tools/perl-5.8.0/man/man3/UNIVERSAL.3

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.IX Title "UNIVERSAL 3"
.TH UNIVERSAL 3 "2002-06-01" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
UNIVERSAL \- base class for ALL classes (blessed references)
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    $is_io = $fd->isa("IO::Handle");
\&    $is_io = Class->isa("IO::Handle");
.Ve
.PP
.Vb 2
\&    $sub = $obj->can("print");
\&    $sub = Class->can("print");
.Ve
.PP
.Vb 4
\&    use UNIVERSAL qw( isa can VERSION );
\&    $yes = isa $ref, "HASH" ;
\&    $sub = can $ref, "fandango" ;
\&    $ver = VERSION $obj ;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`UNIVERSAL\*(C'\fR is the base class which all bless references will inherit from,
see perlobj.
.PP
\&\f(CW\*(C`UNIVERSAL\*(C'\fR provides the following methods and functions:
.IP "$obj\->isa( \s-1TYPE\s0 ), \s-1CLASS\-\s0>isa( \s-1TYPE\s0 ), isa( \s-1VAL\s0, \s-1TYPE\s0 )" 4
.IX Item "$obj->isa( TYPE ), CLASS->isa( TYPE ), isa( VAL, TYPE )"
.Vb 4
\&    C<TYPE> is a package name
\&    $obj is a blessed reference or a string containing a package name
\&    C<CLASS> is a package name
\&    C<VAL> is any of the above or an unblessed reference
.Ve
.Sp
When used as an instance or class method (\f(CW\*(C`$obj\-\*(C'\fRisa( \s-1TYPE\s0 )>), \f(CW\*(C`isa\*(C'\fR
returns \fItrue\fR if \f(CW$obj\fR is blessed into package \f(CW\*(C`TYPE\*(C'\fR or inherits from
package \f(CW\*(C`TYPE\*(C'\fR.
.Sp
When used as a class method (\f(CW\*(C`CLASS\-\*(C'\fRisa( \s-1TYPE\s0 )>; sometimes referred to as a
static method), \f(CW\*(C`isa\*(C'\fR returns \fItrue\fR if \f(CW\*(C`CLASS\*(C'\fR inherits from (or is itself)
the name of the package \f(CW\*(C`TYPE\*(C'\fR or inherits from package \f(CW\*(C`TYPE\*(C'\fR.
.Sp
When used as a function, like
.Sp
.Vb 3
\&   use UNIVERSAL qw( isa ) ;
\&   $yes = isa $h, "HASH";
\&   $yes = isa "Foo", "Bar";
.Ve
.Sp
or
.Sp
.Vb 2
\&   require UNIVERSAL ;
\&   $yes = UNIVERSAL::isa $a, "ARRAY";
.Ve
.Sp
, \f(CW\*(C`isa\*(C'\fR returns \fItrue\fR in the same cases as above and also if \f(CW\*(C`VAL\*(C'\fR is an
unblessed reference to a perl variable of type \f(CW\*(C`TYPE\*(C'\fR, such as \*(L"\s-1HASH\s0\*(R",
\&\*(L"\s-1ARRAY\s0\*(R", or \*(L"Regexp\*(R".
.IP "$obj\->can( \s-1METHOD\s0 ), \s-1CLASS\-\s0>can( \s-1METHOD\s0 ), can( \s-1VAL\s0, \s-1METHOD\s0 )" 4
.IX Item "$obj->can( METHOD ), CLASS->can( METHOD ), can( VAL, METHOD )"
\&\f(CW\*(C`can\*(C'\fR checks if the object or class has a method called \f(CW\*(C`METHOD\*(C'\fR. If it does
then a reference to the sub is returned. If it does not then \fIundef\fR is
returned.  This includes methods inherited or imported by \f(CW$obj\fR, \f(CW\*(C`CLASS\*(C'\fR, or
\&\f(CW\*(C`VAL\*(C'\fR.
.Sp
\&\f(CW\*(C`can\*(C'\fR cannot know whether an object will be able to provide a method
through \s-1AUTOLOAD\s0, so a return value of \fIundef\fR does not necessarily mean
the object will not be able to handle the method call. To get around
this some module authors use a forward declaration (see perlsub)
for methods they will handle via \s-1AUTOLOAD\s0. For such 'dummy' subs, \f(CW\*(C`can\*(C'\fR
will still return a code reference, which, when called, will fall through
to the \s-1AUTOLOAD\s0. If no suitable \s-1AUTOLOAD\s0 is provided, calling the coderef
will cause an error.
.Sp
\&\f(CW\*(C`can\*(C'\fR can be called as a class (static) method, an object method, or a
function.
.Sp
When used as a function, if \f(CW\*(C`VAL\*(C'\fR is a blessed reference or package name which
has a method called \f(CW\*(C`METHOD\*(C'\fR, \f(CW\*(C`can\*(C'\fR returns a reference to the subroutine.
If \f(CW\*(C`VAL\*(C'\fR is not a blessed reference, or if it does not have a method
\&\f(CW\*(C`METHOD\*(C'\fR, \fIundef\fR is returned.
.IP "\s-1VERSION\s0 ( [ \s-1REQUIRE\s0 ] )" 4
.IX Item "VERSION ( [ REQUIRE ] )"
\&\f(CW\*(C`VERSION\*(C'\fR will return the value of the variable \f(CW$VERSION\fR in the
package the object is blessed into. If \f(CW\*(C`REQUIRE\*(C'\fR is given then
it will do a comparison and die if the package version is not
greater than or equal to \f(CW\*(C`REQUIRE\*(C'\fR.
.Sp
\&\f(CW\*(C`VERSION\*(C'\fR can be called as either a class (static) method, an object method or
or a function.
.PP
These subroutines should \fInot\fR be imported via \f(CW\*(C`use\ UNIVERSAL\ qw(...)\*(C'\fR.
If you want simple local access to them you can do
.PP
.Vb 1
\&  *isa = \e&UNIVERSAL::isa;
.Ve
.PP
to import isa into your package.