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. (Note: The reference counts for values in self-referential or cyclic data structures may not go to zero without a little help; see ``Two-Phased Garbage Collection'' for a detailed explanation. 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.
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''.)
Note that taking a reference to an enumerated list is not the same as using square brackets--instead it's the same as creating a list of references!
As a special case, \(@foo) returns a list of references to the contents
of @foo, not a reference to @foo itself. Likewise for %foo.
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 without a subname:
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 SAME
anonymous subroutine.)
Anonymous subroutines act as closures with respect to my() variables, that is, variables visible lexically within the current scope. Closure is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside of the context.
In human terms, it's a funny way of passing arguments to a subroutine when you define it as well as when you call it. It's useful for setting up little bits of code to run later, such as callbacks. You can even do object-oriented stuff with it, though Perl provides a different mechanism to do that already--see the perlobj manpage .
You can also think of closure as a way to write a subroutine template without using eval. (In fact, in version 5.000, eval was the only way to get closures. You may wish to use ``require 5.001'' if you use closures.)
Here's a small example of how closures works:
This prints
Note particularly that $x continues to refer to the value passed into newprint() despite the fact that the ``my $x'' has seemingly gone out of scope by the time the anonymous subroutine runs. That's what closure is all about.
This only applies to lexical variables, by the way. Dynamic variables continue to work as they have always worked. Closure is not something that most Perl programmers need trouble themselves about to begin with.
It's important to understand that we are specifically NOT dereferencing
$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.
However, a ``simple scalar'' includes an identifier that itself uses method
1 recursively. Therefore, the following prints ``howdy''.
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:
Because of being able to omit the curlies for the simple case of $$x,
people often make the mistake of viewing the dereferencing symbols as
proper operators, and wonder about their precedence. If they were,
though, you could use parens instead of braces. That's not the case.
Consider the difference below; case 0 is a short-hand version of case 1,
NOT case 2:
Case 2 is also deceptive in that you're accessing a variable called %hashref, not dereferencing through $hashref to the hash it's presumably referencing. That would be case 3.
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:
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 brackets subscripts, so you can shrink the above down to
Which, in the degenerate case of using only ordinary arrays, gives you multidimensional arrays just like C's:
Well, okay, not entirely like C's arrays, actually. C doesn't know how to grow its arrays on demand. Perl does.
The bless() operator may be used to associate a reference with a package functioning as an object class. See the perlobj manpage .
A typeglob 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:
The way it works is that when the @{...} is seen in the double-quoted
string, it's evaluated as a block. The block creates a reference to an
anonymous array containing the results of the call to mysub(1,2,3). So
the whole block returns a reference to an array, which is then
dereferenced by @{...} and stuck into the double-quoted string. This
chicanery is also useful for arbitrary expressions:
People frequently expect it to work like this. So it does.
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
and then only hard references will be allowed for the rest of the enclosing block. An inner block may countermand that with
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:
This will still print 10, not 20. Remember that local() affects package variables, which are all ``global'' to the package.
has always meant to print ``pop on over'', despite the fact that push is a reserved word. This has been generalized to work the same outside of quotes, so that
and even
will have the same effect. (This would have been a syntax error in 5.000, though Perl 4 allowed it in the spaceless form.) Note that this construct is not considered to be a symbolic reference when you're using strict refs:
Similarly, because of all the subscripting that is done using single words, we've applied the same rule to any bareword that is used for subscripting a hash. So now, instead of writing
you can just write
and not worry about whether the subscripts are reserved words. In the rare event that you do wish to do something like
you can force interpretation as a reserved word by adding anything that makes it more than a bareword:
The -w switch will warn you if it interprets a reserved word as a string. But it will no longer warn you about using lowercase words, since the string is effectively quoted.
If you try to dereference the key, it won't do a hard dereference, and you won't accomplish what you're attempting. You might want to do something more like
And then at least you can use the values(), which will be real refs, instead of the keys(), which won't.
See also the perldsc manpage and the perllol manpage for how to use references to create complex data structures, and the perlobj manpage for how to use them to create objects.