in reply to Re^5: How to safely use $_ in a library function?
in thread How to safely use $_ in a library function?

Yes, Internals::SvREADONLY is there all right:

 1:35 >perl -E "say for sort keys %Internals::"
SvREADONLY
SvREFCNT
V
hv_clear_placeholders

 1:35 >

[download]

But it doesn’t work for me. A Windows issue, maybe?

Thanks for the information on where to find the Internals package in the source code.

Update: I tried the Internals module, but that doesn’t work (for me, on Windows) either:

#! perl
use strict;
use warnings;
use Internals qw(SetReadWrite);

for (qw(literal1 literal2 literal3))
{
    print "$_\n";
    foo('wilma');
}

sub foo
{
    SetReadWrite(\$_);
    $_ = 'fred';
    goto &bar;
}

sub bar
{
    my ($name) = @_;
    print "foo --> name = $name, \$_ = $_\n";
}

[download]

Output:

12:26 >perl 723_SoPW.pl
literal1
Modification of a read-only value attempted at 723_SoPW.pl line 15.

12:28 >

[download]

Even IsWriteProtected fails:

#! perl
use strict;
use warnings;
use Internals qw(IsWriteProtected);

for (qw(literal1 literal2 literal3))
{
    print "$_\n";
    foo('wilma');
}

sub foo
{
    if (IsWriteProtected(\$_))
    {
        warn "Can't modify \$_";
    }
    else
    {
        $_ = 'fred';
    }

    goto &bar;
}

sub bar
{
    my ($name) = @_;
    print "foo --> name = $name, \$_ = $_\n";
}

[download]

Output:

12:28 >perl 723_SoPW.pl
literal1
Modification of a read-only value attempted at 723_SoPW.pl line 20.

12:36 >

[download]

:-(

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^7: How to safely use $_ in a library function?
by tobyink (Canon) on Sep 19, 2013 at 07:10 UTC

    Does Const::Fast work for you?

    use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name
[reply]

      Yes, it seems to work as advertised:

      17:39 >perl -Mstrict -wE "my $s = 42; say $s; ++$s; say $s;"
42
43

17:39 >perl -Mstrict -MConst::Fast -wE "const my $s => 42; say $s; ++$
+s; say $s;"
42
Modification of a read-only value attempted at -e line 1.

17:39 >

      [download]

      How does that help?

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

[reply]
[d/l]

        Const::Fast is basically just a wrapper around Internals::SvREADONLY.

        $ perldoc -m Const::Fast | grep SvREADONLY
    if (my $reftype = reftype $_[0] and not blessed($_[0]) and not &In
+ternals::SvREADONLY($_[0])) {
        &Internals::SvREADONLY($_[0], 1);
    Internals::SvREADONLY($_[0], 1);
    croak 'Attempt to reassign a readonly variable' if &Internals::SvR
+EADONLY($_[0]);

        [download]
        use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name
[reply]
[d/l]

In Section Seekers of Perl Wisdom