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_attribute 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_attribute` 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

Exposed: A type exposed as the native attribute of a component.
Attrib: A type to be used to generate output from.
attr: An attribute instance of type Attrib.
ctx: An instance of type Context.

Expression Semantics

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

Expression

Semantics

extract_from_attribute<Exposed, Attrib>::call(attr, ctx)

Extract the value to generate output from attr 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	Semantics
`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.