Determine the Type to be Stored in a Container (Qi)

container_value

The template container_value is a template meta function used as an attribute customization point. It is invoked by the Qi repetitive parsers (Kleene, Plus, List, and Repeat) to determine the type to store in a container.

Module Headers

#include <boost/spirit/home/support/container.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 Container, typename Enable>
struct container_value
{
    typedef <unspecified> type;
};

Template parameters

Parameter	Description	Default
`Container`	The type `Container` is the type for which the type f the elements has to be deduced.	none
`Enable`	Helper template parameter usable to selectively enable or disable certain specializations of `container_value` utilizing SFINAE (i.e. `boost::enable_if` or `boost::disable_if`).	`void`

C: A type to be tested whether it needs to be treated as a container.
T1, T2, ...: Arbitrary types

Expression Semantics

Expression	Semantics
`container_value<C>::type`	Metafunction that evaluates to the type to be stored in a given container type, `C`.

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
`C`	The non-const `value_type` of the given container type, `C`.
`boost::optional<C>`	Returns `container_value<C>::type`
`boost::variant<T1, T2, ...>`	Returns `container_value<TN>::value` for the first `TN` (out of `T1`, `T2`, ...) for which `is_container<TN>::type` evaluates to `mpl::true_`. Otherwise it will return `unused_type`.
`unused_type`	Returns `unused_type`.

When to implement

The customization point is_container needs to be implemented for a specific type whenever this type is to be used as an attribute in place of a STL container. It is applicable for parsers (Spirit.Qi) only. As a rule of thumb: it has to be implemented whenever a certain type is to be passed as an attribute to a parser normally exposing a STL container and if the type does not expose the interface of a STL container (i.e. no embedded typedef for value_type). These components have an attribute propagation rule in the form:

a: A --> Op(a): vector<A>

where Op(a) stands for any meaningful operation on the component a.

Related Attribute Customization Points

If this customization point is implemented, the following other customization points might need to be implemented as well.

Name	When to implement
`traits::push_back_container`	Qi: List, Kleene, Plus, Repeat.
`traits::clear_value`	Qi: List, Kleene, Plus, Repeat.

Example

Here is an example showing the default implementation of the traits::container_value customization point provided by the library:

template <typename Container, typename Enable/* = void*/>
struct container_value
  : detail::remove_value_const<typename Container::value_type>
{};

This template is instantiated by the library at the appropriate places while using the supplied container type as the template argument. The embedded type is used as the attribute type while parsing the elements to be store in that container.

The following example shows the predefined specialization for unused_type:

template <>
struct container_value<unused_type>
{
    typedef unused_type type;
};

which defines its embedded type to be unused_type as well, this way propagating the 'don't care' attribute status to the embedded parser.