# MIME::Field::ParamVal - subclass of Mail::Field, for structured MIME fi... - perldoc - [phpMan]

## NAME
    [MIME::Field::ParamVal](https://www.chedong.com/phpMan.php/perldoc/MIME%3A%3AField%3A%3AParamVal/markdown) - subclass of [Mail::Field](https://www.chedong.com/phpMan.php/perldoc/Mail%3A%3AField/markdown), for structured MIME fields

## SYNOPSIS
        # Create an object for a content-type field:
        $field = new [Mail::Field](https://www.chedong.com/phpMan.php/perldoc/Mail%3A%3AField/markdown) 'Content-type';

        # Set some attributes:
        $field->param('_'        => 'text/html');
        $field->param('charset'  => 'us-ascii');
        $field->param('boundary' => '---ABC---');

        # Same:
        $field->set('_'        => 'text/html',
                    'charset'  => 'us-ascii',
                    'boundary' => '---ABC---');

        # Get an attribute, or undefined if not present:
        print "no id!"  if defined($field->param('id'));

        # Same, but use empty string for missing values:
        print "no id!"  if ($field->paramstr('id') eq '');

        # Output as string:
        print $field->stringify, "\n";

## DESCRIPTION
    This is an abstract superclass of most MIME fields. It handles fields with a general syntax like
    this:

        Content-Type: Message/Partial;
            number=2; total=3;
            id="oc=<jpbe0M2Yt4s@thumper.bellcore.com>"

    Comments are supported *between* items, like this:

        Content-Type: Message/Partial; (a comment)
            number=2  (another comment) ; (yet another comment) total=3;
            id="oc=<jpbe0M2Yt4s@thumper.bellcore.com>"

## PUBLIC INTERFACE
    set [\%PARAMHASH | KEY=>VAL,...,KEY=>VAL]
        *Instance method.* Set this field. The paramhash should contain parameter names in *all
        lowercase*, with the special "_" parameter name signifying the "default" (unnamed) parameter
        for the field:

           # Set up to be...
           #
           #     Content-type: Message/Partial; number=2; total=3; id="ocj=pbe0M2"
           #
           $conttype->set('_'       => 'Message/Partial',
                          'number'  => 2,
                          'total'   => 3,
                          'id'      => "ocj=pbe0M2");

        Note that a single argument is taken to be a *reference* to a paramhash, while multiple args
        are taken to be the elements of the paramhash themselves.

        Supplying undef for a hashref, or an empty set of values, effectively clears the object.

        The self object is returned.

    parse_params STRING
        *Class/instance utility method.* Extract parameter info from a structured field, and return
        it as a hash reference. For example, here is a field with parameters:

            Content-Type: Message/Partial;
                number=2; total=3;
                id="oc=<jpbe0M2Yt4s@thumper.bellcore.com>"

        Here is how you'd extract them:

            $params = $class->parse_params('content-type');
            if ($$params{'_'} eq 'message/partial') {
                $number = $$params{'number'};
                $total  = $$params{'total'};
                $id     = $$params{'id'};
            }

        Like field names, parameter names are coerced to lowercase. The special '_' parameter means
        the default parameter for the field.

        NOTE: This has been provided as a public method to support backwards compatibility, but you
        probably shouldn't use it.

    parse STRING
        *Class/instance method.* Parse the string into the instance. Any previous information is
        wiped. The self object is returned.

        May also be used as a constructor.

    param PARAMNAME,[VALUE]
        *Instance method.* Return the given parameter, or undef if it isn't there. With argument,
        set the parameter to that VALUE. The PARAMNAME is case-insensitive. A "_" refers to the
        "default" parameter.

    paramstr PARAMNAME,[VALUE]
        *Instance method.* Like param(): return the given parameter, or *empty* if it isn't there.
        With argument, set the parameter to that VALUE. The PARAMNAME is case-insensitive. A "_"
        refers to the "default" parameter.

    stringify
        *Instance method.* Convert the field to a string, and return it.

    tag *Instance method, abstract.* Return the tag for this field.

## SEE ALSO
    [Mail::Field](https://www.chedong.com/phpMan.php/perldoc/Mail%3A%3AField/markdown)