phpMan > man > UNIVERSAL(3pm)

NAME
    UNIVERSAL - base class for ALL classes (blessed references)

SYNOPSIS
        $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");

DESCRIPTION
    "UNIVERSAL" is the base class from which all blessed references inherit. See perlobj.

    "UNIVERSAL" provides the following methods:

    "$obj->isa( TYPE )"
    "CLASS->isa( TYPE )"
    "eval { VAL->isa( TYPE ) }"
        Where

        "TYPE"
            is a package name

        $obj
            is a blessed reference or a package name

        "CLASS"
            is a package name

        "VAL"
            is any of the above or an unblessed reference

        When used as an instance or class method ("$obj->isa( TYPE )"), "isa" returns *true* if $obj
        is blessed into package "TYPE" or inherits from package "TYPE".

        When used as a class method ("CLASS->isa( TYPE )", sometimes referred to as a static
        method), "isa" returns *true* if "CLASS" inherits from (or is itself) the name of the
        package "TYPE" or inherits from package "TYPE".

        If you're not sure what you have (the "VAL" case), wrap the method call in an "eval" block
        to catch the exception if "VAL" is undefined.

        If you want to be sure that you're calling "isa" as a method, not a class, check the
        invocand with "blessed" from Scalar::Util first:

          use Scalar::Util 'blessed';

          if ( blessed( $obj ) && $obj->isa("Some::Class") ) {
              ...
          }

    "$obj->DOES( ROLE )"
    "CLASS->DOES( ROLE )"
        "DOES" checks if the object or class performs the role "ROLE". 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.

        "DOES" and "isa" 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, "DOES" is different
        from "isa" in that it does not care *how* the invocand performs the operations, merely that
        it does. ("isa" of course mandates an inheritance relationship. Other relationships include
        aggregation, delegation, and mocking.)

        By default, classes in Perl only perform the "UNIVERSAL" role, as well as the role of all
        classes in their inheritance. In other words, by default "DOES" responds identically to
        "isa".

        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 "DOES" in place of "isa" safely, as it will return true in all
        places where "isa" will return true (provided that any overridden "DOES" *and* "isa" methods
        behave appropriately).

    "$obj->can( METHOD )"
    "CLASS->can( METHOD )"
    "eval { VAL->can( METHOD ) }"
        "can" checks if the object or class has a method called "METHOD". If it does, then it
        returns a reference to the sub. If it does not, then it returns *undef*. This includes
        methods inherited or imported by $obj, "CLASS", or "VAL".

        "can" cannot know whether an object will be able to provide a method through AUTOLOAD
        (unless the object's class has overridden "can" appropriately), so a return value of *undef*
        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 AUTOLOAD. For such 'dummy' subs, "can" will still return a code reference,
        which, when called, will fall through to the AUTOLOAD. If no suitable AUTOLOAD is provided,
        calling the coderef will cause an error.

        You may call "can" as a class (static) method or an object method.

        Again, the same rule about having a valid invocand applies -- use an "eval" block or
        "blessed" if you need to be extra paranoid.

    "VERSION ( [ REQUIRE ] )"
        "VERSION" will return the value of the variable $VERSION in the package the object is
        blessed into. If "REQUIRE" is given then it will do a comparison and die if the package
        version is not greater than or equal to "REQUIRE", or if either $VERSION or "REQUIRE" is not
        a "lax" version number (as defined by the version module).

        The return from "VERSION" will actually be the stringified version object using the package
        $VERSION scalar, which is guaranteed to be equivalent but may not be precisely the contents
        of the $VERSION scalar. If you want the actual contents of $VERSION, use $CLASS::VERSION
        instead.

        "VERSION" can be called as either a class (static) method or an object method.

WARNINGS
    NOTE: "can" directly uses Perl's internal code for method lookup, and "isa" uses a very similar
    method and cache-ing strategy. This may cause strange effects if the Perl code dynamically
    changes @ISA in any package.

    You may add other methods to the UNIVERSAL class via Perl or XS code. You do not need to "use
    UNIVERSAL" to make these methods available to your program (and you should not do so).

EXPORTS
    None.

    Previous versions of this documentation suggested using "isa" as a function to determine the
    type of a reference:

      $yes = UNIVERSAL::isa($h, "HASH");
      $yes = UNIVERSAL::isa("Foo", "Bar");

    The problem is that this code would *never* call an overridden "isa" method in any class.
    Instead, use "reftype" from Scalar::Util for the first case:

      use Scalar::Util 'reftype';

      $yes = reftype( $h ) eq "HASH";

    and the method form of "isa" for the second:

      $yes = Foo->isa("Bar");