Confused by reference sample code

Melly has asked for the wisdom of the Perl Monks concerning the following question:

In the article on HTML::Template at http://html-template.sourceforge.net/article.html, there is some sample code:

  foreach my $name (sort keys %fruit_data) {
    # get the color and shape from the data hash
    my ($color, $shape) = @{$fruit_data{$name}};
    
    # make a new row for this fruit - the keys are <TMPL_VAR> names
    # and the values are the values to fill in the template.
    my %row = ( 
               name => $name,
               color => $color,
               shape => $shape
              );

    # put this row into the loop by reference             
    push(@loop, \%row);
  }
[download]

... but I'm confused - this looks like (and testing seems to bear it out) that the contents of @loop will all be identical hashes (since they are all references to a single hash, %row).

Here's the code I used to check this:

print "Here we push a hash onto a list, modify the hash and push again
+\n";
my @list;
my %hash;

%hash = (
  val1 => 'foo',
  var2 => 'bar'
);
print "contents of \$hash\n";
foreach(keys %hash){
  print "$_  = $hash{$_}\n";
}
push(@list, \%hash);

my $key;
print "deref contents of \@list\n";
foreach(@list){
  foreach $key(keys %$_){
    print "$key = $$_{$key}\n";
  }
}

%hash = (
  val1 => 'bish',
  val2 => 'bash'
);
print "contents of \$hash\n";
foreach(keys %hash){
  print "$_ = $hash{$_}\n";
}
push(@list, \%hash);
print "deref contents of \@list\n";
foreach(@list){
  foreach $key(keys %$_){
    print "$key = $$_{$key}\n";
  }
}
[download]

Is the article wrong, or have I missed something?

Tom Melly, tom@tomandlu.co.uk

Comment on Confused by reference sample code Select or Download Code

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Re: Confused by reference sample code by sgifford (Prior) on Mar 28, 2006 at 19:50 UTC
The key is the `my` in the sample code. That creates a new variable every time through the loop. Your test uses a global variable instead.	[reply] [d/l]
Re^2: Confused by reference sample code by Melly (Chaplain) on Mar 28, 2006 at 22:04 UTC
Many thanks - I must admit that the difference between 'my' inside a loop and outside in this context had elluded me (the distinction between 'new variable' and 'same variable' is somewhat blurred in this context). Question: if the memory allocated to the hash had not been passed by ref. to the array, would perl have released it's memory? Tom Melly, tom@tomandlu.co.uk	[reply]
Re^3: Confused by reference sample code by ikegami (Patriarch) on Mar 28, 2006 at 22:32 UTC
if the memory allocated to the hash had not been passed by ref. to the array, would perl have released it's memory? Yes. Perl uses reference counting to determine if a variable is still in use. As soon as there are no references to a variable, the memory is released back to Perl (but not necessarilly to the operating system). For example, `{ # Creates a scalar with value 'val1'. my $var1 = 'val1'; # 1 reference to val1. } # $var1 goes out of scope. # 0 references to val1. # No more references to 'val1', so it is released. my $var2; { # Creates a scalar with value 'val1'. my $var3 = 'val2'; # 1 reference to val2. $var2 = \$var3; # 2 references to val2. } # $var3 goes out of scope. # 1 reference to val2. $var2 = 'val3' # 0 references to val2. 1 reference to val3. # No more references to 'val2', so it is released.` [download]	[reply] [d/l]
Re: Confused by reference sample code by duff (Parson) on Mar 28, 2006 at 19:54 UTC
.. but I'm confused - this looks like (and testing seems to bear it out) that the contents of @loop will all be identical hashes (since they are all references to a single hash, %row). Nope. Your tests are wrong. The original code gets a new `%hash` on each iteration of the loop since it's declared with `my` inside the loop. Your test declares a single hash and manipulates the values of that single hash. For the test to be similar to the original code, you'd need to use `my` on both assignments. duff	[reply] [d/l] [select]
Re: Confused by reference sample code by InfiniteSilence (Curate) on Mar 28, 2006 at 21:08 UTC
I read that tutorial and I can see why you are confused. First, check out the HTML::Template perldoc for the section on the TMPL_LOOP tag. It shows the actual structure that is being used for TMPL_LOOP. If you want to use Data::Dumper, you can see it for yourself: use Data::Dumper; use CGI qw\|:standard\|; use HTML::Template; # the fruit data - the keys are the fruit names and the values are # pairs of color and shape contained in anonymous arrays my @loop; my %fruit_data = ( Apple => ['Red, Green or Yellow', 'Round'], Orange => ['Orange', 'Round'], Pear => ['Green or Red', 'Pear-Shaped'], Banana => ['Yellow', 'Curved'], ); print header(); my $tmplObj = HTML::Template->new(filename=>'..\..\tmpl\fruit.tmpl') +; for(keys %fruit_data){ push @loop, {name=>$_, color=>$fruit_data{$_}->[0], shape=>$fruit_data{$_}->[1] }; } my $whatIamReallyPassing = { fruit_loop => \@loop }; warn Dumper $whatIamReallyPassing; $tmplObj->param($whatIamReallyPassing); print $tmplObj->output; [download] Celebrate Intellectual Diversity	[reply] [d/l]