Re-Initialize an Attribute Value before Parsing (Qi)

clear_value

The template clear_value is a type used as an attribute customization point. It is invoked by the Qi repetitive parsers (Kleene, Plus, List, and Repeat) in order to re-initialize the attribute instance passed to the embedded parser after it has been stored in the provided container. This re-initialized attribute instance is reused during the next iteration of the repetitive parser.

Module Headers

#include <boost/spirit/home/support/attributes.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 Attrib, typename Enable>
struct clear_value
{
    static void call(Attrib& val);
};

Template parameters

Parameter	Description	Default
`Attrib`	The type, `Attrib` of the attribute to be re-initialized.	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`

Notation

Attrib: A type to be used as a container to store attribute values in.
attr: An attribute instance of type Attrib.
T1, T2, ...: Arbitrary types

Expression Semantics

Expression	Semantics
clear_value<Attrib>::call(Attrib& attr)	Re-initialize the instance referred to by `attr` in the most efficient way.

Expression

Semantics

clear_value<Attrib>::call(Attrib& attr)

Re-initialize the instance referred to by attr in the most efficient way.

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`	Re-initialize using assignment of default constructed value.
Any type `T` for which `is_container<>::type` is `mpl::true_`	Call the member function `attr.clear()` for the passed attribute instance.
`boost::optional<Attrib>`	Clear the `optional` instance and leave it uninitialized.
`boost::variant<T1, T2, ...>`	Invoke the `clear_value` customization point for the currently held value.
`fusion::tuple<T1, T2, ...>`	Invoke the `clear_value` customization point for all elements of the tuple.
`unused_type`	Do nothing.

When to Implement

The customization point clear_value needs to be implemented for a specific type whenever this type is to be used as an attribute to be stored into a STL container and if the type cannot be re-initialized using one of the specializations listed above. Examples for this might be types not being default constructible or container types not exposing a member function clear().