perl-doc 5.18.2-2ubuntu1.7 / usr / share / perl / 5.18.2 / Scalar / Util.pod

=head1 NAME

Scalar::Util - A selection of general-utility scalar subroutines

=head1 SYNOPSIS

    use Scalar::Util qw(blessed dualvar isdual readonly refaddr reftype
                        tainted weaken isweak isvstring looks_like_number
                        set_prototype);
                        # and other useful utils appearing below

=head1 DESCRIPTION

C<Scalar::Util> contains a selection of subroutines that people have
expressed would be nice to have in the perl core, but the usage would
not really be high enough to warrant the use of a keyword, and the size
so small such that being individual extensions would be wasteful.

By default C<Scalar::Util> does not export any subroutines. The
subroutines defined are

=over 4

=item blessed EXPR

If EXPR evaluates to a blessed reference the name of the package
that it is blessed into is returned. Otherwise C<undef> is returned.

   $scalar = "foo";
   $class  = blessed $scalar;           # undef

   $ref    = [];
   $class  = blessed $ref;              # undef

   $obj    = bless [], "Foo";
   $class  = blessed $obj;              # "Foo"

=item dualvar NUM, STRING

Returns a scalar that has the value NUM in a numeric context and the
value STRING in a string context.

    $foo = dualvar 10, "Hello";
    $num = $foo + 2;                    # 12
    $str = $foo . " world";             # Hello world

=item isdual EXPR

If EXPR is a scalar that is a dualvar, the result is true.

    $foo = dualvar 86, "Nix";
    $dual = isdual($foo);               # true

Note that a scalar can be made to have both string and numeric content
through numeric operations:

    $foo = "10";
    $dual = isdual($foo);               # false
    $bar = $foo + 0;
    $dual = isdual($foo);               # true

Note that although C<$!> appears to be dual-valued variable, it is
actually implemented using a tied scalar:

    $! = 1;
    print("$!\n");                      # "Operation not permitted"
    $dual = isdual($!);                 # false

You can capture its numeric and string content using:

    $err = dualvar $!, $!;
    $dual = isdual($err);               # true

=item isvstring EXPR

If EXPR is a scalar which was coded as a vstring the result is true.

    $vs   = v49.46.48;
    $fmt  = isvstring($vs) ? "%vd" : "%s"; #true
    printf($fmt,$vs);

=item looks_like_number EXPR

Returns true if perl thinks EXPR is a number. See
L<perlapi/looks_like_number>.

=item openhandle FH

Returns FH if FH may be used as a filehandle and is open, or FH is a tied
handle. Otherwise C<undef> is returned.

    $fh = openhandle(*STDIN);           # \*STDIN
    $fh = openhandle(\*STDIN);          # \*STDIN
    $fh = openhandle(*NOTOPEN);         # undef
    $fh = openhandle("scalar");         # undef

=item readonly SCALAR

Returns true if SCALAR is readonly.

    sub foo { readonly($_[0]) }

    $readonly = foo($bar);              # false
    $readonly = foo(0);                 # true

=item refaddr EXPR

If EXPR evaluates to a reference the internal memory address of
the referenced value is returned. Otherwise C<undef> is returned.

    $addr = refaddr "string";           # undef
    $addr = refaddr \$var;              # eg 12345678
    $addr = refaddr [];                 # eg 23456784

    $obj  = bless {}, "Foo";
    $addr = refaddr $obj;               # eg 88123488

=item reftype EXPR

If EXPR evaluates to a reference the type of the variable referenced
is returned. Otherwise C<undef> is returned.

    $type = reftype "string";           # undef
    $type = reftype \$var;              # SCALAR
    $type = reftype [];                 # ARRAY

    $obj  = bless {}, "Foo";
    $type = reftype $obj;               # HASH

=item set_prototype CODEREF, PROTOTYPE

Sets the prototype of the given function, or deletes it if PROTOTYPE is
undef. Returns the CODEREF.

    set_prototype \&foo, '$$';

=item tainted EXPR

Return true if the result of EXPR is tainted

    $taint = tainted("constant");       # false
    $taint = tainted($ENV{PWD});        # true if running under -T

=item weaken REF

REF will be turned into a weak reference. This means that it will not
hold a reference count on the object it references. Also when the reference
count on that object reaches zero, REF will be set to undef.

This is useful for keeping copies of references , but you don't want to
prevent the object being DESTROY-ed at its usual time.

    {
      my $var;
      $ref = \$var;
      weaken($ref);                     # Make $ref a weak reference
    }
    # $ref is now undef

Note that if you take a copy of a scalar with a weakened reference,
the copy will be a strong reference.

    my $var;
    my $foo = \$var;
    weaken($foo);                       # Make $foo a weak reference
    my $bar = $foo;                     # $bar is now a strong reference

This may be less obvious in other situations, such as C<grep()>, for instance
when grepping through a list of weakened references to objects that may have
been destroyed already:

    @object = grep { defined } @object;

This will indeed remove all references to destroyed objects, but the remaining
references to objects will be strong, causing the remaining objects to never
be destroyed because there is now always a strong reference to them in the
@object array.

=item isweak EXPR

If EXPR is a scalar which is a weak reference the result is true.

    $ref  = \$foo;
    $weak = isweak($ref);               # false
    weaken($ref);
    $weak = isweak($ref);               # true

B<NOTE>: Copying a weak reference creates a normal, strong, reference.

    $copy = $ref;
    $weak = isweak($copy);              # false

=back

=head1 DIAGNOSTICS

Module use may give one of the following errors during import.

=over

=item Weak references are not implemented in the version of perl

The version of perl that you are using does not implement weak references, to use
C<isweak> or C<weaken> you will need to use a newer release of perl.

=item Vstrings are not implemented in the version of perl

The version of perl that you are using does not implement Vstrings, to use
C<isvstring> you will need to use a newer release of perl.

=item C<NAME> is only available with the XS version of Scalar::Util

C<Scalar::Util> contains both perl and C implementations of many of its functions
so that those without access to a C compiler may still use it. However some of the functions
are only available when a C compiler was available to compile the XS version of the extension.

At present that list is: weaken, isweak, dualvar, isvstring, set_prototype

=back

=head1 KNOWN BUGS

There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will
show up as tests 8 and 9 of dualvar.t failing

=head1 SEE ALSO

L<List::Util>

=head1 COPYRIGHT

Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.

Except weaken and isweak which are

Copyright (c) 1999 Tuomas J. Lukka <lukka@iki.fi>. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as perl itself.

=cut