✏ 编辑: AnyDBM_File.pm

package AnyDBM_File;
use warnings;
use strict;

use 5.006_001;
our $VERSION = '1.01';
our @ISA = qw(NDBM_File DB_File GDBM_File SDBM_File ODBM_File) unless @ISA;

my $mod;
for $mod (@ISA) {
    if (eval "require $mod") {
	@ISA = ($mod);	# if we leave @ISA alone, warnings abound
	return 1;
    }
}

die "No DBM package was successfully found or installed";

__END__

=head1 NAME

AnyDBM_File - provide framework for multiple DBMs

NDBM_File, DB_File, GDBM_File, SDBM_File, ODBM_File - various DBM implementations

=head1 SYNOPSIS

use AnyDBM_File;

=head1 DESCRIPTION

This module is a "pure virtual base class"--it has nothing of its own.
It's just there to inherit from one of the various DBM packages.  It
prefers ndbm for compatibility reasons with Perl 4, then Berkeley DB (See
L<DB_File>), GDBM, SDBM (which is always there--it comes with Perl), and
finally ODBM.   This way old programs that used to use NDBM via dbmopen()
can still do so, but new ones can reorder @ISA:

BEGIN { @AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File) }
    use AnyDBM_File;

Having multiple DBM implementations makes it trivial to copy database formats:

use Fcntl; use NDBM_File; use DB_File;
    tie %newhash,  'DB_File', $new_filename, O_CREAT|O_RDWR;
    tie %oldhash,  'NDBM_File', $old_filename, 1, 0;
    %newhash = %oldhash;

=head2 DBM Comparisons

Here's a partial table of features the different packages offer:

odbm    ndbm    sdbm    gdbm    bsd-db
			 ----	 ----    ----    ----    ------
 Linkage comes w/ perl   yes     yes     yes     yes     yes
 Src comes w/ perl       no      no      yes     no      no
 Comes w/ many unix os   yes     yes[0]  no      no      no
 Builds ok on !unix      ?       ?       yes     yes     ?
 Code Size               ?       ?       small   big     big
 Database Size           ?       ?       small   big?    ok[1]
 Speed                   ?       ?       slow    ok      fast
 FTPable                 no      no      yes     yes     yes
 Easy to build          N/A     N/A      yes     yes     ok[2]
 Size limits             1k      4k      1k[3]   none    none
 Byte-order independent  no      no      no      no      yes
 Licensing restrictions  ?       ?       no      yes     no

=over 4

=item [0]

on mixed universe machines, may be in the bsd compat library,
which is often shunned.

=item [1]

Can be trimmed if you compile for one access method.

=item [2]

See L<DB_File>. 
Requires symbolic links.

=item [3]

By default, but can be redefined.

=back

=head1 SEE ALSO

dbm(3), ndbm(3), DB_File(3), L<perldbmfilter>

=cut

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