=head1 DESCRIPTION
+=head2 Is this the document you were after?
+
+There are other documents which might contain the information that you're
+looking for:
+
+=over 2
+
+=item This doc
+
+Perl's packages, namespaces, and some info on classes.
+
+=item L<perlnewmod>
+
+Tutorial on making a new module.
+
+=item L<perlmodstyle>
+
+Best practices for making a new module.
+
+=back
+
=head2 Packages
+X<package> X<namespace> X<variable, global> X<global variable> X<global>
Perl provides a mechanism for alternative namespaces to protect
packages from stomping on each other's variables. In fact, there's
in part because it's more readable to B<emacs> macros. It also makes C++
programmers feel like they know what's going on--as opposed to using the
single quote as separator, which was there to make Ada programmers feel
-like they knew what's going on. Because the old-fashioned syntax is still
+like they knew what was going on. Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string like
C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
the $s variable in package C<owner>, which is probably not what you meant.
Use braces to disambiguate, as in C<"This is ${owner}'s house">.
+X<::> X<'>
Packages may themselves contain package separators, as in
C<$OUTER::INNER::var>. This implies nothing about the order of
are either local to the current package, or must be fully qualified
from the outer package name down. For instance, there is nowhere
within package C<OUTER> that C<$INNER::var> refers to
-C<$OUTER::INNER::var>. It would treat package C<INNER> as a totally
+C<$OUTER::INNER::var>. C<INNER> refers to a totally
separate global package.
Only identifiers starting with letters (or underscore) are stored
C<main>, including all punctuation variables, like $_. In addition,
when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
-even when used for other purposes than their built-in one. If you
+even when used for other purposes than their built-in ones. If you
have a package called C<m>, C<s>, or C<y>, then you can't use the
qualified form of an identifier because it would be instead interpreted
as a pattern match, a substitution, or a transliteration.
+X<variable, punctuation>
Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
-$_ is still global though. See also L<perlvar/"Technical Note on the
-Syntax of Variable Names">.
+However, variables and functions named with a single C<_>, such as
+$_ and C<sub _>, are still forced into the package C<main>. See also
+L<perlvar/"The Syntax of Variable Names">.
C<eval>ed strings are compiled in the package in which the eval() was
compiled. (Assignments to C<$SIG{}>, however, assume the signal
from). See L<perldebug>.
The special symbol C<__PACKAGE__> contains the current package, but cannot
-(easily) be used to construct variables.
+(easily) be used to construct variable names.
See L<perlsub> for other scoping issues related to my() and local(),
and L<perlref> regarding closures.
=head2 Symbol Tables
+X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias>
The symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
package mentioned earlier is named C<%OUTER::INNER::>.
The value in each entry of the hash is what you are referring to when you
-use the C<*name> typeglob notation. In fact, the following have the same
-effect, though the first is more efficient because it does the symbol
-table lookups at compile time:
+use the C<*name> typeglob notation.
local *main::foo = *main::bar;
- local $main::{foo} = $main::{bar};
-
-(Be sure to note the B<vast> difference between the second line above
-and C<local $main::foo = $main::bar>. The former is accessing the hash
-C<%main::>, which is the symbol table of package C<main>. The latter is
-simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
-the same package.)
You can use this to print out all the variables in a package, for
instance. The standard but antiquated F<dumpvar.pl> library and
the CPAN module Devel::Symdump make use of this.
+The results of creating new symbol table entries directly or modifying any
+entries that are not already typeglobs are undefined and subject to change
+between releases of perl.
+
Assignment to a typeglob performs an aliasing operation, i.e.,
*dick = *richard;
Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays. Tricky, eh?
-This mechanism may be used to pass and return cheap references
+There is one subtle difference between the following statements:
+
+ *foo = *bar;
+ *foo = \$bar;
+
+C<*foo = *bar> makes the typeglobs themselves synonymous while
+C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
+refer to the same scalar value. This means that the following code:
+
+ $bar = 1;
+ *foo = \$bar; # Make $foo an alias for $bar
+
+ {
+ local $bar = 2; # Restrict changes to block
+ print $foo; # Prints '1'!
+ }
+
+Would print '1', because C<$foo> holds a reference to the I<original>
+C<$bar>. The one that was stuffed away by C<local()> and which will be
+restored when the block ends. Because variables are accessed through the
+typeglob, you can use C<*foo = *bar> to create an alias which can be
+localized. (But be aware that this means you can't have a separate
+C<@foo> and C<@bar>, etc.)
+
+What makes all of this important is that the Exporter module uses glob
+aliasing as the import/export mechanism. Whether or not you can properly
+localize a variable that has been exported from a module depends on how
+it was exported:
+
+ @EXPORT = qw($FOO); # Usual form, can't be localized
+ @EXPORT = qw(*FOO); # Can be localized
+
+You can work around the first case by using the fully qualified name
+(C<$Package::FOO>) where you need a local value, or by overriding it
+by saying C<*FOO = *Package::FOO> in your script.
+
+The C<*x = \$y> mechanism may be used to pass and return cheap references
into or from subroutines if you don't want to copy the whole
thing. It only works when assigning to dynamic variables, not
lexicals.
explicitly.
Another use of symbol tables is for making "constant" scalars.
+X<constant> X<scalar, constant>
*PI = \3.14159265358979;
Now you cannot alter C<$PI>, which is probably a good thing all in all.
This isn't the same as a constant subroutine, which is subject to
optimization at compile-time. A constant subroutine is one prototyped
-to take no arguments and to return a constant expression. See
+to take no arguments and to return a constant expression. See
L<perlsub> for details on these. The C<use constant> pragma is a
convenient shorthand for these.
sub identify_typeglob {
my $glob = shift;
- print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
+ print 'You gave me ', *{$glob}{PACKAGE},
+ '::', *{$glob}{NAME}, "\n";
}
identify_typeglob *foo;
identify_typeglob *bar::baz;
This also has implications for the use of the SUPER:: qualifier
(see L<perlobj>).
-=head2 Package Constructors and Destructors
-
-Four special subroutines act as package constructors and destructors.
-These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The
-C<sub> is optional for these routines.
-
-A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
-it is completely defined, even before the rest of the containing file
-is parsed. You may have multiple C<BEGIN> blocks within a file--they
-will execute in order of definition. Because a C<BEGIN> block executes
-immediately, it can pull in definitions of subroutines and such from other
-files in time to be visible to the rest of the file. Once a C<BEGIN>
-has run, it is immediately undefined and any code it used is returned to
-Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
-
-An C<END> subroutine is executed as late as possible, that is, after
+=head2 BEGIN, UNITCHECK, CHECK, INIT and END
+X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END>
+
+Five specially named code blocks are executed at the beginning and at
+the end of a running Perl program. These are the C<BEGIN>,
+C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks.
+
+These code blocks can be prefixed with C<sub> to give the appearance of a
+subroutine (although this is not considered good style). One should note
+that these code blocks don't really exist as named subroutines (despite
+their appearance). The thing that gives this away is the fact that you can
+have B<more than one> of these code blocks in a program, and they will get
+B<all> executed at the appropriate moment. So you can't execute any of
+these code blocks by name.
+
+A C<BEGIN> code block is executed as soon as possible, that is, the moment
+it is completely defined, even before the rest of the containing file (or
+string) is parsed. You may have multiple C<BEGIN> blocks within a file (or
+eval'ed string); they will execute in order of definition. Because a C<BEGIN>
+code block executes immediately, it can pull in definitions of subroutines
+and such from other files in time to be visible to the rest of the compile
+and run time. Once a C<BEGIN> has run, it is immediately undefined and any
+code it used is returned to Perl's memory pool.
+
+An C<END> code block is executed as late as possible, that is, after
perl has finished running the program and just before the interpreter
is being exited, even if it is exiting as a result of a die() function.
-(But not if it's polymorphing into another program via C<exec>, or
+(But not if it's morphing into another program via C<exec>, or
being blown out of the water by a signal--you have to trap that yourself
(if you can).) You may have multiple C<END> blocks within a file--they
will execute in reverse order of definition; that is: last in, first
out (LIFO). C<END> blocks are not executed when you run perl with the
C<-c> switch, or if compilation fails.
-Inside an C<END> subroutine, C<$?> contains the value that the program is
+Note that C<END> code blocks are B<not> executed at the end of a string
+C<eval()>: if any C<END> code blocks are created in a string C<eval()>,
+they will be executed just as any other C<END> code block of that package
+in LIFO order just before the interpreter is being exited.
+
+Inside an C<END> code block, C<$?> contains the value that the program is
going to pass to C<exit()>. You can modify C<$?> to change the exit
value of the program. Beware of changing C<$?> by accident (e.g. by
running something via C<system>).
+X<$?>
+
+Inside of a C<END> block, the value of C<${^GLOBAL_PHASE}> will be
+C<"END">.
+
+C<UNITCHECK>, C<CHECK> and C<INIT> code blocks are useful to catch the
+transition between the compilation phase and the execution phase of
+the main program.
+
+C<UNITCHECK> blocks are run just after the unit which defined them has
+been compiled. The main program file and each module it loads are
+compilation units, as are string C<eval>s, run-time code compiled using the
+C<(?{ })> construct in a regex, calls to C<do FILE>, C<require FILE>,
+and code after the C<-e> switch on the command line.
+
+C<BEGIN> and C<UNITCHECK> blocks are not directly related to the phase of
+the interpreter. They can be created and executed during any phase.
+
+C<CHECK> code blocks are run just after the B<initial> Perl compile phase ends
+and before the run time begins, in LIFO order. C<CHECK> code blocks are used
+in the Perl compiler suite to save the compiled state of the program.
+
+Inside of a C<CHECK> block, the value of C<${^GLOBAL_PHASE}> will be
+C<"CHECK">.
-Similar to C<BEGIN> blocks, C<INIT> blocks are run just before the
-Perl runtime begins execution, in "first in, first out" (FIFO) order.
-For example, the code generators documented in L<perlcc> make use of
-C<INIT> blocks to initialize and resolve pointers to XSUBs.
+C<INIT> blocks are run just before the Perl runtime begins execution, in
+"first in, first out" (FIFO) order.
-Similar to C<END> blocks, C<CHECK> blocks are run just after the
-Perl compile phase ends and before the run time begins, in
-LIFO order. C<CHECK> blocks are again useful in the Perl compiler
-suite to save the compiled state of the program.
+Inside of an C<INIT> block, the value of C<${^GLOBAL_PHASE}> will be C<"INIT">.
+
+The C<CHECK> and C<INIT> blocks in code compiled by C<require>, string C<do>,
+or string C<eval> will not be executed if they occur after the end of the
+main compilation phase; that can be a problem in mod_perl and other persistent
+environments which use those functions to load code at runtime.
When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
C<END> work just as they do in B<awk>, as a degenerate case.
switch for a compile-only syntax check, although your main code
is not.
+The B<begincheck> program makes it all clear, eventually:
+
+ #!/usr/bin/perl
+
+ # begincheck
+
+ print "10. Ordinary code runs at runtime.\n";
+
+ END { print "16. So this is the end of the tale.\n" }
+ INIT { print " 7. INIT blocks run FIFO just before runtime.\n" }
+ UNITCHECK {
+ print " 4. And therefore before any CHECK blocks.\n"
+ }
+ CHECK { print " 6. So this is the sixth line.\n" }
+
+ print "11. It runs in order, of course.\n";
+
+ BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
+ END { print "15. Read perlmod for the rest of the story.\n" }
+ CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
+ INIT { print " 8. Run this again, using Perl's -c switch.\n" }
+
+ print "12. This is anti-obfuscated code.\n";
+
+ END { print "14. END blocks run LIFO at quitting time.\n" }
+ BEGIN { print " 2. So this line comes out second.\n" }
+ UNITCHECK {
+ print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
+ }
+ INIT { print " 9. You'll see the difference right away.\n" }
+
+ print "13. It only _looks_ like it should be confusing.\n";
+
+ __END__
+
=head2 Perl Classes
+X<class> X<@ISA>
There is no special class syntax in Perl, but a package may act
as a class if it provides subroutines to act as methods. Such a
package may also derive some of its methods from another class (package)
-by listing the other package name(s) in its global @ISA array (which
+by listing the other package name(s) in its global @ISA array (which
must be a package global, not a lexical).
-For more on this, see L<perl
toot> and L<perlobj>.
+For more on this, see L<perl
ootut> and L<perlobj>.
=head2 Perl Modules
+X<module>
A module is just a set of related functions in a library file, i.e.,
-a Perl package with the same name as the file. It is specifically
+a Perl package with the same name as the file. It is specifically
designed to be reusable by other modules or programs. It may do this
by providing a mechanism for exporting some of its symbols into the
-symbol table of any package using it. Or it may function as a class
+symbol table of any package using it, or it may function as a class
definition and make its semantics available implicitly through
method calls on the class and its objects, without explicitly
exporting anything. Or it can do a little of both.
use warnings;
BEGIN {
- use Exporter ();
- our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
+ require Exporter;
# set the version for version checking
- $VERSION = 1.00;
- # if using RCS/CVS, this may be preferred
- $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
+ our $VERSION = 1.00;
- @ISA = qw(Exporter);
- @EXPORT = qw(&func1 &func2 &func4);
- %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
+ # Inherit from Exporter to export functions and variables
+ our @ISA = qw(Exporter);
- # your exported package globals go here,
- # as well as any optionally exported functions
- @EXPORT_OK = qw($Var1 %Hashit &func3);
+ # Functions and variables which are exported by default
+ our @EXPORT = qw(func1 func2);
+
+ # Functions and variables which can be optionally exported
+ our @EXPORT_OK = qw($Var1 %Hashit func3);
}
- our @EXPORT_OK;
# exported package globals go here
- our $Var1;
- our %Hashit;
+ 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 = ();
+ # (they are still accessible as $Some::Module::stuff)
+ our @more = ();
+ our $stuff = '';
- # all file-scoped lexicals must be created before
- # the functions below that use them.
-
- # file-private lexicals go here
+ # file-private lexicals go here, before any functions which use them
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
- # callable as &$priv_func; it cannot be prototyped.
+ # callable as $priv_func->();
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
+ sub func1 { ... }
+ sub func2 { ... }
- END { } # module clean-up code here (global destructor)
+ # this one isn't exported, but could be called directly
+ # as Some::Module::func3()
+ sub func3 { ... }
- ## YOUR CODE GOES HERE
+ END { ... } # module clean-up code here (global destructor)
1; # don't forget to return a true value from the file
This is exactly equivalent to
- BEGIN { require Module; import Module; }
+ BEGIN { require 'Module.pm'; 'Module'->import; }
or
- BEGIN { require Module; import Module LIST; }
+ BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }
As a special case
is exactly equivalent to
- BEGIN { require Module; }
+ BEGIN { require 'Module.pm'; }
All Perl module files have the extension F<.pm>. The C<use> operator
assumes this so you don't have to spell out "F<Module.pm>" in quotes.
The two statements:
require SomeModule;
- require "SomeModule.pm";
+ require "SomeModule.pm";
differ from each other in two ways. In the first case, any double
colons in the module name, such as C<Some::Module>, are translated
because it determines module availability at compile time, not in the
middle of your program's execution. An exception would be if two modules
each tried to C<use> each other, and each also called a function from
-that other module. In that case, it's easy to use C<require>s instead.
+that other module. In that case, it's easy to use C<require> instead.
Perl packages may be nested inside other package names, so we can have
package names containing C<::>. But if we used that package name
although the POSIX module happens to do both dynamic loading and
autoloading, the user can say just C<use POSIX> to get it all.
+=head2 Making your module threadsafe
+X<threadsafe> X<thread safe>
+X<module, threadsafe> X<module, thread safe>
+X<CLONE> X<CLONE_SKIP> X<thread> X<threads> X<ithread>
+
+Perl supports a type of threads called interpreter threads (ithreads).
+These threads can be used explicitly and implicitly.
+
+Ithreads work by cloning the data tree so that no data is shared
+between different threads. These threads can be used by using the C<threads>
+module or by doing fork() on win32 (fake fork() support). When a
+thread is cloned all Perl data is cloned, however non-Perl data cannot
+be cloned automatically. Perl after 5.8.0 has support for the C<CLONE>
+special subroutine. In C<CLONE> you can do whatever
+you need to do,
+like for example handle the cloning of non-Perl data, if necessary.
+C<CLONE> will be called once as a class method for every package that has it
+defined (or inherits it). It will be called in the context of the new thread,
+so all modifications are made in the new area. Currently CLONE is called with
+no parameters other than the invocant package name, but code should not assume
+that this will remain unchanged, as it is likely that in future extra parameters
+will be passed in to give more information about the state of cloning.
+
+If you want to CLONE all objects you will need to keep track of them per
+package. This is simply done using a hash and Scalar::Util::weaken().
+
+Perl after 5.8.7 has support for the C<CLONE_SKIP> special subroutine.
+Like C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is
+called just before cloning starts, and in the context of the parent
+thread. If it returns a true value, then no objects of that class will
+be cloned; or rather, they will be copied as unblessed, undef values.
+For example: if in the parent there are two references to a single blessed
+hash, then in the child there will be two references to a single undefined
+scalar value instead.
+This provides a simple mechanism for making a module threadsafe; just add
+C<sub CLONE_SKIP { 1 }> at the top of the class, and C<DESTROY()> will
+now only be called once per object. Of course, if the child thread needs
+to make use of the objects, then a more sophisticated approach is
+needed.
+
+Like C<CLONE>, C<CLONE_SKIP> is currently called with no parameters other
+than the invocant package name, although that may change. Similarly, to
+allow for future expansion, the return value should be a single C<0> or
+C<1> value.
+
=head1 SEE ALSO
See L<perlmodlib> for general style issues related to building Perl
modules and classes, as well as descriptions of the standard library
and CPAN, L<Exporter> for how Perl's standard import/export mechanism
-works, L<perltoot> and L<perltootc> for an in-depth tutorial on
+works, L<perlootut> and L<perlobj> for in-depth information on
creating classes, L<perlobj> for a hard-core reference document on
objects, L<perlsub> for an explanation of functions and scoping,
and L<perlxstut> and L<perlguts> for more information on writing
https://perl5.git.perl.org / perl5.git / blobdiff