Passing a Perl Hash by reference plus scalar variable - perl

Question

I'm trying to pass from Main.pl to a sub (in ReadConfigFile.pm) a hash by reference and a scalar value. The scalar variable is the path to a config file and once this file is opened I want to fill a hash with some of its values. How do i pass a hash by reference and a scalar so that I then have the hash values available to use in Main.pl
I have done a lot of reading but cant get this to work. I realise i cant do = #_; in my sub as that is creating a new hash.
Ive tried following the prototype method, this fills the hash ok but back in Main.pl the hash is empty.
Main.pl
# Read the config file. Return 3 scalars and a hash
my %apps;
my ($schema, $directory, $staticFile) = readConfigFile(\%apps, $configFilePath);
my %app_list = %apps; # ive tried this in, out and in a variety of states
foreach my $name (sort keys %app_list) {
print "\nMAIN $name";
}
# this is empty
ReadConfigFile.pm
sub readConfigFile (\%$) {
my ($apps_ref, $configFilePath) = #_;
# also tried
# $apps_ref = shift but then configFilePath is empty
# linearray is each line from open config file split by :
$apps_ref{$lineArray[1]}{id} = $lineArray[1];
$apps_ref{$lineArray[1]}{name} = $lineArray[2];
$schema = $lineArray[1];
$directory = $lineArray[1];
$staticFile = $lineArray[1];
return ($schema, $directory, $staticFile);
configFile.txt
APP:1101:ACTIVITY
APP:1102:EVENTS
APP:1103:PERFORMANCE
APP:1104:LOCATION
STATIC_FILE:static_file.sql
SCHEMA:CAASS
DIRECTORY:CAASS
I want to get the 3 scalar variable returned and the hash so i can use them throughout Main.pl and pass to other subs.
I have also tried passing in just the configfilename and returning 4 variables, the 3 scalars and the hash.
I expect someone will crack this in minutes but i just cant work out the combination of \ and # and % and $ to make it work.
Thanks for any help or ideas.
Edit 1:
Main.pl
my %apps;
my ($schema, $directory, $staticFile) = readConfigFile(\%apps, $configFilePath);
foreach my $name (sort keys %apps) {
print "\nMAIN $name";
}
ReadConfigFile
sub readConfigFile () {
my $apps_ref = shift;
my $configFilePath = $_[0];
#Fill It
$apps_ref{$lineArray[1]}{id} = $lineArray[1];
$apps_ref{$lineArray[1]}{name} = $lineArray[2];
# This shows results
foreach my $name (sort keys %apps_ref) {
print "\nreadConfigFile $name";
}
But the values arent coming back into Main.pl
edit 2:
So im still interested in how the above can be made to work. But ive attacked it a different way and it works
Main.pl
my ($schema, $directory, $staticFile, %apps) = readConfigFile($configFilePath);
foreach my $name (sort keys %apps) {
print "\nMAIN $name";
}
ReadConfigFile
sub readConfigFile () {
my $configFilePath = $_[0];
my %apps;
#Fill It
%apps{$lineArray[1]}{id} = $lineArray[1];
$apps{$lineArray[1]}{name} = $lineArray[2];
foreach my $name (sort keys %apps) {
print "\nreadConfigFile $name";
}
return ($schema, $directory, $staticFile, %apps);
Both sets off output show.

Answer 1

There is no implicit 'pass by reference' in Perl*. Everything is passed the same way - as a list of scalars, by alias (thus passing a hash itself will instead pass the list of its keys and values*). But you can create a reference, pass it, and then dereference it to use it - and references can be copied around without copying the underlying structure.
use strict;
use warnings;
my %hash;
my $ref = \%hash;
my $copy = $ref;
$copy->{a} = 1;
print "$ref->{a}\n"; # also 1
References will maintain their referenced structure after a my (...) = #_; or my $foo = shift; assignment in a subroutine.
use strict;
use warnings;
sub foo {
my ($ref, $key) = #_;
$ref->{$key} = 42;
}
my %hash;
foo(\%hash, 'foo');
print "$hash{foo}\n"; # 42
See https://p3rl.org/REF for the relevant documentation on Perl references.
Since you are already passing a reference, there is no need for your (\%$) prototype: you can just remove it from the subroutine definition.
*except sort of with prototypes, but it's better to avoid them in most cases.