FAQ sync

p4raw-id: //depot/perl@24684

1 files changed, 54 insertions, 34 deletions

diff --git a/pod/perlfaq7.pod b/pod/perlfaq7.pod
index b87f0969ba..ac9b31f07a 100644
--- a/pod/perlfaq7.pod
+++ b/pod/perlfaq7.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-perlfaq7 - General Perl Language Issues ($Revision: 1.22 $, $Date: 2005/03/27 07:19:01 $)
+perlfaq7 - General Perl Language Issues ($Revision: 1.23 $, $Date: 2005/04/07 21:39:34 $)
 
 =head1 DESCRIPTION
 
@@ -97,7 +97,7 @@ 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
 get other categories of warnings.  See L<perllexwarn> for the
@@ -411,42 +411,62 @@ You could also investigate the can() method in the UNIVERSAL class
 
 =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.
+(contributed by brian d foy)
 
-Here's code to implement a function-private variable:
+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++ }
+		}
 
-    BEGIN {
-        my $counter = 42;
-        sub prev_counter { return --$counter }
-        sub next_counter { return $counter++ }
-    }
+	my $start = count();
 
-Now prev_counter() and next_counter() share a private variable $counter
-that was initialized at compile time.
+	.... # code that calls 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:
+	my $end = count();
 
-    package Pax;
-    my $started = scalar(localtime(time()));
+In the previous example, you created a function-private variable
+because only one function remembered its reference. You could define
+multiple functions while the variable is in scope, and each function
+can share the "private" variable. It's not really "static" because you
+can access it outside the function while the lexical variable is in
+scope, and even create references to it. In this example,
+C<increment_count> and C<return_count> share the variable. One
+function adds to the value and the other simply returns the value.
+They can both access C<$count>, and since it has gone out of scope,
+there is no other way to access it.
 
-    sub begun { return $started }
+    BEGIN {
+        my $count = 1;
+        sub increment_count { $count++ }
+        sub return_count    { $count }
+		}
 
-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.
+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 details.
+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 @@ with <=end>.
     by everyone
 
 	=end comment
-	
+
     =cut
 
     # program continues
@@ -904,12 +924,12 @@ settings.
 
 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