Atomically Extract an Attribute Value from a Container (Karma)

attribute_as

attribute_as atomically extracts an instance of a type from another type. This customization point is used by the as<T>[] directive.

Module Headers

#include <boost/spirit/home/support/attributes_fwd.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 T, typename Attribute, typename Enable = void>
struct attribute_as;

Template parameters

Parameter	Description	Default
`T`	The type of the attribute natively exposed by the component the `attribute_as` is invoked from.	none
`Attribute`	The type of the attribute to be used to generate output from.	none
`Enable`	Helper template parameter usable to selectively enable or disable certain specializations of `attribute_as` utilizing SFINAE (i.e. `boost::enable_if` or `boost::disable_if`).	`void`

Notation

attr: An instance of type Attrib.

Expression Semantics

Expression	Semantics
`attribute_as<T, Attribute>::type`	The result type of the extraction.
`attribute_as<T, Attribute>::call(attr)`	Extract and return an instance of `type`.
`attribute_as<T, Attribute>::valid_as(attr)`	Determine, at runtime, if the extraction of an instance of `type` from `attr` would cause an error.

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
`unused_type`	The exposed typedef `type` is defined to `unused_type`. The function `call()` returns an instance of `unused_type`.

When to implement

This customization point may need to be implemented when using the as<T>[] directive.