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The @ISA variable is a package variable that is used by all classes to indicate the class's parent (or parents). While this variable can be safely read to learn a class's inheritence inheritance hierarchy, it must not be modified at runtime [Conway 05].

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{
  package Base;

  sub new {
    my $class = shift;
    my $self = {}; # no parent
    bless $self, $class;
    print "new Base\n";
    return $self;
  };

  sub base_value {return 1;}
}


{
  package Derived;
  our @ISA = qw(Base);  # establishes inheritenceinheritance

  sub new {
    my $class = shift;
    my $self = $class->SUPER::new(@_);  # relies on established inheritenceinheritance
    print "new Derived\n";
    return $self;
  };

  sub derived_value {return 2;}
}


BEGIN {
  my $derived = Derived->new();
  my $b = $derived->base_value();
  my $d = $derived->derived_value();
  print "base_value = $d\n";
  print "derived_value = $d\n";
}

When the code is run, we get a program error:

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This error occurs because the BEGIN block is evaluated at the beginning of runtime, before the @ISA statement can be evaluated. ThereforeConsequently, when the Derived::new() constructor is invoked, the Derived class has an empty parents list and therefore fails to invoke Base::new().

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# ... package Base is unchanged

{
  package Derived;
  use parent qw(Base);

  sub new {
    my $class = shift;
    my $self = $class->SUPER::new(@_);  # relies on established inheritenceinheritance
    print "new Derived\n";
    return $self;
  };

  sub derived_value {return 2;}
}

# ... The rest of the code is unchanged

The parent module establishes the inheritence inheritance hierarchy at parse time, before any runtime code, including the BEGIN block, is evaluated. Therefore, when When the Derived::new() constructor is invoked, Perl knows that Derived is an instance of Base, and the program produces the correct output:

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Risk Assessment

Modifying class inheritence inheritance at runtime can introduce subtle bugs , and is usually a sign of poor design.

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Bibliography

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