Re^3: when is destroy function called

[Athanasius]

Sorry, my bad. I added use warnings to my copy of ggoebel’s script; forgot that it wasn’t in the original; and then compounded the confusion by referring to the warning which results as an “error’. The warning in question is:

Use of uninitialized value in print at...

which comes from the inclusion of undef($obj) in the arguments to the print statement.

The evals are there to allow the print statement to complete in spite of the error (exception) which results from the attempt to call a method on an undefined object. That error is:

Can't call method "exterminate" on an undefined value at (eval 2) line
+ 1.

which in the original example was captured by the eval and printed by the statement print $@ if $@;.

I agree that your example “seems to prove” my conclusion; but I wanted to be sure. Hence my provision of an additional resource which is altered within the DESTROY method. Adapting your example:

#! perl
use strict;
use warnings;
use 5.010;

$| = 1;
my $num = 42;

{
    package Dog;

    sub new     { return bless {}; }
    sub bark    { say 'Bow wow!'; }
    sub DESTROY { print "destroy\n"; ++$num; }
}

my $dog = Dog->new();

print 'hello ', 
      $dog->bark(),
      undef $dog, 
      eval('$dog->bark()'),
      "|$num|",
      "world\n";

print $num;

Output:

18:02 >perl 552_SoPW.pl
Bow wow!
Use of uninitialized value in print at 552_SoPW.pl line 31.
hello 1|42|world
destroy
43
18:02 >

which shows that $num is incremented only after the print statement completes, and not at the point where the reference count of $dog falls to zero. (Note that the “1” in the output comes from the say statement in Dog::bark, which returns true on success.)

Hope that helps,