0]; }` [download] Your way isn't "wrong", but (relatively) it does a lot of extra work. hdp.	[reply] [d/l] [select]

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Re: Modifying a reference to a literal string
by hdp (Beadle) on May 02, 2001 at 19:53 UTC

Keep in mind that you can directly modify the parameters passed to a function by messing with @_. You don't need to use \shift; I suggest rewriting modify_string as:

sub modify_string {
  $_[0] =~ s/u/i/;
  return $_[0];
}
[download]

hdp.

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