Inheritance - calling parent constructor

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

Replies are listed 'Best First'.
Re: Inheritance - calling parent constructor by ikegami (Patriarch) on Dec 29, 2009 at 18:12 UTC
or does the use base take care of this? base does not prevent you from overriding the parent's constructor. Nor does it prevent you from doing something silly like blessing the object twice. Fix: `package Child; ... sub new { my ($class, $arg1, $arg2, $arg3) = @_; my $self = $class->SUPER::new($arg1, $arg2); $self->{arg3} => $arg3; return $self; } ...` [download] Also - is there a difference between Yes: base doesn't load Exporter. base will load Parent if it's not loaded (or silently fail). base sets `@ISA` at compile. base doesn't disable strictures for `@ISA`.	[reply] [d/l] [select]
Re: Inheritance - calling parent constructor by kennethk (Abbot) on Dec 29, 2009 at 17:26 UTC
Assuming you are rolling your own constructor, as opposed to using an existing OO framework like Moose or Mouse, you should not explicitly invoke your parent class. Rather, as described in the Overridden Methods section of perltoot, you should invoke the pseudo class SUPER, as in: `package Child; use base qw(Parent); sub new { my ($class, $arg1, $arg2, $arg3) = @_; #my $self = SUPER->new($arg1, $arg2); my $self = $class->SUPER::new($arg1, $arg2); #$self->{arg3} => $arg3; $self->{arg3} = $arg3; #bless $self, $class; return $self; } return 1;` [download] You do need to explicitly invoke the SUPER class, since you are overriding the local new method, and Perl does not a priori know what you've decided to call your constructor. If you were to just say "new", it would call the constructor recursively. Regarding your second question, as documented in base, `use base 'Parent';` will perform the functions intended by the `use Exporter;...` code, though not following the explicitly same approaches. Update: Should have tested before posting. Corrected code above (left original commmented) and fixed an OP typo for argument assignment.	[reply] [d/l] [select]
Re^2: Inheritance - calling parent constructor by ikegami (Patriarch) on Dec 29, 2009 at 18:17 UTC
Wrong syntax. That will attempt to call `new` in package `SUPER`. And you forgot to remove the `bless` that's now useless. See my reply to the OP for the correction.	[reply] [d/l] [select]
Re^3: Inheritance - calling parent constructor by bob_dobalina (Sexton) on Dec 29, 2009 at 19:13 UTC
Wrong syntax. That will attempt to call new in package SUPER. - That's what I thought - i got the correct syntax from the link you had - thanks And you forgot to remove the bless that's now useless. - Could you elaborate here? I thought I have read that re-blessing is bad and was searching for best practices on re-blessing without much luck.	[reply]
Re^4: Inheritance - calling parent constructor by ikegami (Patriarch) on Dec 29, 2009 at 22:55 UTC
Re^5: Inheritance - calling parent constructor by JavaFan (Canon) on Dec 29, 2009 at 23:33 UTC
Some notes below your chosen depth have not been shown here
Re^2: Inheritance - calling parent constructor by bob_dobalina (Sexton) on Dec 29, 2009 at 18:42 UTC
thanks for the info - that works for me. I wonder the difference between using class name and SUPER? Is it OK to not have a constructor in the child object? This seems to work.	[reply]
Re^3: Inheritance - calling parent constructor by kennethk (Abbot) on Dec 29, 2009 at 18:52 UTC
The difference between hard-coding `Parent->` as opposed to invoking `SUPER` is relevant when you have multiple levels of inheritance. If you created a Grandchild, a method using SUPER would climb the `@ISA` tree to resolve `new`, whereas hard-coding `Parent->` will go straight there. It's also useful for code reuse, since SUPER is not package specific. For example, if you had a series of validation routines, you could open `sub validate {` with `my $self = shift; $self->SUPER::validate();` and it would crawl the inheritance tree correctly without refactoring and the associated fagility. You would only need to define a constructor in the child object if it requires additional functionality - in this case, because Child has an additional property. If you are interested in all this or rolling your own OO, a read read through of perlboot and perltoot is merited.	[reply] [d/l] [select]
Re^3: Inheritance - calling parent constructor by ikegami (Patriarch) on Dec 29, 2009 at 23:13 UTC
You could use `my $self = $class->Parent::new(...);` [download] instead of `my $self = $class->SUPER::new(...);` [download] And you might even have to in some weird situations (although `use mro 'c3';` would probably be a better fix). But using `Parent` is redundant. You already specified it's the parent class. To use it here is to needlessly hardcode a value. Nothing good can come of that.	[reply] [d/l] [select]
Re: Inheritance - calling parent constructor by almut (Canon) on Dec 29, 2009 at 18:33 UTC
`use Exporter; use vars qw(@ISA); @ISA = qw(Parent);` [download] Did you actually mean to say `use Exporter`, or rather `use Parent`? Just in case it hasn't become quite clear from the other replies, there is no reason to load Exporter if all you want is inheritance (except if you want to inherit from Exporter itself, of course).	[reply] [d/l] [select]
Re^2: Inheritance - calling parent constructor by bob_dobalina (Sexton) on Dec 29, 2009 at 19:17 UTC
Did you actually mean to say use Exporter, or rather use Parent? Just in case it hasn't become quite clear from the other replies, there is no reason to load Exporter if all you want is inheritance (except if you want to inherit from Exporter itself, of course). - No, I thought you had to use Exporter to use @ISA. Thanks for the correction. i went with use base as it seems like a clearer solution.	[reply]
Re^3: Inheritance - calling parent constructor by ikegami (Patriarch) on Dec 29, 2009 at 22:58 UTC
I prefer parent over base since it doesn't hide errors, but I don't see the point of using a module to replace `use Foo; our @ISA = 'Foo';` [download] It's only one line longer, and I don't buy that the module is any clearer.	[reply] [d/l]