[ZZ][Perl] Export and Export_OK

Export allows to export the functions and variables of modules to user’s namespace using the standard import method. This way, we don’t need to create the objects for the modules to access it’s members.

@EXPORT and @EXPORT_OK are the two main variables used during export operation.

@EXPORT contains list of symbols (subroutines and variables) of the module to be exported into the caller namespace.

@EXPORT_OK does export of symbols on demand basis.

Sample program

Let us use the following sample program to understand Perl exporter

package Arithmetic;

use Exporter;

# base class of this(Arithmetic) module
@ISA = qw(Exporter);

# Exporting the add and subtract routine
@EXPORT = qw(add subtract);
# Exporting the multiply and divide  routine on demand basis.
@EXPORT_OK = qw(multiply divide);

sub add
{
my ($no1,$no2) = @_;
my $result;
$result = $no1+$no2;
return $result;
}

sub subtract
{
my ($no1,$no2) = @_;
my $result;
$result = $no1-$no2;
return $result;

}

sub multiply
{
my ($no1,$no2) = @_;
my $result;
$result = $no1*$no2;
return $result;
}

sub divide
{
my ($no1,$no2) = @_;
my $result;
$result = $no1/$no2;
return $result;

}

In the above example, we defined the arithmetic module with four functions. By default add() and subtract() functions are exported to the user’s/caller’s namespace.

“use Arithmetic” statement imports the subroutines from Arithmetic module that are exported by default.

“use Arithmetic qw(multiply divide)” indicates that these two routines gets exported only when it is specifically requested as shown in the following code snippet.

#! /usr/bin/perl

use strict;
use warnings;

use Arithmetic;
use Arithmetic qw(multiply divide);

print add(1,2),"\n";
print multiply(1,2),"\n";

 

As we seen above, in the main program we used the Arithmetic module with default import ( add and subtract ) and on-demand import ( multiply and divide ).

How Imports Gets Done

Without Exporter, to import functions from Arithmetic module into the main package, we can write a code-snippet like the following to import some of the functions.

sub import {
no strict 'refs';
for (qw(add subtract multiply )) {
*{"main::$_"} = \&$_;
}
}

 

The code in the main package/program are as follows,

#!/usr/bin/perl
use Arithmetic;
# import() subroutine in Arithmetic module gets invoked automatically.
print add(1,2);

 

In the above code-snippet, from the current package ( Arithmetic ), these routines ( add, subtract, multiply ) gets imported into the main package. This provides limited functionality. i.e In case if the use is invoked from packages other than main, this wont work properly. But in the Perl Exporter module, we don’t have this limitation.

 

http://www.thegeekstuff.com/2010/06/perl-exporter-examples/

转载于:https://www.cnblogs.com/sanquanfeng/p/3370515.html