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#### Extract an Attribute Value to Generate Output (Karma)

###### extract_from

Before generating output for a value this value needs to extracted from the attribute instance provided by the user. The customization point `extract_from` is utilized to adapt this extraction for any data type possibly used to store the values to output.

| Note
The interface of this customization point has been changed with Boost V1.44. We added the `Exposed` template parameter to allow for more fine grained specializations of the required Spirit.Karma attribute transformations.

###### Module Headers

    #include <boost/spirit/home/karma/detail/extract_from.hpp>

Also, see Include Structure.

| Note
This header file does not need to be included directly by any user program as it is normally included by other Spirit header files relying on its content.

###### Namespace

Name
`boost::spirit::traits`

###### Synopsis

    template <typename Exposed, typename Attrib, typename Enable>
    struct extract_from_attribute
    {
        typedef <unspecified> type;

        template <typename Context>
        static type call(Attrib const& attr, Context& context);
    };

###### Template parameters

Parameter | Description | Default
--- | --- | ---
`Exposed` | The type, `Exposed` of the attribute natively exposed by the component the `extract_from` is invoked from. | none
`Attrib` | The type, `Attrib` of the attribute to be used to generate output from. | none
`Enable` | Helper template parameter usable to selectively enable or disable certain specializations of `clear_value` utilizing SFINAE (i.e. `boost::enable_if` or `boost::disable_if`). | `void`
`Context` | This is the type of the current generator execution context. |

###### Notation

Notation

`Exposed`

  A type exposed as the native attribute of a component.

`Attrib`

  A type to be used to generate output from.

`attr`

  A attribute instance of type `Attrib`.

###### Expression Semantics

Expression | Semantics
--- | ---
    extract_from_attribute<Exposed, Attrib>::call(attr, ctx)
| Extract the value to generate output from and return it to the caller.

###### Predefined Specializations

Spirit predefines specializations of this customization point for several types. The following table lists those types together with the types exposed and the corresponding semantics:

Template Parameters | Value
--- | ---
`Attrib` | The exposed typedef `type` is defined to `Attrib const&`. The function `call()` returns the argument by reference without change.
`boost::optional<Attrib>` | The exposed typedef `type` is defined to `Attrib const&`. The function `call()` returns the value held by the `optional<>` argument by reference without change.
`boost::reference_wrapper<Attrib>` | The exposed typedef `type` is defined to `Attrib const&`. The function `call()` returns the value held by the `reference_wrapper<>` argument by reference without change.
`unused_type` | The exposed typedef `type` is defined to `unused_type`. The function `call()` returns an instance of `unused_type`.

###### When to implement

The customization point `extract_from_attribute` needs to be implemented for a specific type whenever the default implementation as shown above is not applicable. Examples for this could be that the type to be extracted is different from `Attrib` and is not copy constructible.

###### Example

TBD