---
name: "Object::Pad::MOP::FieldAttr"
section: "3pm"
platform: "linux"
package: "libobject-pad-perl"
version: "0.820-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Object::Pad::MOP::FieldAttr"
---

# Object::Pad::MOP::FieldAttr(3pm)

meta-object representation of a field attribute for "Object::Pad"

## NAME

"Object::Pad::MOP::FieldAttr" - meta-object representation of a field attribute for "Object::Pad"

## DESCRIPTION

This API provides a way for pure-perl implementations of field attributes to be provided. Pure-perl attributes cannot currently add new *behaviour* to the way that fields work, but they do provide a means for class authors to annotate extra metadata onto fields, that can be queried by other code.

Primilarily this is done by using the "get\_attribute\_value" in Object::Pad::MOP::Field accessor method on a field metadata instance.

This API should be considered **experimental**, and will emit warnings to that effect. They can be silenced with

```
   use Object::Pad qw( :experimental(custom_field_attr) );
```

## METHODS

### register

```
   Object::Pad::MOP::FieldAttr->register( $name, %args );
```

*Since version 0.60.*

Creates a new field attribute of the given name. The name must begin with a capital letter, in order to distinguish this from any of the built-in core attributes, whose names are lowercase.

The attribute is only available if the hints hash contains a key of the name given by the attribute's `"permit_hintkey"` argument. This would typically be set in the hints hash by the `"import"` method of the module implementing it, and would be named based on the name of the module providing the attribute:

```
   sub import { $^H{"Some::Package::Name/Attrname"} }
```

Takes the following additional named arguments:

- permit\_hintkey =\> STRING

  Required. A string giving a key that must be found in the hints hash (`"%^H"`) for this attribute name to be visible.

- no\_value =\> BOOL

  An optional flag; if set to true then no value is permitted on the attribute's declaration. A compiletime error will be generated if a value is provided

- must\_value =\> BOOL

  An optional flag; if set to true then a value is required on the attribute's declaration. A compiletime error will be generated if a value is not provided.

  If neither of these flags are provided, then a value is optional. It is not permitted to set both flags at once.

- apply =\> CODE

  An optional code reference for a callback function to invoke when the attribute is applied to a field. If present, it is passed the field metadata instance as a Object::Pad::MOP::Field reference, and a string containing the contents of the attribute's parenthesized value. The return value of the callback will be stored as the attribute's value and can be accessed by the `"get_attribute_value"` method on the field metadata.

  ```
     $result = $apply->( $fieldmeta, $value )
  ```

  If the `"apply"` callback is absent then the string value itself is stored.

## AUTHOR

Paul Evans \<leonerd@leonerd.org.uk\>