# Package =TWiki::Form::FieldDefinition
Base class of all field definition classes.
Type-specific classes are derived from this class to define specific per-type behaviours. This class also provides default behaviours for when a specific type cannot be loaded.
## [[ClassMethod]] **new** `(%...)`
Construct a new [[FieldDefinition]]. Parameters are passed in a hash. See Form.pm for how it is called. Subclasses should pass @\_ on to this class.
## [[ObjectMethod]] **finish** `()`
Break circular references.
## isEditable() -> $boolean
Is the field type editable? Labels aren't, for example. Subclasses may need to redefine this.
## isMultiValued() -> $boolean
Is the field type multi-valued (i.e. does it store multiple values)? Subclasses may need to redefine this.
## isTextMergeable() -> $boolean
Is this field type mergeable using a conventional text merge?
## isMandatory() -> $boolean
Is this field mandatory (required)?
## renderForEdit( $web, $topic, $value ) -> ($col0html, $col1html)
`$web` - the web containing the topic being edited `$topic` - the topic being edited Render the field for editing. Returns two chunks of HTML; the `$col0html` is appended to the HTML for the first column in the form table, and the `$col1html` is used as the content of the second column.
## cssClasses(@classes) -> $classes
Construct a list of the CSS classes for the form field. Adds additional class specifiers related to the attributes of the field e.g mandatory. Pass it a list of the other classnames you want on the field.
## getDefaultValue() -> $value
Try and get a sensible default value for the field from the values stored in the form definition. The result should be a value string.
Some subclasses may not support the definition of defaults in the form definition. In that case this method should return `undef`.
## renderHidden($meta) -> $html
Render the form in `$meta` as a set of hidden fields.
## populateMetaDataFromQuery( $query, $meta, $old ) -> ($bValid, $bPresent)
Given a CGI `$query`, a `$meta` object, and an array of `$old` field entries, then populate the $meta with a row for this field definition, taking the content from the query if it's there, otherwise from $old or failing that, from the default defined for the type. Refuses to update mandatory fields that have an empty value.
Return $bValid true if the value in $meta was updated (either from the query or from a default in the form. Return $bPresent true if a value was present in the query (even it was undef)
## [[ObjectMethod]] \*renderForDisplay `($format,$attrs) -> $html`
Render the field for display, under the control of $attrs.
The following vars in $format are expanded: $title - title of the form field $value - expanded to the **protected** value of the form field
The value is protected by TWiki::Render::protectFormFieldValue.