Superfox il Volpone has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I was thinking to tie the *STDOUT handle to mask the values to be printed in the stardard output. Though, I hit a deep recursion error, probably because I lose the original reference to *STDOUT and invokes my new handle when trying to print: 
package hyundai{
  
  sub TIEHANDLE{
    my $class = shift; 
    my $oldhandle = shift;
    return bless ({"handle" => $oldhandle}, $class); 
  }
  
  my $mask = sub{
    (my $text = shift()) =~ s/a/b/g; # replaces a with b
    return $text;
  };
  
  sub PRINT{
    my $self = shift;
    die("Not a ref") if !ref($self); 
    my $text = join('',@_);
    print STDERR ("STDOUT: ", \*STDOUT, "\n"); # prints GLOB(0x880a20)
+ 
    print STDERR ("STDERR: ", \*STDERR, "\n"); # prints GLOB(0x880a80)
    print STDERR ("handle: ", \$self->{"handle"}, "\n"); # prints GLOB
+(0x8806c0)
    
    $self->{"handle"}->print( &$mask($text) ); # infinite recursion !
  }
  
};


print("Hello world 1 aaaaaaa\n");
my $oldhandle = *STDOUT;
undef(*STDOUT);
tie *STDOUT, "hyundai", $oldhandle;
print("Hello world 2 aaaaaaa\n");

[download]
I was following this (old) post related to the same issue, though it does not seem that a working solution was proposed. Any idea how to proceed ?

s.fox

Replies are listed 'Best First'.
Re: Tie *STDOUT, recursion error
by choroba (Cardinal) on Apr 11, 2015 at 19:31 UTC
    Interestingly, the following works: 
    print { $self->{handle} } $mask->($text);

    [download]
    لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
[reply]
[d/l]
      hi choroba, shouldn't the two syntaxes be equivalent?! also, why is {$self->{"handle"}} and not *{$self->{"handle"}}. My understanding was that { } is used to deference a reference, such as ${ $ref_scalar }, so a sigil was required. But I am wrong as the above code example proves :/

      s.fox
[reply]
Re: Tie *STDOUT, recursion error
by Athanasius (Archbishop) on Apr 12, 2015 at 06:20 UTC

    Hello Superfox il Volpone,

    I get no infinite recursion on my system (Windows 8.1, 64-bit):

    16:17 >perl 1215_SoPW.pl
Hello world 1 aaaaaaa
STDOUT: GLOB(0x1c94bc8)
STDERR: GLOB(0x1c94c28)
handle: GLOB(0x55c2d8)
Hello world 2 bbbbbbb

16:17 >perl -v

This is perl 5, version 20, subversion 2 (v5.20.2) built for MSWin32-x
+64-multi-thread
...

    [download]

    (Strawberry Perl, portable.)

    Hope that is of interest,

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

[reply]
[d/l]
Re: Tie *STDOUT, recursion error
by Anonymous Monk on Apr 11, 2015 at 19:37 UTC
[reply]
      thanks for the reply. I was looking to the source code of Tie::STDOUT. He basically solves it with something like: 
          open(my $stdout, ">&", STDOUT); 
...
# in the overloaded method PRINT
    $stdout->print( @_ ); # this refers to the original STDOUT.

      [download]
      I guess this opens a different underlying file handle, while my $oldhandle = STDOUT is an assignment/alias to the same file handle used by STDOUT.

      s.fox
[reply]
[d/l]
Re: Tie *STDOUT, recursion error
by Superfox il Volpone (Sexton) on Apr 12, 2015 at 09:52 UTC
    hey Athanasius,
    it is interesting indeed. In my case I am under perl v5.16.3 on CentOS 7. I am starting to believe this is a non portable behaviour of perl.

    s.fox
[reply]

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