Re: How to tell if a function exists?

Perl does allow you to take reference of non-exist sub.

Also if this sub becomes defined later, it will be realized.

A piece of code to demo this:

apackage.pm:
use strict;

sub process1 {
    print "abcd\n";
}

1;

test1.pl:
my $a = \&process1;#process1 does not exist at this moment
print $a, "\n";
require apackage;#process1 is realized. If you comment out this, there
+ would be an error when you call process1 later.
print $a, "\n";
&$a;
[download]

Comment on Re: How to tell if a function exists? Download Code

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Re^2: How to tell if a function exists? by aspen (Sexton) on Jan 05, 2003 at 07:06 UTC
pg, thanks, very informative. I gave it a try to play around. If I'm not mistaken, the second line in the test1.pl code should read: `my $a = \&apackage::process1; #process1 does not exist at this moment` [download] This ability to reference a non-existent entity made me wonder, what would happen if you point to the "same" non-existent item twice, will those references be the same? So I gave the following a try: `my $b = \&apackage2::process2; my $c = \&apackage2::process2; my $d = \&apackage2::process3;` [download] Upon running, as should be, $b and $c both contain the same reference (that is, they compare as equal and print the same reference address). $d is a reference to a different not-yet-existent entity. When these entities do get defined - that is, the require is executed such that there is now a real, matching-name subroutine - the pointer address does not change. Also as should be, but interesting (for a newbie) to watch in action. Andy `@_="the journeyman larry disciple keeps learning\n"=~/(.)/gs, print(map$_[$_-77],unpack(q=c*=,q@QSdM[]uRMNV^[ni_\[N]eki^y@))` [download]	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re^2: How to tell if a function exists?
by aspen (Sexton) on Jan 05, 2003 at 07:06 UTC

I gave it a try to play around. If I'm not mistaken, the second line in the test1.pl code should read:

my $a = \&apackage::process1; #process1 does not exist at this moment
[download]

This ability to reference a non-existent entity made me wonder, what would happen if you point to the "same" non-existent item twice, will those references be the same? So I gave the following a try:

my $b = \&apackage2::process2;
my $c = \&apackage2::process2;
my $d = \&apackage2::process3;
[download]

When these entities do get defined - that is, the require is executed such that there is now a real, matching-name subroutine - the pointer address does not change. Also as should be, but interesting (for a newbie) to watch in action.

Andy

@_="the journeyman larry disciple keeps learning\n"=~/(.)/gs,
print(map$_[$_-77],unpack(q=c*=,q@QSdM[]uRMNV^[ni_\[N]eki^y@))
[download]

[reply]
[d/l]
[select]