cvs commit: perlfaq perlfaq7.pod
[email protected] (brian d foy) 7 Apr 2005 21:39:34 -0000
| Newsgroups | perl.cvs.perlfaq |
|---|---|
| Message-ID | <[email protected]> |
cvsuser 05/04/07 14:39:34
Modified: . perlfaq7.pod
Log:
* How do I create a static variable?
I'm completely replacing this answer, so I've included the new
answer as straight text, then the patch at the end.
In the new answer, I define a "static" variable, then show an
example for one function that references an out-of-scope
lexical. The old answer didn't do that.
After that, I show that the out-of-scope lexical can be shared
with other functions (like the previous answer did), so it
isn't really "static".
I cut out most of the discussion of file-scoped variable: it's
enough to remind people that a file is a scope and that lexical
variables can't be seen outside of their scope. If the reader
is looking for static variables, they probably aren't looking
for file-scoped variables.
I also added a pointer to information on closures, which is
where most of the valuable discussion on out-of-scope lexicals
takes place.
Revision Changes Path
1.23 +55 -35 perlfaq/perlfaq7.pod
Index: perlfaq7.pod
===================================================================
RCS file: /cvs/public/perlfaq/perlfaq7.pod,v
retrieving revision 1.22
retrieving revision 1.23
diff -u -r1.22 -r1.23
--- perlfaq7.pod 27 Mar 2005 07:19:01 -0000 1.22
+++ perlfaq7.pod 7 Apr 2005 21:39:34 -0000 1.23
@@ -97,7 +97,7 @@
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
get other categories of warnings. See L<perllexwarn> for the
@@ -411,42 +411,62 @@
=head2 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:
+(contributed by brian d foy)
- BEGIN {
- my $counter = 42;
- sub prev_counter { return --$counter }
- sub next_counter { return $counter++ }
- }
+Perl doesn't have "static" variables, which can only be accessed from
+the function in which they are declared. You can get the same effect
+with lexical variables, though.
+
+You can fake a static variable by using a lexical variable which goes
+of scope. In this example, you define the subroutine C<counter>, and
+it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
+block, C<$count> is defined at compile-time, but also goes out of
+scope at the end of the BEGIN block. The BEGIN block also ensures that
+the subroutine and the value it uses is defined at compile-time so the
+subroutine is ready to use just like any other subroutine, and you can
+put this code in the same place as other subroutines in the program
+text (i.e. at the end of the code, typically). The subroutine
+C<counter> still has a reference to the data, and is the only way you
+can access the value (and each time you do, you increment the value).
+The data in chunk of memory defined by C<$count> is private to
+C<counter>.
+
+ BEGIN {
+ my $count = 1;
+ sub counter { $count++ }
+ }
-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+ my $start = count();
-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:
+ .... # code that calls count();
- package Pax;
- my $started = scalar(localtime(time()));
+ my $end = count();
- sub begun { return $started }
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
-When C<use Pax> or C<require Pax> loads this module, the variable will
-be initialized. It won't get garbage-collected 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.
+ BEGIN {
+ my $count = 1;
+ sub increment_count { $count++ }
+ sub return_count { $count }
+ }
-See L<perlsub/"Persistent Private Variables"> for details.
+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
+cannot be seen from any other file.
+
+See L<perlsub/"Persistent Private Variables"> for more information.
+The discussion of closures in L<perlref> may help you even though we
+did not use anonymous subroutines in this answer. See
+L<perlsub/"Persistent Private Variables"> for details.
=head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
@@ -757,7 +777,7 @@
by everyone
=end comment
-
+
=cut
# program continues
@@ -904,12 +924,12 @@
If you see "bad interpreter - no such file or directory", the first
line in your perl script (the "shebang" line) does not contain the
-right path to perl (or any other program capable of running scripts).
+right path to perl (or any other program capable of running scripts).
Sometimes this happens when you move the script from one machine to
another and each machine has a different path to perl---/usr/bin/perl
versus /usr/local/bin/perl for instance. It may also indicate
-that the source machine has CRLF line terminators and the
-destination machine has LF only: the shell tries to find
+that the source machine has CRLF line terminators and the
+destination machine has LF only: the shell tries to find
/usr/bin/perl<CR>, but can't.
If you see "bad interpreter: Permission denied", you need to make your