Re: static variable hack

This line

my $static = 42 if 0;
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is optimized away after compilation, because the 0 is a constant:

qwurx [shmem] ~ > perl -MO=Deparse
sub foo {
    my $static = 42 if 0;
    print "static is now $static\n";
    $static++;
}

foo() for 1..5;
__END__
sub foo {
    '???';
    print "static is now $static\n";
    $static++;
}
;
foo  foreach (1 .. 5);
__DATA__
- syntax OK
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Note the '???' showing a statement swallowed by the optimizer.

Because optimization happens after compile, the lexical variable declared via my already has been seen and allocated for the subroutine's scope.

That's why that construct passes strictures. And - that' the hack - because the my declaration isn't seen ever again at runtime, the lexical inside that sub doesn't get cleared on subsequent invocations of the sub.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

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Re^2: static variable hack by ikegami (Patriarch) on Oct 29, 2007 at 20:26 UTC
Overall right. Just nitpicking two details. First, optimization is not a factor. It's simply whether the `my` gets executed or not. Second, you're also off on the timing. Clearing occurs at end of scope. It's triggered by the `my`, but it doesn't occurs when the `my` is encountered. (By the way, that's also when a new `my` variable is created if the refcount is greater than 1.) Both points can be observed in `sub foo { my ($reset) = @_; my $x if $reset; # Will reset at end of scope if $reset is true. return ++$x; } print foo(0), "\n"; # 1 print foo(0), "\n"; # 2 print foo(1), "\n"; # 3 print foo(0), "\n"; # 1 print foo(0), "\n"; # 2` [download]	[reply] [d/l] [select]

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Re^2: static variable hack
by ikegami (Patriarch) on Oct 29, 2007 at 20:26 UTC

Overall right. Just nitpicking two details.

First, optimization is not a factor. It's simply whether the my gets executed or not.

Second, you're also off on the timing. Clearing occurs at end of scope. It's triggered by the my, but it doesn't occurs when the my is encountered. (By the way, that's also when a new my variable is created if the refcount is greater than 1.)

Both points can be observed in

sub foo {
   my ($reset) = @_;
   my $x if $reset;   # Will reset at end of scope if $reset is true.
   return ++$x;
}

print foo(0), "\n";  # 1
print foo(0), "\n";  # 2
print foo(1), "\n";  # 3
print foo(0), "\n";  # 1
print foo(0), "\n";  # 2
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