NAME

perlref - Perl references and nested data structures

DESCRIPTION

In Perl 4 it was difficult to represent complex data structures, because all references had to be symbolic, and even that was difficult to do when you wanted to refer to a variable rather than a hash entry. Perl 5 not only makes it easier to use symbolic references to variables, but lets you have "hard" references to any piece of data. Any scalar may hold a hard reference. Since arrays and hashes contain scalars, you can now easily build arrays of arrays, arrays of hashes, hashes of arrays, and so on.

Hard references are smart--they keep track of reference counts for you, automatically freeing the thing referred to when the 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 that have been officially "blessed" into a class package.)

Some rather pathological examples of the use of references can be found in the t/op/ref.t regression test in the Perl source directory.

"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 to explicitly.

References can be constructed in one of 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']];
Note that we've constructed a reference to an anonymous array, the final element of which is a reference to another anonymous array. (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, since these are executable operators, not declarations.

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

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 {}.

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 eval(): 

        {
            my $x = time;
            $coderef = eval "sub { \$x }";
        }
This will generate a unique subroutine each time called, one which accesses the my() variable above it, even when it would otherwise go out of scope.

5.
References are often returned by special subroutines called constructors. Perl objects are just a special kind of reference that happen to know which package they're associated with. Constructors are just special subroutines that know how to make that association, they do it 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.

That's it for creating references. By now you're probably dying to know how to use references to get back to your long-lost data. There are several basic methods.

1.
Anywhere you'd put an identifier as part of a variable or subroutine name, you can replace the identifier with a simple scalar variable containing a reference of the correct type: 

        $bar = $$scalarref;
        push(@$arrayref, $filename);
        $$arrayref[0] = "January";
        $$hashref{"KEY"} = "VALUE";
        &$coderef(1,2,3);
Note that we are specifically not referencing $arrayref[0] or $hashref{"KEY"} there. The dereference of the scalar variable happens before it does any key lookups. Anything more complicated than a simple scalar variable must use methods 2 or 3 below. Note, however, that a simple scalar includes an identifier that use method 1 recursively. The following prints "howdy". 

        $refrefref = \\\"howdy";
        print $$$$refrefref;
2.
Anywhere you'd put an identifier as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct type. In other words, the previous examples could be written like this: 

        $bar = ${$scalarref};
        push(@{$arrayref}, $filename);
        ${$arrayref}[0] = "January";
        ${$hashref}{"KEY"} = "VALUE";
        &{$coderef}(1,2,3);
Admittedly, it's a little silly to use the curlies in this case, but the BLOCK can contain any arbitrary expression, in particular, subscripted expressions: 

        &{ $dispatch{$index} }(1,2,3);        # call correct routine
3.
The case of individual array elements arises often enough that it gets cumbersome to use method 2. As a form of syntactic sugar, the two lines like that above can be written: 

        $arrayref->[0] = "January";
        $hashref->{"KEY} = "VALUE";
The left side of the array can be any expression returning a reference, including a previous dereference. Note that $array[$x] is not the same thing as $array-[$x]> here: 

        $array[$x]->{"foo"}->[0] = "January";
This is one of the cases we mentioned earlier in which references could spring into existence when in an lvalue context. Before this statement, $array[$x] may have been undefined. If so, it's automatically defined with a hash reference so that we can look up {"foo"} in it. Likewise $array[$x]->{"foo"} will automatically get defined with an array reference so that we can look up [0] in it.

One more thing here. The arrow is optional between subscripts, so you can shrink the above down to 

        $array[$x]{"foo"}[0] = "January";
Which, in the degenerate case of using only ordinary arrays, gives you multidimensional arrays just like C's: 

        $score[$x][$y][$z] += 42;
Well, okay, not entirely like C's arrays, actually. C doesn't know how to grow its arrays on demand. Perl does.

4.
If a reference happens to be a reference to an object, then there are probably methods to access the things referred to, and you should probably stick to those methods unless you're in the class package that defines the object's methods. In other words, be nice, and don't violate the object's encapsulation without a very good reason. Note that Perl does not enforce encapsulation. We are not totalitarians here. We do expect some basic civility though.

The ref() operator may be used to determine what type of thing the reference is pointing to. See the perlfunc manpage .

The bless() operator may be used to associate a reference with a package functioning as an object class. See the perlobj manpage .

A type glob may be dereferenced the same way a reference can, since the dereference syntax always indicates the kind of reference desired. So ${*foo} and ${\$foo} both indicate the same scalar variable.

Here's a trick for interpolating a subroutine call into a string: 

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

Symbolic references

We said that references spring into existence as necessary if they are undefined, but we didn't say what happens if a value used as a reference is already defined, but isn't a hard reference. If you use it as a reference in this case, it'll be treated as a symbolic reference. That is, the value of the scalar is taken to be the name of a variable, rather than a direct link to a (possibly) anonymous value.

People frequently expect it to work like this. So it does. 

        $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
This is very powerful, and slightly dangerous, in that it's possible to intend (with the utmost sincerity) to use a hard reference, and accidentally use a symbolic reference instead. To protect against that, you can say 

        use strict 'refs';
and then only hard references will be allowed for the rest of the enclosing block. An inner block may countermand that with 

        no strict 'refs';
Only package variables are visible to symbolic references. Lexical variables (declared with my()) aren't in a symbol table, and thus are invisible to this mechanism. For example: 

        local($value) = 10;
        $ref = \$value;
        {
            my $value = 20;
            print $$ref;
        }
This will still print 10, not 20. Remember that local() affects package variables, which are all "global" to the package.