| Data::Alias(3pm) | User Contributed Perl Documentation | Data::Alias(3pm) |
Data::Alias - Comprehensive set of aliasing operations
use Data::Alias;
alias {
# aliasing instead of copying whenever possible
};
alias $x = $y; # alias $x to $y
alias @x = @y; # alias @x to @y
alias $x[0] = $y; # similar for array and hash elements
alias push @x, $y; # push alias to $y onto @x
$x = alias [ $y, $z ]; # construct array of aliases
alias my ($x, $y) = @_; # named aliases to arguments
alias { ($x, $y) = ($y, $x) }; # swap $x and $y
alias { my @t = @x; @x = @y; @y = @t }; # swap @x and @y
use Data::Alias qw/ alias copy /;
alias { copy $x = $y }; # force copying inside alias-BLOCK
use Data::Alias qw/ deref /;
my @refs = (\$x, \@y, \%z);
foo(deref @refs) # same as foo($x, @y, %z)
Aliasing is the phenomenon where two different expressions actually refer to the same thing. Modifying one will modify the other, and if you take a reference to both, the two values are the same.
Aliasing occurs in Perl for example in for-loops and sub-calls:
for $var ($x) {
# here $var is an alias to $x
}
foo($y);
sub foo {
# here $_[0] is an alias to $y
}
Data::Alias is a module that allows you to apply "aliasing semantics" to a section of code, causing aliases to be made wherever Perl would normally make copies instead. You can use this to improve efficiency and readability, when compared to using references.
The exact details of aliasing semantics are below under "DETAILS".
Perl 5.22 added some support for aliasing to the Perl core. It has a different syntax, and a different set of operations, from that supplied by this module; see "Assigning to References" in perlref. The core's aliasing facilities are implemented more robustly than this module and are better supported. If you can rely on having a sufficiently recent Perl version, you should prefer to use the core facility rather than use this module. If you are already using this module and are now using a sufficiently recent Perl, you should attempt to migrate to the core facility.
Exported by default.
Enables aliasing semantics within the expression or block. Returns an alias to the expression, or the block's return value.
"alias" is context-transparent, meaning that whichever context it is placed in (list, scalar, void), the expression/block is evaluated in the same context.
Restores normal (copying) semantics within the expression or block, and makes a copy of the result value (unless in void context).
Like "alias", "copy" is context-transparent.
Accepts a list of references to scalars, arrays, or hashes. Applies the applicable dereferencing operator to each. This means that:
deref $scalarref, $arrayref, $hashref
behaves like:
$$scalarref, @$arrayref, %$hashref
Where an array or hash reference is given, the returned list does not include the array or hash as an lvalue; the array/hash is expanded and the list includes its elements. Scalars, including the elements of an array/hash, are treated as lvalues, and can be enreferenced using the "\" operator or aliased to using the "alias" operator. This is slightly different from what you'd get using the built-in dereference operators: @$arrayref references the array as an lvalue, so "\" or "alias" can operate on the array itself rather than just its elements.
A common usage of aliasing is to make an abbreviation for an expression, to avoid having to repeat that (possibly verbose or ugly) expression over and over:
alias my $fi = $self->{FrobnitzIndex};
$fi = $fi > 0 ? $fi - $adj : $fi + $adj;
sub rc4 {
alias my ($i, $j, $S) = @_;
my $a = $S->[($i += 1) &= 255];
my $b = $S->[($j += $S->[$i]) &= 255];
$S->[(($S->[$j] = $a) + ($S->[$i] = $b)) & 255]
}
In the second example, the rc4 function updates its first two arguments (two state values) in addition to returning a value.
Aliasing can also be used to avoid copying big strings. This example would work fine without "alias" but would be much slower when passed a big string:
sub middlesection ($) {
alias my $s = shift;
substr $s, length($s)/4, length($s)/2
}
You can also apply aliasing semantics to an entire block. Here this is used to swap two arrays in O(1) time:
alias {
my @temp = @x;
@x = @y;
@y = @temp;
};
The "copy" function is typically used to temporarily reinstate normal semantics, but can also be used to explicitly copy a value when perl would normally not do so:
my $ref = \copy $x;
This section describes exactly what the aliasing semantics are of operations. Anything not listed below has unaltered behaviour.
alias my $lexvar = $foo;
alias $pkgvar = $foo;
alias $array[$i] = $foo;
alias $hash{$k} = $foo;
An attempt to do alias-assignment to an element of a tied (or "magical") array or hash will result in a "Can't put alias into tied array/hash" error.
alias $$ref = $foo;
alias *glob = $reference;
alias substr(...) = $foo; # ERROR!
alias lvalsub() = $foo; # ERROR!
alias $bool ? $var : $var2 = $foo;
alias $var &&= $foo;
alias $var ||= $foo;
alias $var //= $foo; # (perl 5.9.x or later)
alias my @lexarray = @foo;
alias my %lexhash = %foo;
alias @pkgarray = @foo;
alias %pkghash = %foo;
Making the left-side a dereference is also supported:
alias @$ref = @foo;
alias %$ref = %foo;
and analogously to assignment to scalar dereference, these will change $ref to reference the aggregate, if $ref was undef or already a reference. If $ref is a string or glob, the corresponding package variable is aliased.
Anything more complex than a whole-aggregate expression on either side, even just enclosing the aggregate expression in parentheses, will prevent the assignment qualifying for this category. It will instead go into one of the following two categories. Parenthesisation is the recommended way to avoid whole-aggregate aliasing where it is unwanted. If you want to merely replace the contents of the left-side aggregate with aliases to the contents of the right-side aggregate, parenthesise the left side.
alias my @lexfoo = (@foo);
alias my @array = ($x, $y, $z);
alias my %hash = (x => $x, y => $y);
is translated to:
alias my @lexfoo = @{ [@foo] };
alias my @array = @{ [$x, $y, $z] };
alias my %hash = %{ {x => $x, y => $y} };
If you want to merely replace the contents of the aggregate with aliases to the contents of another aggregate, rather than create a new aggregate, you can force list-assignment by parenthesizing the left side, see below.
alias my (@array) = ($x, $y, $z);
alias my (%hash) = (x => $x, y => $y);
alias my ($x, $y, @rest) = @_;
alias @x[0, 1] = @x[1, 0];
Any scalars that appear on the left side must be valid targets for scalar assignment. When an array or hash appears on the left side, normally as the last item, its contents are replaced by the list of all remaining right-side elements. "undef" can also appear on the left side to skip one corresponding item in the right-side list.
Beware when putting a parenthesised list on the left side. Just like Perl parses "print (1+2)*10" as "(print(1+2))*10", it would parse "alias ($x, $y) = ($y, $x)" as "(alias($x, $y)) = ($y, $x)" which does not do any aliasing, and results in the "Useless use of alias" warning, if warnings are enabled.
To circumvent this issue, you can either one of the following:
alias +($x, $y) = ($y, $x);
alias { ($x, $y) = ($y, $x) };
my $arrayref = alias [$x, $y, $z];
my $hashref = alias {x => $x, y => $y};
Note that this also works:
alias my $arrayref = [$x, $y, $z];
alias my $hashref = {x => $x, y => $y};
but this makes the lhs an alias to the temporary, and therefore read-only, reference made by "[]" or "{}". Therefore later attempts to assign to $arrayref or $hashref results in an error. The anonymous aggregate that is referenced behaves the same in both cases obviously.
alias push @array, $foo;
alias unshift @array, $foo;
alias splice @array, 1, 2, $foo;
An attempt to do any of these on tied (or "magical") array will result in a "Can't push/unshift/splice alias onto tied array" error.
A sub call will very often copy the return value(s) immediately after they have been returned. "alias return" can't prevent that. To pass an alias through a sub return and into something else, the call site must process the return value using an aliasing operation, or at least a non-copying one. For example, ordinary assignment with the sub call on the right hand side will copy, but if the call site is in the scope of an "alias" pragma then the assignment will instead alias the return value.
When alias-returning a list of values from a subroutine, each individual value in the list is aliased. The list as a whole is not aliasable; it is not an array. At the call site, a list of aliases can be captured into separate variables or into an array, by an aliasing list assignment.
alias { sub foo { $x } };
my $subref = alias sub { $x };
my $xref1 = \foo;
my $xref2 = \alias eval '$x';
my $xref3 = \$subref->();
Explicitly returning an alias can also be done using "alias return" inside any subroutine or evaluation.
sub foo { alias return $x; }
my $xref = \foo;
alias local $_ = $string;
The aliasing semantics of "local" avoids copying by always localizing the variable itself, regardless of whether it is tied.
This module does not use a source filter, and is therefore safe to use within eval STRING. Instead, Data::Alias hooks into the Perl parser, and replaces operations within the scope of "alias" by aliasing variants.
For those familiar with perl's internals: it triggers on a ck_rv2cv which resolves to the imported "alias" sub, and does a parser hack to allow the "alias BLOCK" syntax. When the ck_entersub is triggered that corresponds to it, the op is marked to be found later. The actual work is done in a peep-hook, which processes the marked entersub and its children, replacing the pp_addrs with aliasing replacements. The peep hook will also take care of any subs defined within the lexical (but not dynamical) scope between the ck_rv2cv and the ck_entersub.
When aliasing a lexical that was declared outside the current subroutine, a compile-time warning is generated "Aliasing of outer lexical variable has limited scope" (warnings category "closure").
Specials thanks go to Elizabeth Mattijsen, Juerd Waalboer, and other members of the Amsterdam Perl Mongers, for their valuable feedback.
Matthijs van Duin <xmath@cpan.org> developed the module originally, and maintained it until 2007. Andrew Main (Zefram) <zefram@fysh.org> updated it to work with Perl versions 5.11.0 and later.
Copyright (C) 2003-2007 Matthijs van Duin. Copyright (C) 2010, 2011, 2013, 2015, 2017 Andrew Main (Zefram) <zefram@fysh.org>. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
| 2023-01-06 | perl v5.36.0 |