Variable::Magic - Associate user-defined magic to variables from Perl.
VERSION
- Version 0.49
+ Version 0.50
SYNOPSIS
use Variable::Magic qw<wizard cast VMG_OP_INFO_NAME>;
write a single line of XS.
You'll realize that these magic variables look a lot like tied
- variables. It's not surprising, as tied variables are implemented as a
+ variables. It is not surprising, as tied variables are implemented as a
special kind of magic, just like any 'irregular' Perl variable : scalars
like $!, $( or $^W, the %ENV and %SIG hashes, the @ISA array, "vec()"
and "substr()" lvalues, threads::shared variables... They all share the
Still, the magic made available by this module differs from tieing and
overloading in several ways :
- * It isn't copied on assignment.
+ * Magic is not copied on assignment.
You attach it to variables, not values (as for blessed references).
- * It doesn't replace the original semantics.
+ * Magic does not replace the original semantics.
Magic callbacks usually get triggered before the original action
- takes place, and can't prevent it from happening. This also makes
+ takes place, and cannot prevent it from happening. This also makes
catching individual events easier than with "tie", where you have to
provide fallbacks methods for all actions by usually inheriting from
the correct "Tie::Std*" class and overriding individual methods in
your own class.
- * It's type-agnostic.
+ * Magic is type-agnostic.
The same magic can be applied on scalars, arrays, hashes, subs or
globs. But the same hook (see below for a list) may trigger
differently depending on the the type of the variable.
- * It's mostly invisible at the Perl level.
+ * Magic is invisible at the Perl level.
Magical and non-magical variables cannot be distinguished with
"ref", "tied" or another trick.
- * It's notably faster.
+ * Magic is notably faster.
Mainly because perl's way of handling magic is lighter by nature,
- and because there's no need for any method resolution. Also, since
+ and because there is no need for any method resolution. Also, since
you don't have to reimplement all the variable semantics, you only
pay for what you actually use.
The operations that can be overloaded are :
- * "get"
+ * *get*
This magic is invoked when the variable is evaluated. It is never
called for arrays and hashes.
- * "set"
+ * *set*
- This one is triggered each time the value of the variable changes.
- It is called for array subscripts and slices, but never for hashes.
+ This magic is called each time the value of the variable changes. It
+ is called for array subscripts and slices, but never for hashes.
- * "len"
+ * *len*
- This magic is a little special : it is called when the 'size' or the
- 'length' of the variable has to be known by Perl. Typically, it's
- the magic involved when an array is evaluated in scalar context, but
- also on array assignment and loops ("for", "map" or "grep"). The
- callback has then to return the length as an integer.
+ This magic only applies to scalars and arrays, and is triggered when
+ the 'size' or the 'length' of the variable has to be known by Perl.
+ This is typically the magic involved when an array is evaluated in
+ scalar context, but also on array assignment and loops ("for", "map"
+ or "grep"). The length is returned from the callback as an integer.
- * "clear"
+ * *clear*
This magic is invoked when the variable is reset, such as when an
array is emptied. Please note that this is different from undefining
result of the undefine (e.g. for an array, but actually a bug
prevent it to work before perl 5.9.5 - see the history).
- * "free"
+ * *free*
- This one can be considered as an object destructor. It happens when
- the variable goes out of scope, but not when it is undefined.
+ This magic is called when an object is destroyed as the result of
+ the variable going out of scope (but not when the variable is
+ undefined).
- * "copy"
+ * *copy*
- This magic only applies to tied arrays and hashes. It fires when you
- try to access or change their elements. It is available on your perl
- iff "MGf_COPY" is true.
+ This magic only applies to tied arrays and hashes, and fires when
+ you try to access or change their elements.
- * "dup"
+ * *dup*
- Invoked when the variable is cloned across threads. Currently not
- available.
+ This magic is invoked when the variable is cloned across threads. It
+ is currently not available.
- * "local"
+ * *local*
When this magic is set on a variable, all subsequent localizations
of the variable will trigger the callback. It is available on your
- perl iff "MGf_LOCAL" is true.
+ perl if and only if "MGf_LOCAL" is true.
- The following actions only apply to hashes and are available iff
- "VMG_UVAR" is true. They are referred to as "uvar" magics.
+ The following actions only apply to hashes and are available if and only
+ if "VMG_UVAR" is true. They are referred to as *uvar* magics.
- * "fetch"
+ * *fetch*
- This magic happens each time an element is fetched from the hash.
+ This magic is invoked each time an element is fetched from the hash.
- * "store"
+ * *store*
This one is called when an element is stored into the hash.
- * "exists"
+ * *exists*
This magic fires when a key is tested for existence in the hash.
- * "delete"
+ * *delete*
- This last one triggers when a key is deleted in the hash, regardless
- of whether the key actually exists in it.
+ This magic is triggered when a key is deleted in the hash,
+ regardless of whether the key actually exists in it.
You can refer to the tests to have more insight of where the different
magics are invoked.
op_info => [ 0 | VMG_OP_INFO_NAME | VMG_OP_INFO_OBJECT ],
)
- This function creates a 'wizard', an opaque type that holds the magic
+ This function creates a 'wizard', an opaque object that holds the magic
information. It takes a list of keys / values as argument, whose keys
can be :
* "data"
A code (or string) reference to a private data constructor. It is
- called each time this magic is cast on a variable, and the scalar
- returned is used as private data storage for it. $_[0] is a
- reference to the magic object and @_[1 .. @_-1] are all extra
- arguments that were passed to "cast".
+ called in scalar context each time the magic is cast onto a
+ variable, with $_[0] being a reference to this variable and @_[1 ..
+ @_-1] being all extra arguments that were passed to "cast". The
+ scalar returned from this call is then attached to the variable and
+ can be retrieved later with "getdata".
* "get", "set", "len", "clear", "free", "copy", "local", "fetch",
"store", "exists" and "delete"
- Code (or string) references to the corresponding magic callbacks.
- You don't have to specify all of them : the magic associated with
- undefined entries simply won't be hooked. In those callbacks, $_[0]
- is always a reference to the magic object and $_[1] is always the
- private data (or "undef" when no private data constructor was
- supplied).
+ Code (or string) references to the respective magic callbacks. You
+ don't have to specify all of them : the magic corresponding to
+ undefined entries will simply not be hooked.
- Moreover, when you pass "op_info => $num" to "wizard", the last
- element of @_ will be the current op name if "$num ==
- VMG_OP_INFO_NAME" and a "B::OP" object representing the current op
- if "$num == VMG_OP_INFO_OBJECT". Both have a performance hit, but
- just getting the name is lighter than getting the op object.
+ When those callbacks are executed, $_[0] is a reference to the magic
+ variable and $_[1] is the associated private data (or "undef" when
+ no private data constructor is supplied with the wizard). Other
+ arguments depend on which kind of magic is involved :
- Other arguments are specific to the magic hooked :
+ * *len*
- * "len"
+ $_[2] contains the natural, non-magical length of the
+ variable (which can only be a scalar or an array as *len*
+ magic is only relevant for these types). The callback is
+ expected to return the new scalar or array length to use, or
+ "undef" to default to the normal length.
- When the variable is an array or a scalar, $_[2] contains
- the non-magical length. The callback can return the new
- scalar or array length to use, or "undef" to default to the
- normal length.
+ * *copy*
- * "copy"
+ $_[2] is a either an alias or a copy of the current key, and
+ $_[3] is an alias to the current element (i.e. the value).
+ Because $_[2] might be a copy, it is useless to try to
+ change it or cast magic on it.
- $_[2] is a either a copy or an alias of the current key,
- which means that it is useless to try to change or cast
- magic on it. $_[3] is an alias to the current element (i.e.
- the value).
+ * *fetch*, *store*, *exists* and *delete*
- * "fetch", "store", "exists" and "delete"
+ $_[2] is an alias to the current key. Note that $_[2] may
+ rightfully be readonly if the key comes from a bareword, and
+ as such it is unsafe to assign to it. You can ask for a copy
+ instead by passing "copy_key => 1" to "wizard" which, at the
+ price of a small performance hit, allows you to safely
+ assign to $_[2] in order to e.g. redirect the action to
+ another key.
- $_[2] is an alias to the current key. Nothing prevents you
- from changing it, but be aware that there lurk dangerous
- side effects. For example, it may rightfully be readonly if
- the key was a bareword. You can get a copy instead by
- passing "copy_key => 1" to "wizard", which allows you to
- safely assign to $_[2] in order to e.g. redirect the action
- to another key. This however has a little performance
- drawback because of the copy.
+ Finally, if "op_info => $num" is also passed to "wizard", then one
+ extra element is appended to @_. Its nature depends on the value of
+ $num :
- All the callbacks are expected to return an integer, which is passed
- straight to the perl magic API. However, only the return value of
- the "len" callback currently holds a meaning.
+ * "VMG_OP_INFO_NAME"
+
+ $_[-1] is the current op name.
+
+ * "VMG_OP_INFO_OBJECT"
+
+ $_[-1] is the "B::OP" object for the current op.
+
+ Both result in a small performance hit, but just getting the name is
+ lighter than getting the op object.
+
+ These callbacks are executed in scalar context and are expected to
+ return an integer, which is then passed straight to the perl magic
+ API. However, only the return value of the *len* magic callback
+ currently holds a meaning.
Each callback can be specified as :
* a reference to "undef", in which case a no-op magic callback is
installed instead of the default one. This may especially be helpful
- for 'local' magic, where an empty callback prevents magic from being
+ for *local* magic, where an empty callback prevents magic from being
copied during localization.
- Note that "free" callbacks are *never* called during global destruction,
- as there's no way to ensure that the wizard and the "free" callback
- weren't destroyed before the variable.
+ Note that *free* magic is never called during global destruction, as
+ there is no way to ensure that the wizard object and the callback were
+ not destroyed before the variable.
- Here's a simple usage example :
+ Here is a simple usage example :
# A simple scalar tracer
my $wiz = wizard(
);
"cast"
- cast [$@%&*]var, $wiz, ...
+ cast [$@%&*]var, $wiz, @args
- This function associates $wiz magic to the variable supplied, without
+ This function associates $wiz magic to the supplied variable, without
overwriting any other kind of magic. It returns true on success or when
- $wiz magic is already present, and croaks on error. All extra arguments
- specified after $wiz are passed to the private data constructor in @_[1
- .. @_-1]. If the variable isn't a hash, any "uvar" callback of the
- wizard is safely ignored.
+ $wiz magic is already attached, and croaks on error. When $wiz provides
+ a data constructor, it is called just before magic is cast onto the
+ variable, and it receives a reference to the target variable in $_[0]
+ and the content of @args in @_[1 .. @args]. Otherwise, @args is ignored.
- # Casts $wiz onto $x, and pass '1' to the data constructor.
+ # Casts $wiz onto $x, passing (\$x, '1') to the data constructor.
my $x;
cast $x, $wiz, 1;
- The "var" argument can be an array or hash value. Magic for those
- behaves like for any other scalar, except that it is dispelled when the
+ The "var" argument can be an array or hash value. Magic for these
+ scalars behaves like for any other, except that it is dispelled when the
entry is deleted from the container. For example, if you want to call
"POSIX::tzset" each time the 'TZ' environment variable is changed in
%ENV, you can use :
use POSIX;
cast $ENV{TZ}, wizard set => sub { POSIX::tzset(); () };
- If you want to overcome the possible deletion of the 'TZ' entry, you
- have no choice but to rely on "store" uvar magic.
+ If you want to handle the possible deletion of the 'TZ' entry, you must
+ also specify *store* magic.
"getdata"
getdata [$@%&*]var, $wiz
This accessor fetches the private data associated with the magic $wiz in
- the variable. It croaks when $wiz do not represent a valid magic object,
- and returns an empty list if no such magic is attached to the variable
- or when the wizard has no data constructor.
+ the variable. It croaks when $wiz does not represent a valid magic
+ object, and returns an empty list if no such magic is attached to the
+ variable or when the wizard has no data constructor.
- # Get the attached data, or undef if the wizard does not attach any.
+ # Get the data attached to $wiz in $x, or undef if $wiz
+ # did not attach any.
my $data = getdata $x, $wiz;
"dispell"
CONSTANTS
"MGf_COPY"
- Evaluates to true iff the 'copy' magic is available.
+ Evaluates to true if and only if the *copy* magic is available. This is
+ the case for perl 5.7.3 and greater, which is ensured by the
+ requirements of this module.
"MGf_DUP"
- Evaluates to true iff the 'dup' magic is available.
+ Evaluates to true if and only if the *dup* magic is available. This is
+ the case for perl 5.7.3 and greater, which is ensured by the
+ requirements of this module.
"MGf_LOCAL"
- Evaluates to true iff the 'local' magic is available.
+ Evaluates to true if and only if the *local* magic is available. This is
+ the case for perl 5.9.3 and greater.
"VMG_UVAR"
- When this constant is true, you can use the "fetch,store,exists,delete"
- callbacks on hashes. Initial VMG_UVAR capability was introduced in perl
- 5.9.5, with a fully functional implementation shipped with perl 5.10.0.
+ When this constant is true, you can use the *fetch*, *store*, *exists*
+ and *delete* magics on hashes. Initial "VMG_UVAR" capability was
+ introduced in perl 5.9.5, with a fully functional implementation shipped
+ with perl 5.10.0.
"VMG_COMPAT_SCALAR_LENGTH_NOLEN"
- True for perls that don't call 'len' magic when taking the "length" of a
+ True for perls that don't call *len* magic when taking the "length" of a
magical scalar.
"VMG_COMPAT_ARRAY_PUSH_NOLEN"
- True for perls that don't call 'len' magic when you push an element in a
+ True for perls that don't call *len* magic when you push an element in a
magical array. Starting from perl 5.11.0, this only refers to pushes in
non-void context and hence is false.
"VMG_COMPAT_ARRAY_PUSH_NOLEN_VOID"
- True for perls that don't call 'len' magic when you push in void context
+ True for perls that don't call *len* magic when you push in void context
an element in a magical array.
"VMG_COMPAT_ARRAY_UNSHIFT_NOLEN_VOID"
- True for perls that don't call 'len' magic when you unshift in void
+ True for perls that don't call *len* magic when you unshift in void
context an element in a magical array.
"VMG_COMPAT_ARRAY_UNDEF_CLEAR"
- True for perls that call 'clear' magic when undefining magical arrays.
+ True for perls that call *clear* magic when undefining magical arrays.
"VMG_COMPAT_HASH_DELETE_NOUVAR_VOID"
- True for perls that don't call 'delete' uvar magic when you delete an
- element from a hash in void context.
+ True for perls that don't call *delete* magic when you delete an element
+ from a hash in void context.
"VMG_COMPAT_GLOB_GET"
- True for perls that call 'get' magic for operations on globs.
+ True for perls that call *get* magic for operations on globs.
"VMG_PERL_PATCHLEVEL"
The perl patchlevel this module was built with, or 0 for non-debugging
perls.
"VMG_THREADSAFE"
- True iff this module could have been built with thread-safety features
- enabled.
+ True if and only if this module could have been built with thread-safety
+ features enabled.
"VMG_FORKSAFE"
- True iff this module could have been built with fork-safety features
- enabled. This will always be true except on Windows where it's false for
- perl 5.10.0 and below .
+ True if and only if this module could have been built with fork-safety
+ features enabled. This is always true except on Windows where it is
+ false for perl 5.10.0 and below.
"VMG_OP_INFO_NAME"
Value to pass with "op_info" to get the current op name in the magic
cast %h, $wiz;
}
- When %h goes out of scope, this will print something among the lines of
- :
+ When %h goes out of scope, this prints something among the lines of :
free HASH at depth 0
free HASH at depth 1
PERL MAGIC HISTORY
The places where magic is invoked have changed a bit through perl
- history. Here's a little list of the most recent ones.
+ history. Here is a little list of the most recent ones.
* 5.6.x
- *p14416* : 'copy' and 'dup' magic.
+ *p14416* : *copy* and *dup* magic.
* 5.8.9
* 5.9.3
- *p25854* : 'len' magic is no longer called when pushing an element
+ *p25854* : *len* magic is no longer called when pushing an element
into a magic array.
- *p26569* : 'local' magic.
+ *p26569* : *local* magic.
* 5.9.5
- *p31064* : Meaningful 'uvar' magic.
+ *p31064* : Meaningful *uvar* magic.
- *p31473* : 'clear' magic wasn't invoked when undefining an array.
+ *p31473* : *clear* magic was not invoked when undefining an array.
The bug is fixed as of this version.
* 5.10.0
Since "PERL_MAGIC_uvar" is uppercased, "hv_magic_check()" triggers
- 'copy' magic on hash stores for (non-tied) hashes that also have
- 'uvar' magic.
+ *copy* magic on hash stores for (non-tied) hashes that also have
+ *uvar* magic.
* 5.11.x
- *p32969* : 'len' magic is no longer invoked when calling "length"
+ *p32969* : *len* magic is no longer invoked when calling "length"
with a magical scalar.
- *p34908* : 'len' magic is no longer called when pushing / unshifting
+ *p34908* : *len* magic is no longer called when pushing / unshifting
an element into a magical array in void context. The "push" part was
already covered by *p25854*.
- *g9cdcb38b* : 'len' magic is called again when pushing into a
+ *g9cdcb38b* : *len* magic is called again when pushing into a
magical array in non-void context.
EXPORT
or by the tags ':consts' and ':all'.
CAVEATS
- If you store a magic object in the private data slot, the magic won't be
- accessible by "getdata" since it's not copied by assignment. The only
- way to address this would be to return a reference.
+ In order to hook hash operations with magic, you need at least perl
+ 5.10.0 (see "VMG_UVAR").
- If you define a wizard with a "free" callback and cast it on itself,
- this destructor won't be called because the wizard will be destroyed
- first.
+ If you want to store a magic object in the private data slot, you will
+ not be able to recover the magic with "getdata", since magic is not
+ copied by assignment. You can work around this gotcha by storing a
+ reference to the magic object instead.
- In order to define magic on hash members, you need at least perl 5.10.0
- (see "VMG_UVAR")
+ If you define a wizard with *free* magic and cast it on itself, it
+ results in a memory cycle, so this destructor will not be called when
+ the wizard is freed.
DEPENDENCIES
perl 5.8.
A C compiler. This module may happen to build with a C++ compiler as
well, but don't rely on it, as no guarantee is made in this regard.
- Carp (standard since perl 5), XSLoader (standard since perl 5.006).
-
- Copy tests need Tie::Array (standard since perl 5.005) and Tie::Hash
- (since 5.002).
-
- Some uvar tests need Hash::Util::FieldHash (standard since perl
- 5.009004).
-
- Glob tests need Symbol (standard since perl 5.002).
+ Carp (core since perl 5), XSLoader (since 5.006).
- Threads tests need threads and threads::shared.
+ Copy tests need Tie::Array (core since perl 5.005) and Tie::Hash (since
+ 5.002). Some uvar tests need Hash::Util::FieldHash (since 5.009004).
+ Glob tests need Symbol (since 5.002). Threads tests need threads and
+ threads::shared (both since 5.007003).
SEE ALSO
perlguts and perlapi for internal information about magic.
projects / perl / modules / Variable-Magic.git / commitdiff