strange problem

[zfang]

Hi,

I have a strange problem with following code.

code1:

#!/usr/local/bin/perl
my $temp;
my $str;
$str = 'test';
$temp = 'str';
print ${$temp}."\n";

Only a blank line was printed, however if I got ride of the declaration of $str, ie
code 2:

#!/usr/local/bin/perl
my $temp;
$str = 'test';
$temp = 'str';
print ${$temp}."\n";

The output is: test
That's what I wanted, but I don't know why code 1 doesn't work?

Thanks,

George

 

[justice41]

It doesn't work because lexical variables (those declared with my()) don't exist in the symbol table, only package variables do. Therefore, lexical variables are not visible to soft references.
In your example ${$temp} evaluates to ${'str'} so Perl looks for $main::str in the symbol table. If you declared $str with my() it won't exist there so ${'str'} returns undef and you print out just the newline.
(Running the script with warnings on will tell you that you are printing an undefined value.)

See the perlref perldoc for more in depth discussion of this.

http://www.perldoc.com/perl5.8.0/pod/perlref.html

jaa

 

[zfang]

jaa, thank you very much for your reply. One more point I want to ask. If I use strict in a code, then I have to declare a variable using my(). So, in this case, are there any ways to get the results as did in code 2?

Thanks,

George

 

[justice41]

Declaring the variable with our() instead of my() writes it into the symbol table but strict will still not allow the soft reference.

One way around this is to locally turn of strict, or even just strict 'refs':

#!/usr/local/bin/perl
use strict;
my $temp;
$str = 'test';
$temp = 'str';
{
    no strict 'refs';
    print "${$temp}\n";
}  # Enclosing block limits scope of 'no strict'

Usually there are better ways to do something than using soft refs, though.

jaa

 

[zfang]

Thanks a lot. Have a nice weekend.