Why doesn't my Perl blessed filehandle doesn't return true with `can('print')`'?

问题

For some reason, I can't get filehandles working with Expect.pm's log_file method. I originally got help on How can I pass a filehandle to Perl Expect's log_file function?, where it was suggested that I use an IO::Handle filehandle to pass to the method. This seems to be a different issue, so I thought I'd start a new question.

This is the offending section of Expect.pm:

if (ref($file) ne 'CODE') {
  croak "Given logfile doesn't have a 'print' method"
    if not $fh->can("print");
  $fh->autoflush(1);        # so logfile is up to date
}

So, then, I tried this sample code:

use IO::Handle;
open $fh, ">>", "file.out" or die "Can't open file";
$fh->print("Hello, world");
if ($fh->can("print"))
{
  print "Yes\n";
}
else
{
  print "No\n";
}

When I run this, I get two (to my mind) conflicting items. A file with a single line that says 'Hello, world', and output of 'No'. To my mind, the $fh->can line should return true. Am I wrong here?

回答1:

Odd, it looks like you need to create a real IO::File object to get the can method to work. Try

use IO::File;

my $fh = IO::File->new("file.out", ">>")
    or die "Couldn't open file: $!";

回答2:

IO::Handle doesn't overload the open() function, so you're not actually getting an IO::Handle object in $fh. I don't know why the $fh->print("Hello, world") line works (probably because you're calling the print() function, and when you do things like $foo->function it's equivalent to function $foo, so you're essentially printing to the filehandle like you'd normally expect).

If you change your code to something like:

use strict;
use IO::Handle;
open my $fh, ">>", "file.out" or die "Can't open file";
my $iofh = new IO::Handle;
$iofh->fdopen( $fh, "w" );
$iofh->print("Hello, world");
if ($iofh->can("print"))
{
  print "Yes\n";
}
else
{
  print "No\n";
}

...then your code will do as you expect. At least, it does for me!

来源：https://stackoverflow.com/questions/3721148/why-doesnt-my-perl-blessed-filehandle-doesnt-return-true-with-canprint

标签

perl

file

handle