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), and even subroutines, and it can be used anywhere in the program.

Creating References

When defining a variable, prepend a \ to the variable name to get a reference to that variable, for example:

$scalarref = \$foo;     # 标量变量引用
$arrayref  = \@ARGV;    # 列表的引用
$hashref   = \%ENV;     # 哈希的引用
$coderef   = \&handler; # 子过程引用
$globref   = \*foo;     # GLOB句柄引用

In arrays, we can use anonymous array references, defined with []:

$aref= [ 1,"foo",undef,13 ];

The elements of an anonymous array can themselves be anonymous arrays, so we can use this method to construct arrays of arrays, and arrays of any dimension.

my $aref = [
        [1, 2, 3],
        [4, 5, 6],
        [7, 8, 9],
]

In hashes, we can use anonymous hash references, defined with {}:

$href= { APR =>4, AUG =>8 };

We can also create an anonymous subroutine reference without a subroutine name:

$coderef = sub { print "Example!\n" };

Dereferencing

Dereferencing can be done using $, @, or % depending on the type, for example:

Examples

#!/usr/bin/perl $var = 10; # $r references $var scalar $r = \$var; # Output the value of the variable stored by $r print "$varis:", $$r, "\n"; @var = (1, 2, 3); # $r references @var array $r = \@var; # Output the value of the variable stored by $r print "@var is:", @$r, "\n"; %var = ('key1' => 10, 'key2' => 20); # $r references %var hash $r = \%var; # Output the value of the variable stored by $r print "\%var is:", %$r, "\n";

Executing the above example produces the following result:

10 为 : 10
1 2 3 为: 123
\%var 为 : key110key220

If you are not sure about the variable type, you can userefto determine it. The return values are as follows. If none of the following values apply, false is returned:

SCALAR
ARRAY
HASH
CODE
GLOB
REF

Example:

Examples

#!/usr/bin/perl $var = 10; $r = \$var; print "Reference type of r:", ref($r), "\n"; @var = (1, 2, 3); $r = \@var; print "Reference type of r:", ref($r), "\n"; %var = ('key1' => 10, 'key2' => 20); $r = \%var; print "Reference type of r:", ref($r), "\n";

Executing the above example produces the following result:

r 的引用类型 : SCALAR
r 的引用类型 : ARRAY
r 的引用类型 : HASH

Circular References

Circular references occur when two references contain each other. You need to use them carefully, otherwise it will cause memory leaks, as in the following example:

Examples

#!/usr/bin/perl my $foo = 100; $foo = \$foo; print "Value of foo is : ", $$foo, "\n";

Executing the above example produces the following result:

Value of foo is : REF(0x9aae38)

Function References

Function reference format: \&

Calling a function reference format: & + the created reference name.

Example:

Examples

#!/usr/bin/perl # Function definition sub PrintHash{ my (%hash) = @_; foreach $item (%hash){ print "Elements:$item\n"; } } %hash = ('name' => 'example', 'age' => 3); # Create a reference to the function $cref = \&PrintHash; # Call the function using the reference &$cref(%hash);

Executing the above example produces the following result:

元素 : age
元素 : 3
元素 : name
元素 : example

Other Extensions