=head1 NAME
perlmod - Perl modules (packages and symbol tables)
=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>
Unlike Perl 4, in which all the variables were dynamic and shared one
global name space, causing maintainability problems, Perl 5 provides two
mechanisms for protecting code from having its variables stomped on by
other code: lexically scoped variables created with C<my> or C<state> and
namespaced global variables, which are exposed via the C<vars> pragma,
or the C<our> keyword. Any global variable is considered to
be part of a namespace and can be accessed via a "fully qualified form".
Conversely, any lexically scoped variable is considered to be part of
that lexical-scope, and does not have a "fully qualified form".
In perl namespaces are called "packages" and
the C<package> declaration tells the compiler which
namespace to prefix to C<our> variables and unqualified dynamic names.
This both protects
against accidental stomping and provides an interface for deliberately
clobbering global dynamic variables declared and used in other scopes or
packages, when that is what you want to do.
The scope of the C<package> declaration is from the
declaration itself through the end of the enclosing block, C<eval>,
or file, whichever comes first (the same scope as the my(), our(), state(), and
local() operators, and also the effect
of the experimental "reference aliasing," which may change), or until
the next C<package> declaration. Unqualified dynamic identifiers will be in
this namespace, except for those few identifiers that, if unqualified,
default to the main package instead of the current one as describedbelow. A C<package> statement affects only dynamic global
symbols, including subroutine names, and variables you've used local()
on, but I<not> lexical variables created with my(), our() or state().
Typically, a C<package> statement is the first declaration in a file
included in a program by one of the C<do>, C<require>, or C<use> operators. You can
switch into a package in more than one place: C<package> has noeffect beyond specifying which symbol table the compiler will use for
dynamic symbols for the rest of that block or until the next C<package> statement.
You can refer to variables and filehandles in other packages
by prefixing the identifier with the package name and a doublecolon: C<$Package::Variable>. If the package name is null, theC<main> package is assumed. That is, C<$::sail> is equivalent toC<$main::sail>.
The old package delimiter was a single quote, but double colon is now thepreferred delimiter, in part because it's more readable to humans, and
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 was going on. Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string likeC<"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 ${owner}'s house">.
X<::> X<'>
Packages may themselves contain package separators, as inC<$OUTER::INNER::var>. This implies nothing about the order of
name lookups, however. There are no relative packages: all symbols
are either local to the current package, or must be fully qualified
from the outer package name down. For instance, there is nowherewithin package C<OUTER> that C<$INNER::var> refers toC<$OUTER::INNER::var>. C<INNER> refers to a totally
separate global package. The custom of treating package names as ahierarchy is very strong, but the language in no way enforces it.
Only identifiers starting with letters (or underscore) are stored
in a package's symbol table. All other symbols are kept in package
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 ones. If you
have a package called C<m>, C<s>, or C<y>, then you can't use thequalified 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 ableto use leading underscore to indicate private variables and method names.However, variables and functions named with a single C<_>, such as
$_ and C<sub _>, are still forced into the package C<main>. See alsoL<perlvar/"The Syntax of Variable Names">.
C<eval>ed strings are compiled in the package in which the eval() wascompiled. (Assignments to C<$SIG{}>, however, assume the signal
handler specified is in the C<main> package. Qualify the signal handler
name if you wish to have a signal handler in a package.) For an
example, examine F<perldb.pl> in the Perl library. It initially switches
to the C<DB> package so that the debugger doesn't interfere with variablesin the program you are trying to debug. At various points, however, it
temporarily switches back to the C<main> package to evaluate variousexpressions in the context of the C<main> package (or wherever you came
from). See L<perldebug>.
The special symbol C<__PACKAGE__> contains the current package, but cannot
(easily) be used to construct variable names. After C<my($foo)> has hidden
package variable C<$foo>, it can still be accessed, without knowing whatpackage you are in, as C<${__PACKAGE__.'::foo'}>.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 thatname with two colons appended. The main symbol table's name is thus
C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
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. local *main::foo = *main::bar;
You can use this to print out all the variables in a package, forinstance. 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;
causes variables, subroutines, formats, and file and directory handles
accessible via the identifier C<richard> also to be accessible via the
identifier C<dick>. If you want to alias only a particular variable or
subroutine, assign a reference instead:
*dick = \$richard;
Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays. Tricky, eh?
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;
{
local $bar = 2;
print $foo;
}
Would print '1', because C<$foo> holds a reference to the I<original>
C<$bar>. The one that was stuffed away by C<local()> and which will be
restored when the block ends. Because variables are accessed through the
typeglob, you can use C<*foo = *bar> to create an alias which can belocalized. (But be aware that this means you can't have a separate
C<@foo> and C<@bar>, etc.)
What makes all of this important is that the Exporter module uses glob
aliasing as the import/export mechanism. Whether or not you can properly
localize a variable that has been exported from a module depends on how
it was exported:
@EXPORT = qw($FOO);
@EXPORT = qw(*FOO);
You can work around the first case by using the fully qualified name
(C<$Package::FOO>) where you need a local value, or by overriding it
by saying C<*FOO = *Package::FOO> in your script.
The C<*x = \$y> mechanism may be used to pass and return cheap references
into or from subroutines if you don't want to copy the whole
thing. It only works when assigning to dynamic variables, not
lexicals.
%some_hash = ();
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
my %nhash = ();
return \%nhash;
}
On return, the reference will overwrite the hash slot in the
symbol table specified by the *some_hash typeglob. This
is a somewhat tricky way of passing around references cheaply
when you don't want to have to remember to dereference variables
explicitly.
Another use of symbol tables is for making "constant" scalars.X<constant> X<scalar, constant>
*PI = \3.14159265358979;
Now you cannot alter C<$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
L<perlsub> for details on these. The C<use constant> pragma is aconvenient shorthand for these.
You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
package the *foo symbol table entry comes from. This may be usefulin a subroutine that gets passed typeglobs as arguments:
sub identify_typeglob {
my $glob = shift;
print 'You gave me ', *{$glob}{PACKAGE},
'::', *{$glob}{NAME}, "\n";
}
identify_typeglob *foo;
identify_typeglob *bar::baz;
This prints
You gave me main::foo
You gave me bar::baz
The C<*foo{THING}> notation can also be used to obtain references to the
individual elements of *foo. See L<perlref>.
Subroutine definitions (and declarations, for that matter) need
not necessarily be situated in the package whose symbol table theyoccupy. You can define a subroutine outside its package byexplicitly qualifying the name of the subroutine:
sub Some_package::foo { ... }
This is just a shorthand for a typeglob assignment at compile time:
BEGIN { *Some_package::foo = sub { ... } }
and is I<not> the same as writing:
{
sub foo { ... }
}
In the first two versions, the body of the subroutine is
lexically in the main package, I<not> in Some_package. So
something like this:
$Some_package::name = "fred";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": \$name is '$name'\n";
}
Some_package::foo();
prints:
in main: $name is 'barney'
rather than:
in Some_package: $name is 'fred'
This also has implications for the use of the SUPER:: qualifier(see L<perlobj>).
=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 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.
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">.
Similar to an C<END> block are C<defer> blocks, though they operate on the
lifetime of individual block scopes, rather than the program as a whole. They
are documented in L<perlsyn/defer>.
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<${^GLOBAL_PHASE}> will be
C<"CHECK">.
C<INIT> blocks are run just before the Perl runtime begins execution, in
"first in, first out" (FIFO) order.
Inside of an C<INIT> block, the value of C<${^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> andC<END> work just as they do in B<awk>, as a degenerate case.
Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>switch for a compile-only syntax check, although your main code
is not.
The B<begincheck> program makes it all clear, eventually:
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";