[SOLVED] When is a new() call necessary for perl XML::LibXML

When is a new() call necessary for perl XML::LibXML

I wrote a program using perl's XML::LibXML

It creates a XML::LibXML using load_xml().

I was surprised that it doesn't need a call to new().

When is a call to new() necessary?

Here is the code that surprisingly (to me) works:

#!/usr/bin/env perl
use 5.020;
use warnings;
use XML::LibXML;

#the input xml
my $inputstr = <<XML;
<a>
<b class="type1">some type 1 data</b>
<b class="type2">some type 2 data</b>
<b class="type3">some type 3 data</b>
<b class="type4">some type 4 data</b>
<b notaclass="type1">don't change this</b>
<c class="type1">don't change this either</c>
</a>
XML

my $dom = XML::LibXML->load_xml(
string => $inputstr
);

say $dom->toString();

Solution

XML::LibXML uses a style where methods work as both object and class methods. This is for convenience. If you call XML::LibXML->load_xml it will create a parser object for you, configure it using your arguments, load the XML, and then throw the parser object out.

sub load_xml {
  my $class_or_self = shift;
  my %args = map { ref($_) eq 'HASH' ? (%$_) : $_ } @_;
 
  my $URI = delete($args{URI});
  $URI = "$URI"  if defined $URI; # stringify in case it is an URI object
  my $parser;
  # if called as an object method
  if (ref($class_or_self)) { 
    $parser = $class_or_self->_clone();
    $parser->{XML_LIBXML_PARSER_OPTIONS} = $parser->_parser_options(\%args);
  # if called as a class method
  } else {
    $parser = $class_or_self->new(\%args);
  }

That means these two pieces of code are equivalent.

my $dom = XML::LibXML->load_xml(
  string => $inputstr
);

my $dom = XML::LibXML->new(
  string => $inputstr
)->load_xml(
  string => $inputstr
);

When is a call to new() necessary?

When you want to change the default parser options and reuse the parser object. Or when you want to use a method that is only available as an object method.

For example, you might want to pass a configured XML parser into another function, or use it to configure the behavior of another object.

my $parser = XML::LibXML->new(pedantic_parser => 1);
my $object = Some::Class->new(xml_parser => $parser);

# This will use $parser which has been configured to be pedantic.
$object->do_something_with_an_xml_document($document);