blessed

    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.

refaddr

    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

reftype

    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

    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

    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).

isweak

    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

dualvar

    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

isdual

    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

isvstring

    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);

looks_like_number

    my $isnum = looks_like_number( $var );

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

openhandle

    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

readonly

    my $ro = readonly( $var );

Returns true if $var is readonly.

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

set_prototype

    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, '$$';

tainted

    my $t = tainted( $var );

Return true if $var is tainted.

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