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

OK create 3 modules A, B & C in 3 separate files all in the same directory like so: A.pm: 
package A;

sub foo { print "xyzzy\n"; }

1;

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B.pm: 
package B;
use base 'A';

1;

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and C.pm: 
package C;
use base 'B';

1;

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and now set up a 'test.pl': 
#!/usr/bin/perl

require 'C.pm';

C::foo();

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Run it and you get:

22. perl test.pl
Can't locate object method "foo" via package "C" at C.pm line 5.

Why doesn't it find the sub foo in 'A.pm'? I must be making a newbie mistake, but for the life of me I can't see what.

Replies are listed 'Best First'.
Re: Inheritance strangeness
by Joost (Canon) on Sep 08, 2004 at 07:58 UTC
[reply]
[d/l]
Re: Inheritance strangeness
by Zaxo (Archbishop) on Sep 08, 2004 at 07:57 UTC

    Do $ perl -e'print "@INC"' from the command line to see whether the core lib location or '.' (dot) is searched first. B is perl's core backend module, and doesn't have a foo() function.

    After Compline,
    Zaxo

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[d/l]
Re: Inheritance strangeness
by BUU (Prior) on Sep 08, 2004 at 07:49 UTC
    The mistake is simple. Bar::foo() just tries to call the function foo() from package Bar. If you want to call an inherited method, you must use the class method call: Bar->foo()
[reply]
[d/l]
[select]
      Change test.pl to 
      #!/usr/bin/perl

require 'C.pm';

C->foo();

      [download]
      and you get:

      55. perl test.pl
      Can't locate object method "foo" via package "C" at test.pl line 5.

[reply]
[d/l]
        Watch for the bold 
        C:\e>echo package A;sub foo{666};1; >A.pm

C:\e>echo package B;use base 'A';1; >B.pm

C:\e>echo package C;use base 'B';1; >C.pm

C:\e>perl -e"require 'C.pm';die C->foo()"
Can't locate object method "foo" via package "C" at -e line 1.


C:\e>perl -le"require 'C.pm';print for %INC"
XSLoader.pm
C:/Perl/lib/XSLoader.pm
Carp.pm
C:/Perl/lib/Carp.pm
warnings/register.pm
C:/Perl/lib/warnings/register.pm
Exporter.pm
C:/Perl/lib/Exporter.pm
vars.pm
C:/Perl/lib/vars.pm
strict.pm
C:/Perl/lib/strict.pm
C.pm
C.pm
warnings.pm
C:/Perl/lib/warnings.pm
base.pm
C:/Perl/lib/base.pm
B.pm
C:/Perl/lib/B.pm

C:\e>

        MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
        I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
        ** The third rule of perl club is a statement of fact: pod is sexy.

[reply]

        This does what you'd expect:

        use C;
C->foo();

        [download]

        Update: In my test code, I had a use lib "[directory with A.pm, B.pm, C.pm]" before use C;. That means I explicitly got the new B.pm instead of the system one. Everyone else is right that if you're picking up the system B.pm first, that's a problem.

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Re: Inheritance strangeness
by cleverett (Friar) on Sep 08, 2004 at 08:07 UTC
    Damn,

    I tried to make simple test case for a problem I'm having with mod_perl spread out over 5 modules and using multiple inheritance, and I ran into the stupid B module.

    The test case now works, but my mod_perl issue still exists ... Grr!

[reply]
      And the same answer as above applies...

      C::foo is a function call, C->foo is a method call.

      C::foo is procedural style and doesn't inherit, C->foo is an OO style and does inherit.
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Re: Inheritance strangeness
by cleverett (Friar) on Sep 08, 2004 at 07:48 UTC
    Whoops I meant:

    run it and you get:

    53. perl test.pl
    Undefined subroutine &C::foo called at test.pl line 5.

[reply]

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