perlfaq7.pod

ARM上的如果你对底层感兴趣

or someone can sneak shell escapes into the regexp due to the double
interpolation of the eval and the double-quoted string.  For example:

    $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';

    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
Build_MatchMany_Function() is particularly interesting.  A complete
citation of this book is given in L<perlfaq2>.

=item Passing Methods

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();
        }
    }

Or you can use a closure to bundle up the object and its method call
and arguments:

    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).

=back

=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:

    BEGIN {
        my $counter = 42;
        sub prev_counter { return --$counter }
        sub next_counter { return $counter++ }
    }

Now prev_counter() and next_counter() share a private variable $counter
that was initialized at compile time.

To declare a file-private variable, you'll still use a my(), putting
it at the outer scope level at the top of the file.  Assume this is in
file Pax.pm:

    package Pax;
    my $started = scalar(localtime(time()));

    sub begun { return $started }

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.

See L<perlsub/"Peristent Private Variables"> for details.

=head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?

C<local($x)> saves away the old value of the global variable C<$x>,
and assigns a new value for the duration of the subroutine, I<which is
visible in other functions called from that subroutine>.  This is done
at run-time, so is called dynamic scoping.  local() always affects global
variables, also called package variables or dynamic variables.

C<my($x)> creates a new variable that is only visible in the current
subroutine.  This is done at compile-time, so is called lexical or
static scoping.  my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.

For instance:

    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 lexical {
	my $var = 'private';    # new private variable, $var
	visible();              # (invisible outside of sub scope)
    }

    $var = '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()
function, and it is hidden from called subroutine.

In summary, local() doesn't make what you think of as private, local
variables.  It gives a global variable a temporary value.  my() is
what you're looking for if you want private variables.

See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary
Values via local()"> for excruciating details.

=head2 How can I access a dynamic variable while a similarly named lexical is in scope?

You can do this via symbolic references, provided you haven't set
C<use strict "refs">.  So instead of $var, use C<${'var'}>.

    local $var = "global";
    my    $var = "lexical";

    print "lexical is $var\n";

    no strict 'refs';
    print "global  is ${'var'}\n";

If you know your package, you can just mention it explicitly, as in
$Some_Pack::var.  Note that the notation $::var is I<not> the dynamic
$var in the current package, but rather the one in the C<main>
package, as though you had written $main::var.  Specifying the package
directly makes you hard-code its name, but it executes faster and
avoids running afoul of C<use strict "refs">.

=head2 What's the difference between deep and shallow binding?

In deep binding, lexical variables mentioned in anonymous subroutines
are the same ones that were in scope when the subroutine was created.
In shallow binding, they are whichever variables with the same names
happen to be in scope when the subroutine is called.  Perl always uses
deep binding of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package variables)
are effectively shallowly bound.  Consider this just one more reason
not to use them.  See the answer to L<"What's a closure?">.

=head2 Why doesn't "my($foo) = <FILE>;" work right?

C<my()> and C<local()> give list context to the right hand side
of C<=>.  The E<lt>FHE<gt> read operation, like so many of Perl's
functions and operators, can tell which context it was called in and
behaves appropriately.  In general, the scalar() function can help.
This function does nothing to the data itself (contrary to popular myth)
but rather tells its argument to behave in whatever its scalar fashion is.
If that function doesn't have a defined scalar behavior, this of course
doesn't help you (such as with sort()).

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

You should probably be using lexical variables anyway, although the
issue is the same here:

    my($foo) = <FILE>;	# WRONG
    my $foo  = <FILE>;	# right

=head2 How do I redefine a builtin function, operator, or method?

Why do you want to do that? :-)

If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
module.  See L<perlsub/"Overriding Builtin Functions">.  There's
also an example in L<perltoot/"Class::Template">.

If you want to overload a Perl operator, such as C<+> or C<**>,
then you'll want to use the C<use overload> pragma, documented
in L<overload>.

If you're talking about obscuring method calls in parent classes,
see L<perltoot/"Overridden Methods">.

=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 by-pass prototypes.  That means that
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.

When you call your function as C<&foo()>, then you I<do> get a new @_,
but prototyping is still circumvented.

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.

=head2 How do I create a switch or case statement?

This is explained in more depth in the L<perlsyn>.  Briefly, there's
no official case statement, because of the variety of tests possible
in Perl (numeric comparison, string comparison, glob comparison,
regexp matching, overloaded comparisons, ...).  Larry couldn't decide
how best to do this, so he left it out, even though it's been on the
wish list since perl1.

The general answer is to write a construct like this:

    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, this
time lined up in a way to make it look more like a switch statement.
We'll do a multi-way conditional based on the type of reference stored
in $whatchamacallit:

    SWITCH: for (ref $whatchamacallit) {

	/^$/		&& die "not a reference";

	/SCALAR/	&& do {
				print_scalar($$ref);
				last SWITCH;
			};

	/ARRAY/		&& do {
				print_array(@$ref);
				last SWITCH;
			};

	/HASH/		&& do {
				print_hash(%$ref);
				last SWITCH;
			};

	/CODE/		&& do {
				warn "can't print function ref";
				last SWITCH;
			};

	# DEFAULT

	warn "User defined type skipped";

    }

See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other 
examples in this style.

Sometimes you should change the positions of the constant and the variable.
For example, let's say you wanted to test which of many answers you were
given, but in a case-insensitive way that also allows abbreviations.
You can use the following technique if the strings all start with
different characters, or if you want to arrange the matches so that
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"  }

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";
    } 

=head2 How can I catch accesses to undefined variables/functions/methods?

The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
undefined functions and methods.

When it comes to undefined variables that would trigger a warning
under C<-w>, you can use a handler to trap the pseudo-signal
C<__WARN__> like this:

    $SIG{__WARN__} = sub {

	for ( $_[0] ) {		# voici un switch statement 

	    /Use of uninitialized value/  && do {
		# promote warning to a fatal
		die $_;
	    };

	    # other warning cases to catch could go here;

	    warn $_;
	}

    };

=head2 Why can't a method included in this same file be found?

Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type.  Check
out L<perltoot> for details on these.  You may also use C<print
ref($object)> to find out the class C<$object> was blessed into.

Another possible reason for problems is because you've used the
indirect object syntax (eg, C<find Guru "Samy">) on a class name
before Perl has seen that such a package exists.  It's wisest to make
sure your packages are all defined before you start using them, which
will be taken care of if you use the C<use> statement instead of
C<require>.  If not, make sure to use arrow notation (eg,
C<Guru-E<gt>find("Samy")>) instead.  Object notation is explained in
L<perlobj>.

Make sure to read about creating modules in L<perlmod> and
the perils of indirect objects in L<perlobj/"WARNING">.

=head2 How can I find out my current package?

If you're just a random program, you can do this to find
out what the currently compiled package is:

    my $packname = __PACKAGE__;

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):

    sub amethod {
	my $self  = shift;
	my $class = ref($self) || $self;
	warn "called me from a $class object";
    }

=head2 How can I comment out a large block of perl code?

Use embedded POD to discard it:

    # program is here

    =for nobody
    This paragraph is commented out

    # program continues

    =begin comment text

    all of this stuff

    here will be ignored
    by everyone

    =end comment text

    =cut

This can't go just anywhere.  You have to put a pod directive where
the parser is expecting a new statement, not just in the middle
of an expression or some other arbitrary yacc grammar production.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
All rights reserved.

When included as part of the Standard Version of Perl, or as part of
its complete documentation whether printed or otherwise, this work
may be distributed only under the terms of Perl's Artistic License.
Any distribution of this file or derivatives thereof I<outside>
of that package require that special arrangements be made with
copyright holder.

Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.