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Re: Multiple "constructors", possible to call several.

by $code or die (Deacon)
on Feb 17, 2002 at 15:49 UTC ( #146000=note: print w/replies, xml ) Need Help??


in reply to Multiple "constructors", possible to call several.

As you say, programming like this probably causes some maintenance problems. Having said that, you should look at UNIVERSAL::isa()

Your test of $invokant also won't work if called by a class inheriting from MyClass:

package MyClass; sub new { print shift; } package MySubClass; @MySubClass::ISA = qw(MyClass); package main my $test = new MySubClass; __DATA__ prints "MySubClass"

So having said that, I probably would write the constructor like this:

sub _new_or_old { my $invokant = shift; defined( $invokant ) or return undef; if( UNIVERSAL::isa($invokant, 'MyClass' ) { return $invokant; } else { my $self = {}; bless( $self, $invokant ); return $self; } }
___ Simon Flack ($code or die)
$,=reverse'"ro_';s,$,\$,;s,$,lc ref sub{},e;$,
=~y'_"' ';eval"die";print $_,lc substr$@,0,3;

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Re: Re: Multiple "constructors", possible to call several.
by Dog and Pony (Priest) on Feb 17, 2002 at 16:24 UTC
    Yeah, I know that inheritance might be a problem, but decided to ignore that for now. I know, I know, bad toad...

    Actually, I did look at UNIVERSAL::isa(), but I didn't get that to work properly. Matter of factly, your code is very much like I started to write it - and your code breaks my testcases for this. This is what I get from them:

    1..11 Testing to construct via 'start_stuff()' Can't use string ("MyClass") as a HASH ref while "strict refs" in use +at MyClass.pm line 61 (#1) (F) Only hard references are allowed by "strict refs". Symbolic references are disallowed. See perlref. Uncaught exception from user code: Can't use string ("MyClass") as a HASH ref while "strict refs" + in use at MyClass.pm line 61. MyClass::start_stuff('MyClass') called at testmyclass.pl line +10
    And these are the very simple testcases:
    #!/usr/bin/perl -w use strict; use Test; BEGIN { plan tests => 11 }; use MyClass; print "Testing to construct via 'start_stuff()'\n"; my $object_via_start = MyClass->start_stuff(); ok( ref( $object_via_start ), 'MyClass' ); ok( UNIVERSAL::isa( $object_via_start, 'MyClass' )); print "Testing to construct via 'init_stuff()'\n"; my $object_via_init = MyClass->init_stuff(); ok( ref( $object_via_init ), 'MyClass' ); ok( UNIVERSAL::isa( $object_via_init , 'MyClass' )); print "Testing to invoke 'start_stuff()' and 'use_stuff()' \n"; print "via object from 'init_stuff()'\n"; print "(Should not return a new object).\n"; my $start_stuff_after_init = $object_via_init->start_stuff(); my $start_stuff_after_use = $object_via_init->use_stuff(); ok( ref( $start_stuff_after_init ), 'MyClass' ); ok( UNIVERSAL::isa( $start_stuff_after_init , 'MyClass' )); ok( ref( $start_stuff_after_use ), 'MyClass' ); ok( UNIVERSAL::isa( $start_stuff_after_use , 'MyClass' )); ok( $object_via_init, $start_stuff_after_init ); ok( $object_via_init, $start_stuff_after_use ); ok( $start_stuff_after_init, $start_stuff_after_use );
    So my question now is, are my testcases faulty? I would bet on that, since I am no expert at this, but no matter how I tried, I concluded that UNIVERSAL::isa() did not work.

    But this might be why that approach doesn't work. From the docs of UNIVERSAL:

    UNIVERSAL::isa ( VAL, TYPE )
    isa returns true if one of the following statements is true.

    • VAL is a reference blessed into either package TYPE or a package which inherits from package TYPE.
    • VAL is a reference to a TYPE of Perl variable (e.g. 'HASH').
    • VAL is the name of a package that inherits from (or is itself) package TYPE.

    The last point there being the catch. Since, when invoked for the first time, VAL will indeed be "MyClass" (the string). Which is what throws the exception you see above.

    This did indeed bother me, so any ideas on why this is happening, or what I am doing wrong, or how I could redesign, would still be helpful.

    Minor detail: Due to the way my constructor was finally written, it was well possible to fall off the end, that is why return undef was at the end.

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