=head1 NAME
-perlfaq7 - General Perl Language Issues ($Revision: 9309 $)
+perlfaq7 - General Perl Language Issues
=head1 DESCRIPTION
They are type specifiers, as detailed in L<perldata>:
- $ 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.
+ $ 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.
+ <> are used for inputting a record from a filehandle.
+ \ takes a reference to something.
Note that <FILE> is I<neither> the type specifier for files
nor the name of the handle. It is the C<< <> >> operator applied
subroutine) and the left-hand operand to the C<< => >> operator both
count as though they were quoted:
- This is like this
- ------------ ---------------
- $foo{line} $foo{'line'}
- bar => stuff 'bar' => stuff
+ This is like this
+ ------------ ---------------
+ $foo{line} $foo{'line'}
+ bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final comma in a
list. Good style (see L<perlstyle>) says to put them in except for
one-liners:
- if ($whoops) { exit 1 }
- @nums = (1, 2, 3);
+ if ($whoops) { exit 1 }
+ @nums = (1, 2, 3);
- if ($whoops) {
- exit 1;
- }
- @lines = (
+ if ($whoops) {
+ exit 1;
+ }
+
+ @lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
- );
+ );
=head2 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];
+ $dir = (getpwnam($user))[7];
Another way is to use undef as an element on the left-hand-side:
- ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements that
you need:
allows fine control of what warning are produced.
See L<perllexwarn> for more details.
- {
+ {
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
- }
+ }
Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
If you have an older version of Perl, the C<$^W> variable (documented
in L<perlvar>) controls runtime warnings for a block:
- {
+ {
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
- }
+ }
Note that like all the punctuation variables, you cannot currently
use my() on C<$^W>, only local().
A common mistake is to write:
- unlink $file || die "snafu";
+ unlink $file || die "snafu";
This gets interpreted as:
- unlink ($file || die "snafu");
+ unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or use the
super low precedence C<or> operator:
- (unlink $file) || die "snafu";
- unlink $file or die "snafu";
+ (unlink $file) || die "snafu";
+ unlink $file or die "snafu";
The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
deliberately have precedence lower than that of list operators for
just such situations as the one above.
Another operator with surprising precedence is exponentiation. It
-binds more tightly even than unary minus, making C<-2**2> product a
+binds more tightly even than unary minus, making C<-2**2> produce a
negative not a positive four. It is also right-associating, meaning
that C<2**3**2> is two raised to the ninth power, not eight squared.
produces an lvalue. This assigns $x to either $a or $b, depending
on the trueness of $maybe:
- ($maybe ? $a : $b) = $x;
+ ($maybe ? $a : $b) = $x;
=head2 How do I declare/create a structure?
anonymous) hash reference. See L<perlref> and L<perldsc> for details.
Here's an example:
- $person = {}; # new anonymous hash
- $person->{AGE} = 24; # set field AGE to 24
- $person->{NAME} = "Nat"; # set field NAME to "Nat"
+ $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 L<perltoot>.
which is the best hands-on guide to creating module
distributions.
+=head2 How do I adopt or take over a module already on CPAN?
+
+(contributed by brian d foy)
+
+The full answer to this can be found at
+http://cpan.org/modules/04pause.html#takeover
+
+The easiest way to take over a module is to have the current
+module maintainer either make you a co-maintainer or transfer
+the module to you.
+
+If you can't reach the author for some reason (e.g. email bounces),
+the PAUSE admins at modules@perl.org can help. The PAUSE admins
+treat each case individually.
+
+=over 4
+
+=item
+
+Get a login for the Perl Authors Upload Server (PAUSE) if you don't
+already have one: http://pause.perl.org
+
+=item
+
+Write to modules@perl.org explaining what you did to contact the
+current maintainer. The PAUSE admins will also try to reach the
+maintainer.
+
+=item
+
+Post a public message in a heavily trafficked site announcing your
+intention to take over the module.
+
+=item
+
+Wait a bit. The PAUSE admins don't want to act too quickly in case
+the current maintainer is on holiday. If there's no response to
+private communication or the public post, a PAUSE admin can transfer
+it to you.
+
+=back
+
=head2 How do I create a class?
+X<class, creation> X<package>
+
+(contributed by brian d foy)
+
+In Perl, a class is just a package, and methods are just subroutines.
+Perl doesn't get more formal than that and lets you set up the package
+just the way that you like it (that is, it doesn't set up anything for
+you).
-See L<perltoot> for an introduction to classes and objects, as well as
-L<perlobj> and L<perlbot>.
+The Perl documentation has several tutorials that cover class
+creation, including L<perlboot> (Barnyard Object Oriented Tutorial),
+L<perltoot> (Tom's Object Oriented Tutorial), L<perlbot> (Bag o'
+Object Tricks), and L<perlobj>.
=head2 How can I tell if a variable is tainted?
hard-to-explain meaning. Usually, closures are implemented in Perl as
anonymous subroutines with lasting references to lexical variables
outside their own scopes. These lexicals magically refer to the
-variables that were around when the subroutine was defined (deep
+variables that were around when the subroutine was defined (deep
binding).
Closures are most often used in programming languages where you can
Here's a classic non-closure function-generating function:
- sub add_function_generator {
- return sub { shift() + shift() };
- }
+ sub add_function_generator {
+ return sub { shift() + shift() };
+ }
- $add_sub = add_function_generator();
- $sum = $add_sub->(4,5); # $sum is 9 now.
+ $add_sub = add_function_generator();
+ $sum = $add_sub->(4,5); # $sum is 9 now.
The anonymous subroutine returned by add_function_generator() isn't
technically a closure because it refers to no lexicals outside its own
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 };
- }
+ sub make_adder {
+ my $addpiece = shift;
+ return sub { shift() + $addpiece };
+ }
- $f1 = make_adder(20);
- $f2 = make_adder(555);
+ $f1 = make_adder(20);
+ $f2 = make_adder(555);
Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
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> } );
+ my $line;
+ timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
C<< '$line = <STDIN>' >>, there would have been no way for the
sure a variable doesn't get meddled with during the lifetime of the
package:
- BEGIN {
- my $id = 0;
- sub next_id { ++$id }
- }
+ BEGIN {
+ my $id = 0;
+ sub next_id { ++$id }
+ }
This is discussed in more detail in L<perlsub>, see the entry on
I<Persistent Private 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";
+ my $f = 'foo';
+ sub T {
+ while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
+ }
+
+ T;
+ print "Finally $f\n";
If you are experiencing variable suicide, that C<my $f> in the subroutine
doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
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_scalar );
- func( \@some_array );
- func( [ 1 .. 10 ] );
+ func( \@some_array );
+ func( [ 1 .. 10 ] );
- func( \%some_hash );
- func( { this => 10, that => 20 } );
+ func( \%some_hash );
+ func( { this => 10, that => 20 } );
- func( \&some_func );
- func( sub { $_[0] ** $_[1] } );
+ func( \&some_func );
+ func( sub { $_[0] ** $_[1] } );
=item Passing Filehandles
sub func {
my $passed_fh = shift;
- my $line = <$fh>;
+ my $line = <$passed_fh>;
}
Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
Here's an example of how to pass in a string to be regex compared
using C<qr//>:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = $val1 =~ /$regex/;
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = $val1 =~ /$regex/;
return $retval;
- }
- $match = compare("old McDonald", qr/d.*D/i);
+ }
+ $match = compare("old McDonald", qr/d.*D/i);
Notice how C<qr//> allows flags at the end. That pattern was compiled
at compile time, although it was executed later. The nifty C<qr//>
had to approach this problem much less intuitively. For example, here
it is again if you don't have C<qr//>:
- sub compare($$) {
- my ($val1, $regex) = @_;
- my $retval = eval { $val1 =~ /$regex/ };
+ sub compare($$) {
+ my ($val1, $regex) = @_;
+ my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
- }
+ }
- $match = compare("old McDonald", q/($?i)d.*D/);
+ $match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
- return eval "\$val =~ /$regex/"; # WRONG
+ return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due to the double
interpolation of the eval and the double-quoted string. For example:
- $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
+ $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
- eval "\$string =~ /$pattern_of_evil/";
+ eval "\$string =~ /$pattern_of_evil/";
Those preferring to be very, very clever might see the O'Reilly book,
I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
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();
- }
- }
+ 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();
- }
+ 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 distribution).
(contributed by brian d foy)
-Perl doesn't have "static" variables, which can only be accessed from
-the function in which they are declared. You can get the same effect
-with lexical variables, though.
+In Perl 5.10, declare the variable with C<state>. The C<state>
+declaration creates the lexical variable that persists between calls
+to the subroutine:
+
+ sub counter { state $count = 1; $counter++ }
You can fake a static variable by using a lexical variable which goes
out of scope. In this example, you define the subroutine C<counter>, and
The data in chunk of memory defined by C<$count> is private to
C<counter>.
- BEGIN {
- my $count = 1;
- sub counter { $count++ }
- }
+ BEGIN {
+ my $count = 1;
+ sub counter { $count++ }
+ }
- my $start = counter();
+ my $start = counter();
- .... # code that calls counter();
+ .... # code that calls counter();
- my $end = counter();
+ my $end = counter();
In the previous example, you created a function-private variable
because only one function remembered its reference. You could define
They can both access C<$count>, and since it has gone out of scope,
there is no other way to access it.
- BEGIN {
- my $count = 1;
- sub increment_count { $count++ }
- sub return_count { $count }
- }
+ BEGIN {
+ my $count = 1;
+ sub increment_count { $count++ }
+ sub return_count { $count }
+ }
To declare a file-private variable, you still use a lexical variable.
A file is also a scope, so a lexical variable defined in the file
For instance:
- sub visible {
- print "var has value $var\n";
- }
+ 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 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)
- }
+ sub lexical {
+ my $var = 'private'; # new private variable, $var
+ visible(); # (invisible outside of sub scope)
+ }
- $var = 'global';
+ $var = 'global';
- visible(); # prints global
- dynamic(); # prints local
- lexical(); # prints 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()
print "lexical is $var\n";
{
- our $var;
- print "global is $var\n";
+ our $var;
+ print "global is $var\n";
}
=head2 What's the difference between deep and shallow binding?
To enforce scalar context in this particular case, however, you need
merely omit the parentheses:
- local($foo) = <FILE>; # WRONG
- local($foo) = scalar(<FILE>); # ok
- local $foo = <FILE>; # right
+ local($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 $foo = <FILE>; # right
+ my($foo) = <FILE>; # WRONG
+ my $foo = <FILE>; # right
=head2 How do I redefine a builtin function, operator, or method?
=head2 What's the difference between calling a function as &foo and foo()?
-When you call a function as C<&foo>, you allow that function access to
-your current @_ values, and you bypass prototypes.
-The function doesn't get an empty @_--it gets yours! While not
-strictly speaking a bug (it's documented that way in L<perlsub>), it
-would be hard to consider this a feature in most cases.
+(contributed by brian d foy)
+
+Calling a subroutine as C<&foo> with no trailing parentheses ignores
+the prototype of C<foo> and passes it the current value of the argumet
+list, C<@_>. Here's an example; the C<bar> subroutine calls C<&foo>,
+which prints what its arguments list:
+
+ sub bar { &foo }
+
+ sub foo { print "Args in foo are: @_\n" }
-When you call your function as C<&foo()>, then you I<do> get a new @_,
-but prototyping is still circumvented.
+ bar( qw( a b c ) );
-Normally, you want to call a function using C<foo()>. You may only
-omit the parentheses if the function is already known to the compiler
-because it already saw the definition (C<use> but not C<require>),
-or via a forward reference or C<use subs> declaration. Even in this
-case, you get a clean @_ without any of the old values leaking through
-where they don't belong.
+When you call C<bar> with arguments, you see that C<foo> got the same C<@_>:
+
+ Args in foo are: a b c
+
+Calling the subroutine with trailing parentheses, with or without arguments,
+does not use the current C<@_> and respects the subroutine prototype. Changing
+the example to put parentheses after the call to C<foo> changes the program:
+
+ sub bar { &foo() }
+
+ sub foo { print "Args in foo are: @_\n" }
+
+ bar( qw( a b c ) );
+
+Now the output shows that C<foo> doesn't get the C<@_> from its caller.
+
+ Args in foo are:
+
+The main use of the C<@_> pass-through feature is to write subroutines
+whose main job it is to call other subroutines for you. For further
+details, see L<perlsub>.
=head2 How do I create a switch or case statement?
+In Perl 5.10, use the C<given-when> construct described in L<perlsyn>:
+
+ use 5.010;
+
+ given ( $string ) {
+ when( 'Fred' ) { say "I found Fred!" }
+ when( 'Barney' ) { say "I found Barney!" }
+ when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
+ default { say "I don't recognize the name!" }
+ };
+
If one wants to use pure Perl and to be compatible with Perl versions
-prior to 5.10, the general answer is to write a construct like this:
+prior to 5.10, the general answer is to use C<if-elsif-else>:
- for ($variable_to_test) {
- if (/pat1/) { } # do something
- elsif (/pat2/) { } # do something else
- elsif (/pat3/) { } # do something else
- else { } # default
- }
+ for ($variable_to_test) {
+ if (/pat1/) { } # do something
+ elsif (/pat2/) { } # do something else
+ elsif (/pat3/) { } # do something else
+ else { } # default
+ }
Here's a simple example of a switch based on pattern matching,
lined up in a way to make it look more like a switch statement.
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:
- chomp($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$answer/i) { print "Action is edit\n" }
+ chomp($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$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function references.
- my %commands = (
- "happy" => \&joy,
- "sad", => \&sullen,
- "done" => sub { die "See ya!" },
- "mad" => \&angry,
- );
-
- print "How are you? ";
- chomp($string = <STDIN>);
- if ($commands{$string}) {
- $commands{$string}->();
- } else {
- print "No such command: $string\n";
- }
-
-Note that starting from version 5.10, Perl has now a native switch
-statement. See L<perlsyn>.
+ my %commands = (
+ "happy" => \&joy,
+ "sad", => \&sullen,
+ "done" => sub { die "See ya!" },
+ "mad" => \&angry,
+ );
+
+ print "How are you? ";
+ chomp($string = <STDIN>);
+ if ($commands{$string}) {
+ $commands{$string}->();
+ } else {
+ print "No such command: $string\n";
+ }
Starting from Perl 5.8, a source filter module, C<Switch>, can also be
used to get switch and case. Its use is now discouraged, because it's
Make sure to read about creating modules in L<perlmod> and
the perils of indirect objects in L<perlobj/"Method Invocation">.
-=head2 How can I find out my current package?
+=head2 How can I find out my current or calling package?
-If you're just a random program, you can do this to find
-out what the currently compiled package is:
+(contributed by brian d foy)
- my $packname = __PACKAGE__;
+To find the package you are currently in, use the special literal
+C<__PACKAGE__>, as documented in L<perldata>. You can only use the
+special literals as separate tokens, so you can't interpolate them
+into strings like you can with variables:
-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):
+ my $current_package = __PACKAGE__;
+ print "I am in package $current_package\n";
- sub amethod {
- my $self = shift;
- my $class = ref($self) || $self;
- warn "called me from a $class object";
- }
+This is different from finding out the package an object is blessed
+into, which might not be the current package. For that, use C<blessed>
+from C<Scalar::Util>, part of the Standard Library since Perl 5.8:
-=head2 How can I comment out a large block of perl code?
+ use Scalar::Util qw(blessed);
+ my $object_package = blessed( $object );
-You can use embedded POD to discard it. Enclose the blocks you want
-to comment out in POD markers. The <=begin> directive marks a section
-for a specific formatter. Use the C<comment> format, which no formatter
-should claim to understand (by policy). Mark the end of the block
-with <=end>.
+Most of the time, you shouldn't care what package an object is blessed
+into, however, as long as it claims to inherit from that class:
- # program is here
+ my $is_right_class = eval { $object->isa( $package ) }; # true or false
- =begin comment
+If you want to find the package calling your code, perhaps to give better
+diagnostics as C<Carp> does, use the C<caller> built-in:
- all of this stuff
+ sub foo {
+ my @args = ...;
+ my( $package, $filename, $line ) = caller;
- here will be ignored
- by everyone
+ print "I was called from package $package\n";
+ );
- =end comment
+By default, your program starts in package C<main>, so you should
+always be in some package unless someone uses the C<package> built-in
+with no namespace. See the C<package> entry in L<perlfunc> for the
+details of empty packges.
+
+=head2 How can I comment out a large block of Perl code?
+
+(contributed by brian d foy)
+
+The quick-and-dirty way to comment out more than one line of Perl is
+to surround those lines with Pod directives. You have to put these
+directives at the beginning of the line and somewhere where Perl
+expects a new statement (so not in the middle of statements like the #
+comments). You end the comment with C<=cut>, ending the Pod section:
+
+ =pod
+
+ my $object = NotGonnaHappen->new();
+
+ ignored_sub();
- =cut
+ $wont_be_assigned = 37;
- # program continues
+ =cut
-The pod directives cannot go just anywhere. You must put a
-pod directive where the parser is expecting a new statement,
-not just in the middle of an expression or some other
-arbitrary grammar production.
+The quick-and-dirty method only works well when you don't plan to
+leave the commented code in the source. If a Pod parser comes along,
+you're multiline comment is going to show up in the Pod translation.
+A better way hides it from Pod parsers as well.
-See L<perlpod> for more details.
+The C<=begin> directive can mark a section for a particular purpose.
+If the Pod parser doesn't want to handle it, it just ignores it. Label
+the comments with C<comment>. End the comment using C<=end> with the
+same label. You still need the C<=cut> to go back to Perl code from
+the Pod comment:
+
+ =begin comment
+
+ my $object = NotGonnaHappen->new();
+
+ ignored_sub();
+
+ $wont_be_assigned = 37;
+
+ =end comment
+
+ =cut
+
+For more information on Pod, check out L<perlpod> and L<perlpodspec>.
=head2 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 $$fullname;
- undef @$fullname;
- undef %$fullname;
- undef &$fullname;
- undef *$fullname;
+ sub scrub_package {
+ no strict 'refs';
+ my $pack = shift;
+ die "Shouldn't delete main package"
+ if $pack eq "" || $pack eq "main";
+ my $stash = *{$pack . '::'}{HASH};
+ my $name;
+ foreach $name (keys %$stash) {
+ my $fullname = $pack . '::' . $name;
+ # Get rid of everything with that name.
+ undef $$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.
Beginners often think they want to have a variable contain the name
of a variable.
- $fred = 23;
- $varname = "fred";
- ++$$varname; # $fred now 24
+ $fred = 23;
+ $varname = "fred";
+ ++$$varname; # $fred now 24
This works I<sometimes>, but it is a very bad idea for two reasons.
(like C<%main::>) instead of a user-defined hash. The solution is to
use your own hash or a real reference instead.
- $USER_VARS{"fred"} = 23;
- $varname = "fred";
- $USER_VARS{$varname}++; # not $$varname++
+ $USER_VARS{"fred"} = 23;
+ $varname = "fred";
+ $USER_VARS{$varname}++; # not $$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
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
+ $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
+ $str =~ s/\$(\w+)/$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
+ $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
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";
- $$name{WIFE} = "wilma"; # set %fred
+ $name = "fred";
+ $$name{WIFE} = "wilma"; # set %fred
- $name = "barney";
- $$name{WIFE} = "betty"; # set %barney
+ $name = "barney";
+ $$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled with the
problems enumerated above. It would be far better to write:
- $folks{"fred"}{WIFE} = "wilma";
- $folks{"barney"}{WIFE} = "betty";
+ $folks{"fred"}{WIFE} = "wilma";
+ $folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
In those cases, you would turn off C<strict 'refs'> temporarily 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>" };
- }
+ @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.
=head1 REVISION
-Revision: $Revision: 9309 $
+Revision: $Revision$
-Date: $Date: 2007-03-23 15:28:16 +0100 (Fri, 23 Mar 2007) $
+Date: $Date$
See L<perlfaq> for source control details and availability.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
+Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or modify it
projects / p5sagit/p5-mst-13.2.git / blobdiff