PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1)NAMEperlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date:
1999/05/23 20:36:18 $)
DESCRIPTION
This section deals with general Perl language issues that
don't clearly fit into any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc
grammar in perly.y in the source distribution if you're
particularly brave. The grammar relies on very smart tok
enizing code, so be prepared to venture into toke.c as
well.
In the words of Chaim Frenkel: "Perl's grammar can not be
reduced to BNF. The work of parsing perl is distributed
between yacc, the lexer, smoke and mirrors."
What are all these $@%&* punctuation signs, and how do I
know when to use them?
They are type specifiers, as detailed in the perldata man
page:
$ for scalar values (number, string or reference)
@ for arrays
% for hashes (associative arrays)
& for subroutines (aka functions, procedures, methods)
* for all types of that symbol name. In version 4 you used them like
pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to
encounter that aren't really type specifiers:
<> are used for inputting a record from a filehandle.
\ takes a reference to something.
Note that <FILE> is neither the type specifier for files
nor the name of the handle. It is the "<>" operator
applied to the handle FILE. It reads one line (well,
record--see the section on "$/" in the perlvar manpage)
from the handle FILE in scalar context, or all lines in
list context. When performing open, close, or any other
operation besides "<>" on files, or even when talking
about the handle, do not use the brackets. These are cor
rect: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN
to FILE".
Do I always/never have to quote my strings or use semi
colons and commas?
Normally, a bareword doesn't need to be quoted, but in
most cases probably should be (and must be under "use
strict"). But a hash key consisting of a simple word
(that isn't the name of a defined subroutine) and the
left-hand operand to the "=>" operator both count as
though they were quoted:
This is like this
---------------------------
$foo{line} $foo{"line"}
bar => stuff "bar" => stuff
The final semicolon in a block is optional, as is the
final comma in a list. Good style (see the perlstyle man
page) says to put them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
How do I skip some return values?
One way is to treat the return values as a list and index
into it:
$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-
hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the "use warn
ings" pragma allows fine control of what warning are pro
duced. See the perllexwarn manpage for more details.
{
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
If you have an older version of Perl, the "$^W" variable
(documented in the perlvar manpage) controls runtime warn
ings for a block:
{
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Note that like all the punctuation variables, you cannot
currently use my() on "$^W", only local().
What's an extension?
An extension is a way of calling compiled C code from
Perl. Reading the perlxstut manpage is a good place to
learn more about extensions.
Why do Perl operators have different precedence than C
operators?
Actually, they don't. All C operators that Perl copies
have the same precedence in Perl as they do in C. The
problem is with operators that C doesn't have, especially
functions that give a list context to everything on their
right, eg. print, chmod, exec, and so on. Such functions
are called "list operators" and appear as such in the
precedence table in the perlop manpage.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or
use the super low precedence "or" operator:
(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "not")
deliberately have precedence lower than that of list oper
ators for just such situations as the one above.
Another operator with surprising precedence is exponentia
tion. It binds more tightly even than unary minus, making
"-2**2" product a negative not a positive four. It is
also right-associating, meaning that "2**3**2" is two
raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:"
operator produces an lvalue. This assigns $x to either $a
or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a
(probably anonymous) hash reference. See the perlref man
page and the perldsc manpage for details. Here's an exam
ple:
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try
the perltoot manpage.
How do I create a module?
A module is a package that lives in a file of the same
name. For example, the Hello::There module would live in
Hello/There.pm. For details, read the perlmod manpage.
You'll also find the Exporter manpage helpful. If you're
writing a C or mixed-language module with both C and Perl,
then you should study the perlxstut manpage.
Here's a convenient template you might wish you use when
starting your own module. Make sure to change the names
appropriately.
package Some::Module; # assumes Some/Module.pm
use strict;
use warnings;
BEGIN {
use Exporter ();
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
## set the version for version checking; uncomment to use
## $VERSION = 1.00;
# if using RCS/CVS, this next line may be preferred,
# but beware two-digit versions.
$VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func3);
%EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit);
}
our @EXPORT_OK;
# exported package globals go here
our $Var1;
our %Hashit;
# non-exported package globals go here
our @more;
our $stuff;
# initialize package globals, first exported ones
$Var1 = '';
%Hashit = ();
# then the others (which are still accessible as $Some::Module::stuff)
$stuff = '';
@more = ();
# all file-scoped lexicals must be created before
# the functions below that use them.
# file-private lexicals go here
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
# callable as &$priv_func; it cannot be prototyped.
my $priv_func = sub {
# stuff goes here.
};
# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
sub func1 {} # no prototype
sub func2() {} # proto'd void
sub func3($$) {} # proto'd to 2 scalars
# this one isn't exported, but could be called!
sub func4(\%) {} # proto'd to 1 hash ref
END { } # module clean-up code here (global destructor)
1; # modules must return true
The h2xs program will create stubs for all the important
stuff for you:
% h2xs -XA -n My::Module
How do I create a class?
See the perltoot manpage for an introduction to classes
and objects, as well as the perlobj manpage and the perl
bot manpage.
How can I tell if a variable is tainted?
See the Laundering and Detecting Tainted Data entry in the
perlsec manpage. Here's an example (which doesn't use any
system calls, because the kill() is given no processes to
signal):
sub is_tainted {
return ! eval { join('',@_), kill 0; 1; };
}
This is not "-w" clean, however. There is no "-w" clean
way to detect taintedness--take this as a hint that you
should untaint all possibly-tainted data.
What's a closure?
Closures are documented in the perlref manpage.
Closure is a computer science term with a precise but
hard-to-explain meaning. Closures are implemented in Perl
as anonymous subroutines with lasting references to lexi
cal variables outside their own scopes. These lexicals
magically refer to the variables that were around when the
subroutine was defined (deep binding).
Closures make sense in any programming language where you
can have the return value of a function be itself a func
tion, as you can in Perl. Note that some languages pro
vide anonymous functions but are not capable of providing
proper closures: the Python language, for example. For
more information on closures, check out any textbook on
functional programming. Scheme is a language that not
only supports but encourages closures.
Here's a classic function-generating function:
sub add_function_generator {
return sub { shift + shift };
}
$add_sub = add_function_generator();
$sum = $add_sub->(4,5); # $sum is 9 now.
The closure works as a function template with some cus
tomization slots left out to be filled later. The anony
mous subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals
outside its own scope.
Contrast this with the following make_adder() function, in
which the returned anonymous function contains a reference
to a lexical variable outside the scope of that function
itself. Such a reference requires that Perl return a
proper closure, thus locking in for all time the value
that the lexical had when the function was created.
sub make_adder {
my $addpiece = shift;
return sub { shift + $addpiece };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in,
whereas "&$f2($n)" is always 555 plus whatever $n you pass
in. The $addpiece in the closure sticks around.
Closures are often used for less esoteric purposes. For
example, when you want to pass in a bit of code into a
function:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
"'$line = <STDIN>'", there would have been no way for the
hypothetical timeout() function to access the lexical
variable $line back in its caller's scope.
What is variable suicide and how can I prevent it?
Variable suicide is when you (temporarily or permanently)
lose the value of a variable. It is caused by scoping
through my() and local() interacting with either closures
or aliased foreach() iterator variables and subroutine
arguments. It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder. Take
this code:
my $f = "foo";
sub T {
while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
}
T;
print "Finally $f\n";
The $f that has "bar" added to it three times should be a
new "$f" ("my $f" should create a new local variable each
time through the loop). It isn't, however. This was a
bug, now fixed in the latest releases (tested against
5.004_05, 5.005_03, and 5.005_56).
How can I pass/return a {Function, FileHandle, Array,
Hash, Method, Regex}?
With the exception of regexes, you need to pass references
to these objects. See the Pass by Reference entry in the
perlsub manpage for this particular question, and the
perlref manpage for information on references.
See ``Passing Regexes'', below, for information on passing
regular expressions.
Passing Variables and Functions
Regular variables and functions are quite easy to
pass: just pass in a reference to an existing or
anonymous variable or function:
func( \$some_scalar );
func( \@some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
To pass filehandles to subroutines, use the "*FH" or
"\*FH" notations. These are "typeglobs"--see the
Typeglobs and Filehandles entry in the perldata man
page and especially the Pass by Reference entry in the
perlsub manpage for more information.
Here's an excerpt:
If you're passing around filehandles, you could usu
ally just use the bare typeglob, like *STDOUT, but
typeglobs references would be better because they'll
still work properly under "use strict 'refs'". For
example:
splutter(\*STDOUT);
sub splutter {
my $fh = shift;
print $fh "her um well a hmmm\n";
}
$rec = get_rec(\*STDIN);
sub get_rec {
my $fh = shift;
return scalar <$fh>;
}
If you're planning on generating new filehandles, you
could do this:
sub openit {
my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
$fh = openit('< /etc/motd');
print <$fh>;
Passing Regexes
To pass regexes around, you'll need to be using a
release of Perl sufficiently recent as to support the
"qr//" construct, pass around strings and use an
exception-trapping eval, or else be very, very clever.
Here's an example of how to pass in a string to be
regex compared using "qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
Notice how "qr//" allows flags at the end. That pat
tern was compiled at compile time, although it was
executed later. The nifty "qr//" notation wasn't
introduced until the 5.005 release. Before that, you
had to approach this problem much less intuitively.
For example, here it is again if you don't have
"qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
}
$match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due
to the double interpolation of the eval and the dou
ble-quoted string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the
O'Reilly book, Mastering Regular Expressions, by Jef
frey Friedl. Page 273's Build_MatchMany_Function() is
particularly interesting. A complete citation of this
book is given in the perlfaq2 manpage.
Passing Methods
To pass an object method into a subroutine, you can do
this:
call_a_lot(10, $some_obj, "methname")
sub call_a_lot {
my ($count, $widget, $trick) = @_;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
}
}
Or, you can use a closure to bundle up the object, its
method call, and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args) };
func($whatnot);
sub func {
my $code = shift;
&$code();
}
You could also investigate the can() method in the
UNIVERSAL class (part of the standard perl distribu
tion).
How do I create a static variable?
As with most things in Perl, TMTOWTDI. What is a "static
variable" in other languages could be either a function-
private variable (visible only within a single function,
retaining its value between calls to that function), or a
file-private variable (visible only to functions within
the file it was declared in) in Perl.
Here's code to implement a function-private variable:
BEGIN {
my $counter = 42;
sub prev_counter { return --$counter }
sub next_counter { return $counter++ }
}
Now prev_counter() and next_counter() share a private
variable $counter that was initialized at compile time.
To declare a file-private variable, you'll still use a
my(), putting the declaration at the outer scope level at
the top of the file. Assume this is in file Pax.pm:
package Pax;
my $started = scalar(localtime(time()));
sub begun { return $started }
When "use Pax" or "require Pax" loads this module, the
variable will be initialized. It won't get garbage-col
lected the way most variables going out of scope do,
because the begun() function cares about it, but no one
else can get it. It is not called $Pax::started because
its scope is unrelated to the package. It's scoped to the
file. You could conceivably have several packages in that
same file all accessing the same private variable, but
another file with the same package couldn't get to it.
See the Persistent Private Variables entry in the perlsub
manpage for details.
What's the difference between dynamic and lexical (static)
scoping? Between local() and my()?
"local($x)" saves away the old value of the global vari
able "$x" and assigns a new value for the duration of the
subroutine which is visible in other functions called from
that subroutine. This is done at run-time, so is called
dynamic scoping. local() always affects global variables,
also called package variables or dynamic variables.
"my($x)" creates a new variable that is only visible in
the current subroutine. This is done at compile-time, so
it is called lexical or static scoping. my() always
affects private variables, also called lexical variables
or (improperly) static(ly scoped) variables.
For instance:
sub visible {
print "var has value $var\n";
}
sub dynamic {
local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
}
sub lexical {
my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get
printed. That's because $var only has that value within
the block of the lexical() function, and it is hidden from
called subroutine.
In summary, local() doesn't make what you think of as pri
vate, local variables. It gives a global variable a tem
porary value. my() is what you're looking for if you want
private variables.
See the Private Variables via my() entry in the perlsub
manpage and the Temporary Values via local() entry in the
perlsub manpage for excruciating details.
How can I access a dynamic variable while a similarly
named lexical is in scope?
You can do this via symbolic references, provided you
haven't set "use strict "refs"". So instead of $var, use
"${'var'}".
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
no strict 'refs';
print "global is ${'var'}\n";
If you know your package, you can just mention it explic
itly, as in $Some_Pack::var. Note that the notation
$::var is not the dynamic $var in the current package, but
rather the one in the "main" package, as though you had
written $main::var. Specifying the package directly makes
you hard-code its name, but it executes faster and avoids
running afoul of "use strict "refs"".
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous
subroutines are the same ones that were in scope when the
subroutine was created. In shallow binding, they are
whichever variables with the same names happen to be in
scope when the subroutine is called. Perl always uses
deep binding of lexical variables (i.e., those created
with my()). However, dynamic variables (aka global,
local, or package variables) are effectively shallowly
bound. Consider this just one more reason not to use
them. See the answer to the section on "What's a
closure?".
Why doesn't "my($foo) = <FILE>;" work right?
"my()" and "local()" give list context to the right hand
side of "=". The <FH> read operation, like so many of
Perl's functions and operators, can tell which context it
was called in and behaves appropriately. In general, the
scalar() function can help. This function does nothing to
the data itself (contrary to popular myth) but rather
tells its argument to behave in whatever its scalar fash
ion is. If that function doesn't have a defined scalar
behavior, this of course doesn't help you (such as with
sort()).
To enforce scalar context in this particular case, how
ever, you need merely omit the parentheses:
local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway,
although the issue is the same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
How do I redefine a builtin function, operator, or method?
Why do you want to do that? :-)
If you want to override a predefined function, such as
open(), then you'll have to import the new definition from
a different module. See the Overriding Built-in Functions
entry in the perlsub manpage. There's also an example in
the Class::Template entry in the perltoot manpage.
If you want to overload a Perl operator, such as "+" or
"**", then you'll want to use the "use overload" pragma,
documented in the overload manpage.
If you're talking about obscuring method calls in parent
classes, see the Overridden Methods entry in the perltoot
manpage.
What's the difference between calling a function as &foo
and foo()?
When you call a function as "&foo", you allow that func
tion access to your current @_ values, and you bypass pro
totypes. The function doesn't get an empty @_--it gets
yours! While not strictly speaking a bug (it's documented
that way in the perlsub manpage), it would be hard to con
sider this a feature in most cases.
When you call your function as "&foo()", then you do get a
new @_, but prototyping is still circumvented.
Normally, you want to call a function using "foo()". You
may only omit the parentheses if the function is already
known to the compiler because it already saw the defini
tion ("use" but not "require"), or via a forward reference
or "use subs" declaration. Even in this case, you get a
clean @_ without any of the old values leaking through
where they don't belong.
How do I create a switch or case statement?
This is explained in more depth in the the perlsyn man
page. Briefly, there's no official case statement,
because of the variety of tests possible in Perl (numeric
comparison, string comparison, glob comparison, regex
matching, overloaded comparisons, ...). Larry couldn't
decide how best to do this, so he left it out, even though
it's been on the wish list since perl1.
The general answer is to write a construct like this:
for ($variable_to_test) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
}
Here's a simple example of a switch based on pattern
matching, this time lined up in a way to make it look more
like a switch statement. We'll do a multi-way conditional
based on the type of reference stored in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
See "perlsyn/"Basic BLOCKs and Switch Statements"" for
many other examples in this style.
Sometimes you should change the positions of the constant
and the variable. For example, let's say you wanted to
test which of many answers you were given, but in a case-
insensitive way that also allows abbreviations. You can
use the following technique if the strings all start with
different characters or if you want to arrange the matches
so that one takes precedence over another, as ""SEND"" has
precedence over ""STOP"" here:
chomp($answer = <>);
if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of func
tion references.
my %commands = (
"happy" => \&joy,
"sad", => \&sullen,
"done" => sub { die "See ya!" },
"mad" => \&angry,
);
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string\n";
}
How can I catch accesses to undefined variables/func
tions/methods?
The AUTOLOAD method, discussed in the Autoloading entry in
the perlsub manpage and the AUTOLOAD: Proxy Methods entry
in the perltoot manpage, lets you capture calls to unde
fined functions and methods.
When it comes to undefined variables that would trigger a
warning under "-w", you can use a handler to trap the
pseudo-signal "__WARN__" like this:
$SIG{__WARN__} = sub {
for ( $_[0] ) { # voici un switch statement
/Use of uninitialized value/ && do {
# promote warning to a fatal
die $_;
};
# other warning cases to catch could go here;
warn $_;
}
};
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting con
fused, you've misspelled the method name, or the object is
of the wrong type. Check out the perltoot manpage for
details about any of the above cases. You may also use
"print ref($object)" to find out the class "$object" was
blessed into.
Another possible reason for problems is because you've
used the indirect object syntax (eg, "find Guru "Samy"")
on a class name before Perl has seen that such a package
exists. It's wisest to make sure your packages are all
defined before you start using them, which will be taken
care of if you use the "use" statement instead of
"require". If not, make sure to use arrow notation (eg.,
"Guru->find("Samy")") instead. Object notation is
explained in the perlobj manpage.
Make sure to read about creating modules in the perlmod
manpage and the perils of indirect objects in the WARNING
entry in the perlobj manpage.
How can I find out my current package?
If you're just a random program, you can do this to find
out what the currently compiled package is:
my $packname = __PACKAGE__;
But, if you're a method and you want to print an error
message that includes the kind of object you were called
on (which is not necessarily the same as the one in which
you were compiled):
sub amethod {
my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
How can I comment out a large block of perl code?
Use embedded POD to discard it:
# program is here
=for nobody
This paragraph is commented out
# program continues
=begin comment text
all of this stuff
here will be ignored
by everyone
=end comment text
=cut
This can't go just anywhere. You have to put a pod direc
tive where the parser is expecting a new statement, not
just in the middle of an expression or some other arbi
trary yacc grammar production.
How do I clear a package?
Use this code, provided by Mark-Jason Dominus:
sub scrub_package {
no strict 'refs';
my $pack = shift;
die "Shouldn't delete main package"
if $pack eq "" || $pack eq "main";
my $stash = *{$pack . '::'}{HASH};
my $name;
foreach $name (keys %$stash) {
my $fullname = $pack . '::' . $name;
# Get rid of everything with that name.
undef $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
}
}
Or, if you're using a recent release of Perl, you can just
use the Symbol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain
the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two
reasons.
The first reason is that this technique only works on
global variables. That means that if $fred is a lexical
variable created with my() in the above example, the code
wouldn't work at all: you'd accidentally access the global
and skip right over the private lexical altogether.
Global variables are bad because they can easily collide
accidentally and in general make for non-scalable and con
fusing code.
Symbolic references are forbidden under the "use strict"
pragma. They are not true references and consequently are
not reference counted or garbage collected.
The other reason why using a variable to hold the name of
another variable is a bad idea is that the question often
stems from a lack of understanding of Perl data struc
tures, particularly hashes. By using symbolic references,
you are just using the package's symbol-table hash (like
"%main::") instead of a user-defined hash. The solution
is to use your own hash or a real reference instead.
$fred = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic
references. Sometimes this comes up in reading strings
from the user with variable references and wanting to
expand them to the values of your perl program's vari
ables. This is also a bad idea because it conflates the
program-addressable namespace and the user-addressable
one. Instead of reading a string and expanding it to the
actual contents of your program's own variables:
$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
it would be better to keep a hash around like %USER_VARS
and have variable references actually refer to entries in
that hash:
$str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous
approach. Of course, you don't need to use a dollar sign.
You could use your own scheme to make it less confusing,
like bracketed percent symbols, etc.
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a
variable to contain the name of a variable is because they
don't know how to build proper data structures using
hashes. For example, let's say they wanted two hashes in
their program: %fred and %barney, and that they wanted to
use another scalar variable to refer to those by name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled
with the problems enumerated above. It would be far bet
ter to write:
$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic ref
erences are when you really must refer to the symbol
table. This may be because it's something that can't take
a real reference to, such as a format name. Doing so may
also be important for method calls, since these always go
through the symbol table for resolution.
In those cases, you would turn off "strict 'refs'" tem
porarily so you can play around with the symbol table.
For example:
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
no strict 'refs'; # renege for the block
*$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
All those functions (red(), blue(), green(), etc.) appear
to be separate, but the real code in the closure actually
was compiled only once.
So, sometimes you might want to use symbolic references to
directly manipulate the symbol table. This doesn't matter
for formats, handles, and subroutines, because they are
always global--you can't use my() on them. For scalars,
arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan Tork
ington. All rights reserved.
When included as part of the Standard Version of Perl, or
as part of its complete documentation whether printed or
otherwise, this work may be distributed only under the
terms of Perl's Artistic License. Any distribution of
this file or derivatives thereof outside of that package
require that special arrangements be made with copyright
holder.
Irrespective of its distribution, all code examples in
this file are hereby placed into the public domain. You
are permitted and encouraged to use this code in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit would be courteous but
is not required.
2001-04-07 perl v5.6.1 PERLFAQ7(1)
