X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlmod.pod;h=d3b087caad215d09dae5295659e4acd29ce45d7e;hb=3e79b69bf4e5ee29a68ea7ec363a1195dc7fddf5;hp=994c3eb5dc477cc2798425f196e3cc4b44a487af;hpb=7d30b5c4c60a798b772f5d7bd3b85d21016359c7;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlmod.pod b/pod/perlmod.pod index 994c3eb..d3b087c 100644 --- a/pod/perlmod.pod +++ b/pod/perlmod.pod @@ -8,7 +8,7 @@ perlmod - Perl modules (packages and symbol tables) Perl provides a mechanism for alternative namespaces to protect packages from stomping on each other's variables. In fact, there's -really no such thing as a global variable in Perl . The package +really no such thing as a global variable in Perl. The package statement declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block, C, @@ -61,8 +61,8 @@ as a pattern match, a substitution, or a transliteration. 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. -$_ is still global though. See also L. +$_ is still global though. See also +L. Ced strings are compiled in the package in which the eval() was compiled. (Assignments to C<$SIG{}>, however, assume the signal @@ -85,7 +85,7 @@ and L regarding closures. 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 -C<%main::>, or C<%::> for short. Likewise symbol table for the nested +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 @@ -96,8 +96,14 @@ table lookups at compile time: local *main::foo = *main::bar; local $main::{foo} = $main::{bar}; +(Be sure to note the B difference between the second line above +and C. The former is accessing the hash +C<%main::>, which is the symbol table of package C
. The latter is +simply assigning scalar C<$bar> in package C
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 antequated F library and +instance. The standard but antiquated F library and the CPAN module Devel::Symdump make use of this. Assignment to a typeglob performs an aliasing operation, i.e., @@ -114,8 +120,44 @@ subroutine, assign a reference instead: 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 -into or from subroutines if you won't want to copy the whole +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 +C<$bar> -- the one that was stuffed away by C and which will be +restored when the block ends. Because variables are accessed through the +typeglob, you can use C<*foo = *bar> to create an alias which can be +localized. (But be aware that this means you can't have a separate +C<@foo> and C<@bar>, etc.) + +What makes all of this important is that the Exporter module uses glob +aliasing as the import/export mechanism. Whether or not you can properly +localize a variable that has been exported from a module depends on how +it was exported: + + @EXPORT = qw($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. @@ -132,18 +174,18 @@ lexicals. 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 won't want to have to remember to dereference variables +when you don't want to have to remember to dereference variables explicitly. Another use of symbol tables is for making "constant" scalars. *PI = \3.14159265358979; -Now you cannot alter $PI, which is probably a good thing all in all. +Now you cannot alter C<$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. This isn't. A constant subroutine is one -prototyped to take no arguments and to return a constant expression. -See L for details on these. The C pragma is a +optimization at compile-time. A constant subroutine is one prototyped +to take no arguments and to return a constant expression. See +L for details on these. The C pragma is a convenient shorthand for these. You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and @@ -163,7 +205,7 @@ This prints 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. +individual elements of *foo. See L. Subroutine definitions (and declarations, for that matter) need not necessarily be situated in the package whose symbol table they @@ -233,7 +275,7 @@ being blown out of the water by a signal--you have to trap that yourself (if you can).) You may have multiple C blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO). C blocks are not executed when you run perl with the -C<-c> switch. +C<-c> switch, or if compilation fails. Inside an C subroutine, C<$?> contains the value that the program is going to pass to C. You can modify C<$?> to change the exit @@ -251,10 +293,10 @@ LIFO order. C blocks are again useful in the Perl compiler suite to save the compiled state of the program. When you use the B<-n> and B<-p> switches to Perl, C and -C work just as they do in B, as a degenerate case. As currently -implemented (and subject to change, since its inconvenient at best), -both C and blocks are run when you use the B<-c> switch -for a compile-only syntax check, although your main code is not. +C work just as they do in B, as a degenerate case. +Both C and C blocks are run when you use the B<-c> +switch for a compile-only syntax check, although your main code +is not. =head2 Perl Classes @@ -268,14 +310,14 @@ For more on this, see L and L. =head2 Perl Modules -A module is just a set of related function in a library file 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 +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 +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 definition and make its semantics available implicitly through method calls on the class and its objects, without explicitly -exportating anything. Or it can do a little of both. +exporting anything. Or it can do a little of both. For example, to start a traditional, non-OO module called Some::Module, create a file called F and start with this template: @@ -283,6 +325,7 @@ create a file called F and start with this template: package Some::Module; # assumes Some/Module.pm use strict; + use warnings; BEGIN { use Exporter (); @@ -303,6 +346,10 @@ create a file called F and start with this template: } our @EXPORT_OK; + # exported package globals go here + our $Var1; + our %Hashit; + # non-exported package globals go here our @more; our $stuff; @@ -418,26 +465,45 @@ that other module. In that case, it's easy to use Cs 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 -directly as a filename it would makes for unwieldy or impossible +directly as a filename it would make for unwieldy or impossible filenames on some systems. Therefore, if a module's name is, say, C, then its definition is actually found in the library file F. Perl modules always have a F<.pm> file, but there may also be dynamically linked executables (often ending in F<.so>) or autoloaded -subroutine definitions (often ending in F<.al> associated with the +subroutine definitions (often ending in F<.al>) associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the F<.pm> file to load (or arrange to autoload) any additional functionality. For example, although the POSIX module happens to do both dynamic loading and -autoloading, but the user can say just C to get it all. +autoloading, the user can say just C to get it all. + +=head2 Making your module threadsafe + +Perl has since 5.6.0 support for a new type of threads called +interpreter threads. 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 using the 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 5.7.2 has support for the C +special subroutine . In C you can do whatever you need to do, +like for example handle the cloning of non-Perl data, if necessary. +C will be executed once 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. + +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(). =head1 SEE ALSO See L for general style issues related to building Perl modules and classes, as well as descriptions of the standard library and CPAN, L for how Perl's standard import/export mechanism -works, L and L for an in-depth tutorial on +works, L and L for an in-depth tutorial on creating classes, L for a hard-core reference document on objects, L for an explanation of functions and scoping, and L and L for more information on writing