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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] 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.


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