YouTipPerl References
# Perl References
A reference is a pointer. A Perl reference is a scalar type that can point to variables, arrays, hashes (also called associative arrays), or even subroutines. It can be used anywhere in a program.
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## Creating References
When defining a variable, you can get a reference to that variable by adding a backslash `` before the variable name. For example:
```perl
$scalarref = $foo; # Reference to a scalar variable
$arrayref = @ARGV; # Reference to a list
$hashref = %ENV; # Reference to a hash
$coderef = &handler; # Reference to a subroutine
$globref = *foo; # Reference to a GLOB handle
In arrays, we can use anonymous array references, defined using `[]`:
```perl
$aref = [ 1, "foo", undef, 13 ];
Elements of an anonymous array can themselves be anonymous arrays, so we can use this method to construct arrays of arrays, creating arrays of any dimension.
```perl
my $aref = [ [1, 2, 3], [4, 5, 6], [7, 8, 9], ];
In hashes, we can use anonymous hash references, defined using `{}`:
```perl
$href = { APR => 4, AUG => 8 };
We can also create an anonymous subroutine reference without a subroutine name:
```perl
$coderef = sub { print "Tutorial!n" };
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## Dereferencing
Dereferencing can be done using `$`, `@`, or `%` depending on the type. Examples are as follows:
## Example
```perl
#!/usr/bin/perl
$var = 10;
# $r is a reference to $var scalar
$r = $var;
# Print the value of the variable stored in $r
print "$var is : ", $$r, "n";
@var = (1, 2, 3);
# $r is a reference to @var array
$r = @var;
# Print the value of the variable stored in $r
print "@var is: ", @$r, "n";
%var = ('key1' => 10, 'key2' => 20);
# $r is a reference to %var hash
$r = %var;
# Print the value of the variable stored in $r
print "%var is : ", %$r, "n";
The execution result of the above example is:
10 is : 10
1 2 3 is: 1 2 3
%var is : key110key220
If you are unsure of the variable type, you can use **ref** to determine it. The return value list is as follows; if none of these values are returned, it returns false:
SCALAR
ARRAY
HASH
CODE
GLOB
REF
Example:
## Example
```perl
#!/usr/bin/perl
$var = 10;
$r = $var;
print "r Reference Types : ", ref($r), "n";
@var = (1, 2, 3);
$r = @var;
print "r Reference Types : ", ref($r), "n";
%var = ('key1' => 10, 'key2' => 20);
$r = %var;
print "r Reference Types : ", ref($r), "n";
The execution result of the above example is:
r Reference Types : SCALAR
r Reference Types : ARRAY
r Reference Types : HASH
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## Circular References
Circular references occur when two references contain each other. You need to use them carefully, otherwise it can lead to memory leaks, as shown in the following example:
## Example
```perl
#!/usr/bin/perl
my $foo = 100;
$foo = $foo;
print "Value of foo is : ", $$foo, "n";
The execution result of the above example is:
Value of foo is : REF(0x9aae38)
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## Reference to Functions
Function reference format: `&`
Calling a referenced function format: `&` + the created reference name.
Example:
## Example
```perl
#!/usr/bin/perl
# Function definition
sub PrintHash {
my (%hash) = @_;
foreach $item (%hash) {
print "Elements : $itemn";
}
}
%hash = ('name' => 'tutorial', 'age' => 3);
# Create a reference to the function
$cref = &PrintHash;
# Call the function using the reference
&$cref(%hash);
The execution result of the above example is:
Elements : age
Elements : 3
Elements : name
Elements : tutorial