NAME

DESCRIPTION

Hard references are smart--they keep track of reference counts for you, automatically freeing the thing referred to when its reference count goes to zero. If that thing happens to be an object, the object is destructed. See the perlobj manpage for more about objects. (In a sense, everything in Perl is an object, but we usually reserve the word for references to objects that have been officially "blessed" into a class package.)

A symbolic reference contains the name of a variable, just as a symbolic link in the filesystem merely contains the name of a file. The *glob notation is a kind of symbolic reference. Hard references are more like hard links in the file system: merely another way at getting at the same underlying object, irrespective of its name.

"Hard" references are easy to use in Perl. There is just one overriding principle: Perl does no implicit referencing or dereferencing. When a scalar is holding a reference, it always behaves as a scalar. It doesn't magically start being an array or a hash unless you tell it so explicitly by dereferencing it.

References can be constructed several ways.

1.

By using the backslash operator on a variable, subroutine, or value. (This works much like the & (address-of) operator works in C.) Note that this typically creates ANOTHER reference to a variable, since there's already a reference to the variable in the symbol table. But the symbol table reference might go away, and you'll still have the reference that the backslash returned. Here are some examples:

        $scalarref = \$foo;
        $arrayref  = \@ARGV;
        $hashref   = \%ENV;
        $coderef   = \&handler;

2.

A reference to an anonymous array can be constructed using square brackets:

        $arrayref = [1, 2, ['a', 'b', 'c']];

Here we've constructed a reference to an anonymous array of three elements whose final element is itself reference to another anonymous array of three elements. (The multidimensional syntax described later can be used to access this. For example, after the above, $arrayref->[2][1] would have the value "b".)

3.

A reference to an anonymous hash can be constructed using curly brackets:

        $hashref = {
            'Adam'  => 'Eve',
            'Clyde' => 'Bonnie',
        };

Anonymous hash and array constructors can be intermixed freely to produce as complicated a structure as you want. The multidimensional syntax described below works for these too. The values above are literals, but variables and expressions would work just as well, because assignment operators in Perl (even within local() or my()) are executable statements, not compile-time declarations.

Because curly brackets (braces) are used for several other things including BLOCKs, you may occasionally have to disambiguate braces at the beginning of a statement by putting a + or a return in front so that Perl realizes the opening brace isn't starting a BLOCK. The economy and mnemonic value of using curlies is deemed worth this occasional extra hassle.

For example, if you wanted a function to make a new hash and return a reference to it, you have these options:

        sub hashem {        { @_ } }   # silently wrong
        sub hashem {       +{ @_ } }   # ok
        sub hashem { return { @_ } }   # ok

4.

A reference to an anonymous subroutine can be constructed by using sub without a subname:

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

Note the presence of the semicolon. Except for the fact that the code inside isn't executed immediately, a sub {} is not so much a declaration as it is an operator, like do{} or eval{}. (However, no matter how many times you execute that line (unless you're in an eval("...")), $coderef will still have a reference to the SAM&/I; anonymous subroutine.)

For those who worry about these things, the current implementation uses shallow binding of local() variables; my() variables are not accessible. This precludes true closures. However, you can work around this with a run-time (rather than a compile-time) eval():

        {
            my $x = time;
            $coderef = eval "sub { \$x }";
        }

Normally--if you'd used just sub{} or even eval{}--your unew sub would only have been able to access the global $x. But because you've used a run-time eval(), this will not only generate a brand new subroutine reference each time called, it will all grant access to the my() variable lexically above it rather than the global one. The particular $x accessed will be different for each new sub you create. This mechanism yields deep binding of variables. (If you don't know what closures, deep binding, or shallow binding are, don't worry too much about it.)

5.

References are often returned by special subroutines called constructors. Perl objects are just reference a special kind of object that happens to know which package it's associated with. Constructors are just special subroutines that know how to create that association. They do so by starting with an ordinary reference, and it remains an ordinary reference even while it's also being an object. Constructors are customarily named new(), but don't have to be:

        $objref = new Doggie (Tail => 'short', Ears => 'long');

6.

References of the appropriate type can spring into existence if you dereference them in a context that assumes they exist. Since we haven't talked about dereferencing yet, we can't show you any examples yet.

        print "My sub returned ${\mysub(1,2,3)}\n";

        $name = "foo";
        $$name = 1;                        # Sets $foo
        ${$name} = 2;                # Sets $foo
        ${$name x 2} = 3;                # Sets $foofoo
        $name->[0] = 4;                # Sets $foo[0]
        @$name = ();                # Clears @foo
        &$name();                        # Calls &foo() (as in Perl 4)
        $pack = "THAT";
        ${"${pack}::$name"} = 5;        # Sets $THAT::foo without eval

        use strict 'refs';

        no strict 'refs';

        local($value) = 10;
        $ref = \$value;
        {
            my $value = 20;
            print $$ref;
        }