Re: What's a reference? What's a variable? What's scope?

[Juerd]

Perl uses a garbage collector. The technique for collecting garbage that Perl uses is called 'reference counting'.

Every variable in Perl has a counter, the reference count. When the variable is created, the counter is set to 1. When the variable is referenced, it is increased by one.

use strict;                        # 1
{                                  # 2
    my $foo = "Hello, world!\n";   # 3
    print $foo;                    # 4
}                                  # 5
print $foo;                        # 6

Line 3: $foo is created, its reference count is now 1.
Line 4: $foo is used for print, its reference count is increased. The reference count is now 2.
Line 4: When print returns, the reference count is decreased. The reference count is now 1 again.
Line 5: $foo's scope ends, the reference count is decreased. Because the reference count is 0 now, the variable is destroyed.
Line 6: There is no $foo in this scope, and because the $foo in the block's scope no longer exists, there is no way to access it.

use strict;                        # 1
my $bar;                           # 2
{                                  # 3
    my $foo = "Hello, world!\n";   # 4
    print $foo;                    # 5
    $bar = \$foo;                  # 6
}                                  # 7
print $$bar;                       # 8

Line 4: $foo is created, its reference count is now 1.
Line 5: $foo is used for print, its reference count is increased. The reference count is now 2.
Line 5: When print returns, the reference count is decreased. The reference count is now 1 again.
Line 6: $bar is assigned a refence to $foo. $foo's reference count is now increased, and has a new value of 2.
Line 7: $foo's scope ends, the reference count is decreased. Because the reference count is 1 (not 0, as in the previous example) now, the variable is not destroyed.
Line 8: There is no $foo in this scope, but the variable that was called $foo in the block's scope can still be accessed through $bar. An extra $ is used to dereference it.

Further reading:

• MJD's reference tutorial: perlreftut
• MJD's scoping tutorial: Coping with Scoping

Additional nitpicking:

• Don't use an & when calling a subroutine. If you do so without using parens, the current @_ is implicitly passed.
• There's no need to exit() explicitly. If you want to make clear that no code follows, put your subs first (which has more advantages: your $hash_ref and %hash are shared throughout the program in the current situation - is that really what you want?)

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

[vek]

Juerd++

Nice explanation of reference counting.

-- vek --