If I want to instantiate an object, but don't know whether I have a Sedan or an SUV, can I ask the base class to figure that out for me and return the right object?

You certainly can.

Is that good practice?

This probably debatable. From a theoretical perspective, a parent class should not depend on its child classes, just the other way. Or formulated differently, you want to avoid cycles in your dependency graphs.

From a practical point of view, there often needs to be some piece of code that needs to create as-specific-as-possible objects. Why not have put it in the parent class?

An approach I found practical is to have some kind of "type registry", which is usually just a hash. On object creation, some kind of key looks into the hash, and then simply re-dispatches to new method of that type.

package Car;

has %car_by_type;

sub new {
   my ($self, %opts) = @_;
   if ($opts{name} && $car_by_type{$opts{name}}) {
      $car_by_type{$opts{name}}->new(%opts);
   }
   else {
      die "Don't know how to make a new car";
   }
}

method REGISTER_TYPE {
    my ($self, $name, $type) = @_;
    $car_by_type{$name} = $type;

}

package Car::BMW;
our @ISA = qw/Car/;
Car::BMW->REGISTER_TYPE('Z1', __PACKAGE__);
Car::BMW->REGISTER_TYPE('Z3', __PACKAGE__);

package main;
my $z1 = Car->new(name => 'Z1');

[download]

(untested)

Perl 6 - the future is here, just unevenly distributed

In reply to Re: Inheritance: parent class determine which child class to use? by moritz
in thread Inheritance: parent class determine which child class to use? by fbicknel

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.