Manpages - Scalar_Util.3perl

my $pkg = blessed( $ref );

If $ref is a blessed reference, the name of the package that it is blessed into is returned. Otherwise undef is returned.

$scalar = “foo”; $class = blessed $scalar; # undef $ref = []; $class = blessed $ref; # undef $obj = bless [], “Foo”; $class = blessed $obj; # “Foo”

Take care when using this function simply as a truth test (such as in if(blessed $ref)...) because the package name "0" is defined yet false.

my $addr = refaddr( $ref );

If $ref is reference, the internal memory address of the referenced value is returned as a plain integer. Otherwise undef is returned.

$addr = refaddr “string”; # undef $addr = refaddr \$var; # eg 12345678 $addr = refaddr []; # eg 23456784 $obj = bless {}, “Foo”; $addr = refaddr $obj; # eg 88123488

my $type = reftype( $ref );

If $ref is a reference, the basic Perl type of the variable referenced is returned as a plain string (such as ARRAY or HASH). Otherwise undef is returned.

$type = reftype “string”; # undef $type = reftype \$var; # SCALAR $type = reftype []; # ARRAY $obj = bless {}, “Foo”; $type = reftype $obj; # HASH

Note that for internal reasons, all precompiled regexps (qr/.../) are blessed references; thus ref() returns the package name string "Regexp" on these but reftype() will return the underlying C structure type of "REGEXP" in all capitals.

weaken( $ref );

The lvalue $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, the reference will be set to undef. This function mutates the lvalue passed as its argument and returns no value.

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 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.

unweaken( $ref );

Since version 1.36.

The lvalue REF will be turned from a weak reference back into a normal (strong) reference again. This function mutates the lvalue passed as its argument and returns no value. This undoes the action performed by weaken.

This function is slightly neater and more convenient than the otherwise-equivalent code

my $tmp = $REF; undef $REF; $REF = $tmp;

(because in particular, simply assigning a weak reference back to itself does not work to unweaken it; $REF = $REF does not work).

my $weak = isweak( $ref );

Returns true if $ref is a weak reference.

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

NOTE: Copying a weak reference creates a normal, strong, reference.

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

my $var = 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

my $dual = isdual( $var );

Since version 1.26.

If $var is a scalar that has both numeric and string values, 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 $! appears to be a dual-valued variable, it is actually implemented as a magical variable inside the interpreter:

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

You can capture its numeric and string content using:

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

my $vstring = isvstring( $var );

If $var 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);

my $isnum = looks_like_number( $var );

Returns true if perl thinks $var is a number. See looks_like_number in perlapi.

my $fh = openhandle( $fh );

Returns $fh itself, if $fh may be used as a filehandle and is open, or if it is a tied handle. Otherwise undef is returned.

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

my $ro = readonly( $var );

Returns true if $var is readonly.

sub foo { readonly($_[0]) } $readonly = foo($bar); # false $readonly = foo(0); # true

my $code = set_prototype( $code, $prototype );

Sets the prototype of the function given by the $code reference, or deletes it if $prototype is undef. Returns the $code reference itself.

set_prototype \&foo, $$;

my $t = tainted( $var );

Return true if $var is tainted.

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