References

• Sample usage   Output

• A reference is a scalar value, preceded by $
• Can use references to build lists and hashes within lists and hashes

Getting a reference value

• \  : Precede a variable name with a blackslash to get a reference value for the variable; the value can then be assigned to a reference variable.

Number

$num = 10;
$num_ref = \$num;                 # Reference to a named number (scalar)

$num_ref = \10;                   # Reference to an anonymous number (scalar)

String

$str = "Hello";
$str_ref = \$str;                 # Reference to a named string (scalar)

$ref = \"Hello";                  # Reference to an anonymous string (scalar)

Array

@arr = ('one', 'two');
$arr_ref = \@arr;                 # Reference to a named array

$arr_ref = ['one', 'two'];        # Reference to an anonymous array

@arr = ('one', 'two');
$arr_ref = [@arr];                # Reference to an anonymous copy of a named array

Hash

%hash = (
    two => 'deux',
    three => 'trois',
);
$hash_ref = \%hash;               # Reference to a named hash

$hash_ref = {                     # Reference to an anonymous hash
    two => 'deux',
    three => 'trois',
};

%hash = (
    two => 'deux',
    three => 'trois',
);
$hash_ref = {%hash};              # Reference to an anonymous copy of a named hash

Dereference operator

• ->  : Used for references to arrays and hashes to get the value that the reference refers to (like dereferencing a pointer to a struct or object in C / C++)

Dereferencing numerics and strings

• If $ref is a reference to a simple scalar variable, $$ref or ${$ref} can be used anywhere the name of the scalar variable would be used (the braces are optional).

With out a      Substitution       Substitution
reference       with braces        without braces
----------      ------------       --------------
$int            ${$int_ref}        $$int_ref
$str            ${$str_ref}        $$str_ref

Dereferencing arrays and hashes

• If $ref is a reference to an array or hash, {$ref} or ref-> can be used anywhere the name of the array or hash would be used (the braces are optional).

With out a    Dereference        Substitution          Substitution
reference     operator           with braces           without braces
----------    -----------        ------------          --------------
@arr                             @{$arr_ref}           @$arr_ref

%hash1                           %{$hash1_ref}         %$hash1_ref

$arr[3]       $arr_ref->[3]      ${$arr_ref}[3]        $$arr_ref[3]
$hash1{red}   $hash1_ref->{red}  ${$hash1_ref}{red}    $$hash1_ref{red}

Multiple levels of indirection

$ref = \"Hello";
$str = $$ref;

$ref_ref = \\"Hello";
$str2 = $$$ref_ref;