base.pm revision 7c478bd95313f5f23a4c958a745db2134aa03244
=head1 NAME
base - Establish IS-A relationship with base class at compile time
=head1 SYNOPSIS
package Baz;
use base qw(Foo Bar);
=head1 DESCRIPTION
Roughly similar in effect to
BEGIN {
require Foo;
require Bar;
push @ISA, qw(Foo Bar);
}
Will also initialize the %FIELDS hash if one of the base classes has
it. Multiple inheritance of %FIELDS is not supported. The 'base'
pragma will croak if multiple base classes have a %FIELDS hash. See
L<fields> for a description of this feature.
When strict 'vars' is in scope I<base> also let you assign to @ISA
without having to declare @ISA with the 'vars' pragma first.
If any of the base classes are not loaded yet, I<base> silently
C<require>s them. Whether to C<require> a base class package is
determined by the absence of a global $VERSION in the base package.
If $VERSION is not detected even after loading it, <base> will
define $VERSION in the base package, setting it to the string
C<-1, set by base.pm>.
=head1 HISTORY
This module was introduced with Perl 5.004_04.
=head1 SEE ALSO
L<fields>
=cut
package base;
use 5.005_64;
our $VERSION = "1.01";
sub import {
my $class = shift;
my $fields_base;
my $pkg = caller(0);
foreach my $base (@_) {
push @{"$pkg\::ISA"}, $base;
unless (exists ${"$base\::"}{VERSION}) {
eval "require $base";
# Only ignore "Can't locate" errors from our eval require.
# Other fatal errors (syntax etc) must be reported.
die if $@ && $@ !~ /^Can't locate .*? at \(eval /;
unless (%{"$base\::"}) {
require Carp;
Carp::croak("Base class package \"$base\" is empty.\n",
"\t(Perhaps you need to 'use' the module ",
"which defines that package first.)");
}
${"$base\::VERSION"} = "-1, set by base.pm"
unless exists ${"$base\::"}{VERSION};
}
# A simple test like (defined %{"$base\::FIELDS"}) will
# sometimes produce typo warnings because it would create
# the hash if it was not present before.
my $fglob;
if ($fields_base) {
require Carp;
Carp::croak("Can't multiply inherit %FIELDS");
} else {
$fields_base = $base;
}
}
}
if ($fields_base) {
require fields;
}
}
1;