Perl Quote

Reference is a pointer. Perl reference is a scalar type that can point to variables, arrays, hash tables (also called associative arrays) and even subroutines, and can be applied anywhere in the program.

Create a reference

When defining a variable, add \ in front of the variable name to get a reference to the 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

We can use anonymous array references in arrays, using [] definition:

$aref= [ 1,"foo",undef,13 ];

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

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

In the hash we can use anonymous hash references, using {} definition:

$href= { APR =>4, AUG =>8 };

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

$coderef = sub { print "php!\n" };

Cancel reference

Cancellation of application can be canceled using $, @ or % according to different types. The example is as follows:

#!/usr/bin/perl

$var = 10;

# $r 引用 $var 标量
$r = $var;

# 输出本地存储的 $r 的变量值
print "$var 为 : ", $$r, "\n";

@var = (1, 2, 3);
# $r 引用  @var 数组
$r = \@var;
# 输出本地存储的 $r 的变量值
print "@var 为: ",  @$r, "\n";

%var = ('key1' => 10, 'key2' => 20);
# $r 引用  %var 数组
$r = \%var;
# 输出本地存储的 $r 的变量值
print "%var 为 : ", %$r, "\n";

The execution result of the above example is:

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

If you cannot determine the variable type, you can use ref to determine. The return value list is as follows. If there is no following value, return false:

SCALAR
ARRAY
HASH
CODE
GLOB
REF

The example is as follows:

#!/usr/bin/perl

$var = 10;
$r = $var;
print "r 的引用类型 : ", ref($r), "\n";

@var = (1, 2, 3);
$r = \@var;
print "r 的引用类型 : ", ref($r), "\n";

%var = ('key1' => 10, 'key2' => 20);
$r = \%var;
print "r 的引用类型 : ", ref($r), "\n";

The execution result of the above example is:

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

Circular reference

Circular reference occurs when two references include each other. You need to use it carefully, otherwise it will cause memory leaks, as shown in the following example:

#!/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)

Reference function

Function reference format: \&

Call reference function format: & + Created reference name.

The example is as follows:

#!/usr/bin/perl

# 函数定义
sub PrintHash{
   my (%hash) = @_;
   
   foreach $item (%hash){
      print "元素 : $item\n";
   }
}
%hash = ('name' => 'php', 'age' => 3);

# 创建函数的引用
$cref = \&PrintHash;

# 使用引用调用函数
&$cref(%hash);

The execution result of the above example is:

元素 : age
元素 : 3
元素 : name
元素 : php