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问题:
I've written some Perl code which compose two classes inherent from a base one. I suppose it would print something like this
Mik: Meow! Meow! Sat: Woof! Woof!
But it actually print this way:
Sat: Woof! Woof! Sat: Woof! Woof!
,
package Animal; sub new { my $obj = shift; my $name = shift; our %pkg = ( 'name' => $name ); bless \%pkg, $obj; return \%pkg; } package Cat; @ISA = ("Animal"); sub new { my $obj = shift; my $name = shift; my $self = $obj->SUPER::new($name); return $self; } sub get_name { my $obj = shift; return $obj->{'name'}; } sub talk { my $obj = shift; return "Meow! Meow!"; } package Dog; @ISA = ("Animal"); sub new { my $obj = shift; my $name = shift; my $self = $obj->SUPER::new( $name ); return $self; } sub get_name { my $obj = shift; return $obj->{'name'}; } sub talk { my $obj = shift; return "Woof! Woof!"; } package Main; my $cat = new Cat('Mike'); my $dog = new Dog('Sat'); print $cat->get_name() . ": " . $cat->talk() , "\n"; print $dog->get_name() . ": " . $dog->talk() , "\n";
But if I change the caller in this way, it prints what I suppose to be. So it is quite strange why the $cat object was overwritten after the $dog was instantiated?
package Main; my $cat = new Cat('Mily'); print $cat->get_name() . ": " . $cat->talk() , "\n"; my $dog = new Dog('Sat'); print $dog->get_name() . ": " . $dog->talk() , "\n";
回答1:
Why do you bless into a global variable? Change your constructor to:
sub new { my $obj = shift; my $name = shift; my %pkg = ( 'name' => $name ); bless \%pkg, $obj; return \%pkg; }
Better yet, change it to something more idiomatic:
sub new { my $class = shift; my $name = shift; my $self = { name => $name }; return bless $self, $class; }
Moving on:
Why implement new and get_name in each kind of animal? Both methods can be inherited. While we're at it, we might as well get rid off the messing around with @ISA:
package Animal; sub new { my $class = shift; my $name = shift; my $self = { name => $name }; return bless $self, $class; } sub get_name { my $self = shift; return $self->{'name'}; } package Cat; use base qw/ Animal /; sub talk { my $self = shift; return "Meow! Meow!"; } package Dog; use base qw/ Animal /; sub talk { my $self = shift; return "Woof! Woof!"; } package Main; my $cat = Cat->new('Mike'); my $dog = Dog->new('Sat'); print $cat->get_name() . ": " . $cat->talk() , "\n"; print $dog->get_name() . ": " . $dog->talk() , "\n";
May I ask which tutorial or book you are following?
While the above is perfectly fine, you might as well do it the Modern Perl way:
package Animal; use Moose; has name => ( required => 1, is => 'rw', isa => 'Str' ); package Cat; use Moose; extends 'Animal'; has talk => ( default => "Meow! Meow!", is => 'ro' ); package Dog; use Moose; extends 'Animal'; has talk => ( default => "Woof! Woof!", is => 'ro' ); package Main; my $cat = Cat->new( name => 'Mike'); my $dog = Dog->new( name => 'Sat'); print $cat->name . ": " . $cat->talk , "\n"; print $dog->name . ": " . $dog->talk , "\n";
回答2:
You have declared the variable to store the instance data using
our %pkg
This is an alias for a single data structure (%Animal::pkg), so all your objects are using the same hash. Change our to my in order to create a new hash each time.
It might be worth noting that "inside-out" objects in Perl can and do use a shared data structure in the package to store instance data, but there is an additional level of abstraction required to make that work, and I wouldn't recommend starting OO Perl with them, they are an acquired taste.
回答3:
In a nutshell: our declares package variables, so every time our %pkg = (...) is executed, you assign a new value to the same variable. As all \%pkg references point to the same var, all return values of new are the same object. A reference can only be blessed into one class, so the last one wins.
Just change the our to my, and it should work as expected.
