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.\" ========================================================================
.\"
.IX Title "base 3pm"
.TH base 3pm "2009-02-12" "perl v5.10.1" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
base \- Establish an ISA relationship with base classes at compile time
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    package Baz;
\&    use base qw(Foo Bar);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Unless you are using the \f(CW\*(C`fields\*(C'\fR pragma, consider this module discouraged
in favor of the lighter-weight \f(CW\*(C`parent\*(C'\fR.
.PP
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
.PP
.Vb 6
\&    package Baz;
\&    BEGIN {
\&        require Foo;
\&        require Bar;
\&        push @ISA, qw(Foo Bar);
\&    }
.Ve
.PP
\&\f(CW\*(C`base\*(C'\fR employs some heuristics to determine if a module has already been
loaded, if it has it doesn't try again. If \f(CW\*(C`base\*(C'\fR tries to \f(CW\*(C`require\*(C'\fR 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:
.PP
.Vb 2
\&        package Foo;
\&        sub exclaim { "I can have such a thing?!" }
\&        
\&        package Bar;
\&        use base "Foo";
.Ve
.PP
If \f(CW$VERSION\fR is not detected even after loading it, <base> will define \f(CW$VERSION\fR
in the base package, setting it to the string \f(CW\*(C`\-1, set by base.pm\*(C'\fR.
.PP
\&\f(CW\*(C`base\*(C'\fR will also initialize the fields if one of the base classes has it.
Multiple inheritance of fields is \fB\s-1NOT\s0\fR 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.
.PP
The base class' \f(CW\*(C`import\*(C'\fR method is \fBnot\fR called.
.SH "DIAGNOSTICS"
.IX Header "DIAGNOSTICS"
.ie n .IP "Base class package ""%s"" is empty." 4
.el .IP "Base class package ``%s'' is empty." 4
.IX Item "Base class package %s is empty."
base.pm was unable to require the base package, because it was not
found in your path.
.IP "Class 'Foo' tried to inherit from itself" 4
.IX Item "Class 'Foo' tried to inherit from itself"
Attempting to inherit from yourself generates a warning.
.Sp
.Vb 2
\&    use Foo;
\&    use base \*(AqFoo\*(Aq;
.Ve
.SH "HISTORY"
.IX Header "HISTORY"
This module was introduced with Perl 5.004_04.
.SH "CAVEATS"
.IX Header "CAVEATS"
Due to the limitations of the implementation, you must use
base \fIbefore\fR you declare any of your own fields.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
fields
