autovivification - Lexically disable autovivification.
VERSION
- Version 0.01
+ Version 0.18
SYNOPSIS
no autovivification;
- my $x;
- $x->{foo} = 1; # croaks
+
+ my $hashref;
+
+ my $a = $hashref->{key_a}; # $hashref stays undef
+
+ if (exists $hashref->{option}) { # Still undef
+ ...
+ }
+
+ delete $hashref->{old}; # Still undef again
+
+ $hashref->{new} = $value; # Vivifies to { new => $value }
DESCRIPTION
+ When an undefined variable is dereferenced, it gets silently upgraded to
+ an array or hash reference (depending of the type of the dereferencing).
+ This behaviour is called *autovivification* and usually does what you
+ mean (e.g. when you store a value) but it may be unnatural or surprising
+ because your variables gets populated behind your back. This is
+ especially true when several levels of dereferencing are involved, in
+ which case all levels are vivified up to the last, or when it happens in
+ intuitively read-only constructs like "exists".
+
+ This pragma lets you disable autovivification for some constructs and
+ optionally throws a warning or an error when it would have happened.
+
METHODS
"unimport"
- Magically called when "no autovivification" is encountered. Turns the
- module on.
+ no autovivification; # defaults to qw<fetch exists delete>
+ no autovivification qw<fetch store exists delete>;
+ no autovivification warn => @categories;
+ no autovivification strict => @categories;
+
+ Magically called when "no autovivification @opts" is encountered.
+ Enables the features given in @opts, which can be :
+
+ * 'fetch'
+
+ Turns off autovivification for rvalue dereferencing expressions,
+ such as :
+
+ $value = $arrayref->[$idx]
+ $value = $hashref->{$key}
+ keys %$hashref
+ values %$hashref
+
+ Starting from perl 5.11, it also covers "keys" and "values" on array
+ references :
+
+ keys @$arrayref
+ values @$arrayref
+
+ When the expression would have autovivified, "undef" is returned for
+ a plain fetch, while "keys" and "values" return 0 in scalar context
+ and the empty list in list context.
+
+ * 'exists'
+
+ Turns off autovivification for dereferencing expressions that are
+ parts of an "exists", such as :
+
+ exists $arrayref->[$idx]
+ exists $hashref->{$key}
+
+ '' is returned when the expression would have autovivified.
+
+ * 'delete'
+
+ Turns off autovivification for dereferencing expressions that are
+ parts of a "delete", such as :
+
+ delete $arrayref->[$idx]
+ delete $hashref->{$key}
+
+ "undef" is returned when the expression would have autovivified.
+
+ * 'store'
+
+ Turns off autovivification for lvalue dereferencing expressions,
+ such as :
+
+ $arrayref->[$idx] = $value
+ $hashref->{$key} = $value
+ for ($arrayref->[$idx]) { ... }
+ for ($hashref->{$key}) { ... }
+ function($arrayref->[$idx])
+ function($hashref->{$key})
+
+ An exception is thrown if vivification is needed to store the value,
+ which means that effectively you can only assign to levels that are
+ already defined. In the example, this would require $arrayref (resp.
+ $hashref) to already be an array (resp. hash) reference.
+
+ * 'warn'
+
+ Emits a warning when an autovivification is avoided for the
+ categories specified in @opts.
+
+ Note that "no autovivification 'warn'" currently does nothing by
+ itself, in particular it does not make the default categories warn.
+ This behaviour may change in a future version of this pragma.
+
+ * 'strict'
+
+ Throws an exception when an autovivification is avoided for the
+ categories specified in @opts.
+
+ Note that "no autovivification 'strict'" currently does nothing by
+ itself, in particular it does not make the default categories die.
+ This behaviour may change in a future version of this pragma.
+
+ Each call to "unimport" adds the specified features to the ones already
+ in use in the current lexical scope.
+
+ When @opts is empty, it defaults to "qw<fetch exists delete>".
"import"
- Magically called when "use autovivification" is encountered. Turns the
- module off.
+ use autovivification; # default Perl behaviour
+ use autovivification qw<fetch store exists delete>;
+
+ Magically called when "use autovivification @opts" is encountered.
+ Disables the features given in @opts, which can be the same as for
+ "unimport".
+
+ Each call to "import" removes the specified features to the ones already
+ in use in the current lexical scope.
+
+ When @opts is empty, it defaults to restoring the original Perl
+ autovivification behaviour.
+
+CONSTANTS
+ "A_THREADSAFE"
+ True if and only if the module could have been built with thread-safety
+ features enabled. This constant only has a meaning when your perl is
+ threaded, otherwise it will always be false.
+
+ "A_FORKSAFE"
+ True if and only if this module could have been built with fork-safety
+ features enabled. This constant will always be true, except on Windows
+ where it is false for perl 5.10.0 and below.
+
+CAVEATS
+ Using this pragma will cause a slight global slowdown of any subsequent
+ compilation phase that happens anywere in your code - even outside of
+ the scope of use of "no autovivification" - which may become noticeable
+ if you rely heavily on numerous calls to "eval STRING".
+
+ The pragma doesn't apply when one dereferences the returned value of an
+ array or hash slice, as in "@array[$id]->{member}" or
+ @hash{$key}->{member}. This syntax is valid Perl, yet it is discouraged
+ as the slice is here useless since the dereferencing enforces scalar
+ context. If warnings are turned on, Perl will complain about one-element
+ slices.
+
+ Autovivifications that happen in code "eval"'d during the global
+ destruction phase of a spawned thread or pseudo-fork (the processes used
+ internally for the "fork" emulation on Windows) are not reported.
DEPENDENCIES
- perl 5.8.
+ perl 5.8.3.
+
+ 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.
+
+ XSLoader (standard since perl 5.6.0).
- XSLoader (standard since perl 5.006).
+SEE ALSO
+ perlref.
AUTHOR
Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
perldoc autovivification
- Tests code coverage report is available at
- <http://www.profvince.com/perl/cover/autovivification>.
+ACKNOWLEDGEMENTS
+ Matt S. Trout asked for it.
COPYRIGHT & LICENSE
- Copyright 2009 Vincent Pit, all rights reserved.
+ Copyright 2009,2010,2011,2012,2013,2014,2015,2017 Vincent Pit, all
+ rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
projects / perl / modules / autovivification.git / blobdiff