base

Section: Perl Programmers Reference Guide (3pm)
Updated: 2007-12-18
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NAME

base - Establish an ISA relationship with base classes at compile time  

SYNOPSIS

    package Baz;
    use base qw(Foo Bar);
 

DESCRIPTION

Allows you to both load one or more modules, while setting up inheritance from those modules at the same time. Roughly similar in effect to 

    package Baz;
    BEGIN {
        require Foo;
        require Bar;
        push @ISA, qw(Foo Bar);
    }

"base" employs some heuristics to determine if a module has already been loaded, if it has it doesn't try again. If "base" tries to "require" the module it will not die if it cannot find the module's file, but will die on any other error. After all this, should your base class be empty, containing no symbols, it will die. This is useful for inheriting from classes in the same file as yourself, like so: 

        package Foo;
        sub exclaim { "I can have such a thing?!" }
        
        package Bar;
        use base "Foo";

If $VERSION is not detected even after loading it, <base> will define $VERSION in the base package, setting it to the string "-1, set by base.pm".

"base" will also initialize the fields if one of the base classes has it. Multiple inheritance of fields is NOT supported, if two or more base classes each have inheritable fields the 'base' pragma will croak. See fields, public and protected for a description of this feature.

The base class' "import" method is not called.  

DIAGNOSTICS

Base class package %s is empty.
base.pm was unable to require the base package, because it was not found in your path.
Class 'Foo' tried to inherit from itself
Attempting to inherit from yourself generates a warning. 

    use Foo;
    use base 'Foo';
 

HISTORY

This module was introduced with Perl 5.004_04.  

CAVEATS

Due to the limitations of the implementation, you must use base before you declare any of your own fields.  

SEE ALSO

fields