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.\" ========================================================================
.\"
.IX Title "UNIVERSAL 3pm"
.TH UNIVERSAL 3pm "2009-07-03" "perl v5.10.1" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.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");
\&
\&    $does_log = $obj\->DOES("Logger");
\&    $does_log = Class\->DOES("Logger");
\&
\&    $sub      = $obj\->can("print");
\&    $sub      = Class\->can("print");
\&
\&    $sub      = eval { $ref\->can("fandango") };
\&    $ver      = $obj\->VERSION;
\&
\&    # but never do this!
\&    $is_io    = UNIVERSAL::isa($fd, "IO::Handle");
\&    $sub      = UNIVERSAL::can($obj, "print");
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`UNIVERSAL\*(C'\fR is the base class from which all blessed references inherit.
See perlobj.
.PP
\&\f(CW\*(C`UNIVERSAL\*(C'\fR provides the following methods:
.ie n .IP """$obj\->isa( TYPE )""" 4
.el .IP "\f(CW$obj\->isa( TYPE )\fR" 4
.IX Item "$obj->isa( TYPE )"
.PD 0
.ie n .IP """CLASS\->isa( TYPE )""" 4
.el .IP "\f(CWCLASS\->isa( TYPE )\fR" 4
.IX Item "CLASS->isa( TYPE )"
.ie n .IP """eval { VAL\->isa( TYPE ) }""" 4
.el .IP "\f(CWeval { VAL\->isa( TYPE ) }\fR" 4
.IX Item "eval { VAL->isa( TYPE ) }"
.PD
Where
.RS 4
.ie n .IP """TYPE""" 4
.el .IP "\f(CWTYPE\fR" 4
.IX Item "TYPE"
is a package name
.ie n .IP "$obj" 4
.el .IP "\f(CW$obj\fR" 4
.IX Item "$obj"
is a blessed reference or a package name
.ie n .IP """CLASS""" 4
.el .IP "\f(CWCLASS\fR" 4
.IX Item "CLASS"
is a package name
.ie n .IP """VAL""" 4
.el .IP "\f(CWVAL\fR" 4
.IX Item "VAL"
is any of the above or an unblessed reference
.RE
.RS 4
.Sp
When used as an instance or class method (\f(CW\*(C`$obj\->isa( TYPE )\*(C'\fR),
\&\f(CW\*(C`isa\*(C'\fR returns \fItrue\fR if \f(CW$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\->isa( TYPE )\*(C'\fR, 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
If you're not sure what you have (the \f(CW\*(C`VAL\*(C'\fR case), wrap the method call in an
\&\f(CW\*(C`eval\*(C'\fR block to catch the exception if \f(CW\*(C`VAL\*(C'\fR is undefined.
.Sp
If you want to be sure that you're calling \f(CW\*(C`isa\*(C'\fR as a method, not a class,
check the invocant with \f(CW\*(C`blessed\*(C'\fR from Scalar::Util first:
.Sp
.Vb 1
\&  use Scalar::Util \*(Aqblessed\*(Aq;
\&
\&  if ( blessed( $obj ) && $obj\->isa("Some::Class") {
\&      ...
\&  }
.Ve
.RE
.ie n .IP """$obj\->DOES( ROLE )""" 4
.el .IP "\f(CW$obj\->DOES( ROLE )\fR" 4
.IX Item "$obj->DOES( ROLE )"
.PD 0
.ie n .IP """CLASS\->DOES( ROLE )""" 4
.el .IP "\f(CWCLASS\->DOES( ROLE )\fR" 4
.IX Item "CLASS->DOES( ROLE )"
.PD
\&\f(CW\*(C`DOES\*(C'\fR checks if the object or class performs the role \f(CW\*(C`ROLE\*(C'\fR.  A role is a
named group of specific behavior (often methods of particular names and
signatures), similar to a class, but not necessarily a complete class by
itself.  For example, logging or serialization may be roles.
.Sp
\&\f(CW\*(C`DOES\*(C'\fR and \f(CW\*(C`isa\*(C'\fR are similar, in that if either is true, you know that the
object or class on which you call the method can perform specific behavior.
However, \f(CW\*(C`DOES\*(C'\fR is different from \f(CW\*(C`isa\*(C'\fR in that it does not care \fIhow\fR the
invocant performs the operations, merely that it does.  (\f(CW\*(C`isa\*(C'\fR of course
mandates an inheritance relationship.  Other relationships include aggregation,
delegation, and mocking.)
.Sp
By default, classes in Perl only perform the \f(CW\*(C`UNIVERSAL\*(C'\fR role, as well as the
role of all classes in their inheritance.  In other words, by default \f(CW\*(C`DOES\*(C'\fR
responds identically to \f(CW\*(C`isa\*(C'\fR.
.Sp
There is a relationship between roles and classes, as each class implies the
existence of a role of the same name.  There is also a relationship between
inheritance and roles, in that a subclass that inherits from an ancestor class
implicitly performs any roles its parent performs.  Thus you can use \f(CW\*(C`DOES\*(C'\fR in
place of \f(CW\*(C`isa\*(C'\fR safely, as it will return true in all places where \f(CW\*(C`isa\*(C'\fR will
return true (provided that any overridden \f(CW\*(C`DOES\*(C'\fR \fIand\fR \f(CW\*(C`isa\*(C'\fR methods behave
appropriately).
.ie n .IP """$obj\->can( METHOD )""" 4
.el .IP "\f(CW$obj\->can( METHOD )\fR" 4
.IX Item "$obj->can( METHOD )"
.PD 0
.ie n .IP """CLASS\->can( METHOD )""" 4
.el .IP "\f(CWCLASS\->can( METHOD )\fR" 4
.IX Item "CLASS->can( METHOD )"
.ie n .IP """eval { VAL\->can( METHOD ) }""" 4
.el .IP "\f(CWeval { VAL\->can( METHOD ) }\fR" 4
.IX Item "eval { VAL->can( METHOD ) }"
.PD
\&\f(CW\*(C`can\*(C'\fR checks if the object or class has a method called \f(CW\*(C`METHOD\*(C'\fR. If it does,
then it returns a reference to the sub.  If it does not, then it returns
\&\fIundef\fR.  This includes methods inherited or imported by \f(CW$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 (unless the object's class has overriden \f(CW\*(C`can\*(C'\fR appropriately), 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
You may call \f(CW\*(C`can\*(C'\fR as a class (static) method or an object method.
.Sp
Again, the same rule about having a valid invocant applies \*(-- use an \f(CW\*(C`eval\*(C'\fR
block or \f(CW\*(C`blessed\*(C'\fR if you need to be extra paranoid.
.ie n .IP """VERSION ( [ REQUIRE ] )""" 4
.el .IP "\f(CWVERSION ( [ REQUIRE ] )\fR" 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 or an object
method.
.SH "WARNINGS"
.IX Header "WARNINGS"
\&\fB\s-1NOTE:\s0\fR \f(CW\*(C`can\*(C'\fR directly uses Perl's internal code for method lookup, and
\&\f(CW\*(C`isa\*(C'\fR uses a very similar method and cache-ing strategy. This may cause
strange effects if the Perl code dynamically changes \f(CW@ISA\fR in any package.
.PP
You may add other methods to the \s-1UNIVERSAL\s0 class via Perl or \s-1XS\s0 code.
You do not need to \f(CW\*(C`use UNIVERSAL\*(C'\fR to make these methods
available to your program (and you should not do so).
.SH "EXPORTS"
.IX Header "EXPORTS"
None by default.
.PP
You may request the import of three functions (\f(CW\*(C`isa\*(C'\fR, \f(CW\*(C`can\*(C'\fR, and \f(CW\*(C`VERSION\*(C'\fR),
however it is usually harmful to do so.  Please don't do this in new code.
.PP
For example, previous versions of this documentation suggested using \f(CW\*(C`isa\*(C'\fR as
a function to determine the type of a reference:
.PP
.Vb 1
\&  use UNIVERSAL \*(Aqisa\*(Aq;
\&
\&  $yes = isa $h, "HASH";
\&  $yes = isa "Foo", "Bar";
.Ve
.PP
The problem is that this code will \fInever\fR call an overridden \f(CW\*(C`isa\*(C'\fR method in
any class.  Instead, use \f(CW\*(C`reftype\*(C'\fR from Scalar::Util for the first case:
.PP
.Vb 1
\&  use Scalar::Util \*(Aqreftype\*(Aq;
\&
\&  $yes = reftype( $h ) eq "HASH";
.Ve
.PP
and the method form of \f(CW\*(C`isa\*(C'\fR for the second:
.PP
.Vb 1
\&  $yes = Foo\->isa("Bar");
.Ve
