EQ2EMu/EQ2/source/depends/boost_1_72_0/libs/spirit/doc/advanced/customization_points.qbk

[/==============================================================================
    Copyright (C) 2001-2011 Hartmut Kaiser
    Copyright (C) 2001-2011 Joel de Guzman

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[import ../example/karma/customize_embedded_container.cpp]    [/ this pulls in the embedded_container example]
[import ../example/karma/customize_counter.cpp]               [/ this pulls in the counter example]
[import ../example/karma/customize_use_as_container.cpp]      [/ this pulls in the use_as_container example]

[def __customize_embedded_container_example__ [link spirit.advanced.customize.iterate.container_iterator.example embedded_container_example]]
[def __customize_counter_example__ [link spirit.advanced.customize.iterate.deref_iterator.example counter_example]]
[def __customize_use_as_container_example__ [link spirit.advanced.customize.iterate.next_iterator.example use_as_container]]

[section:customize Customization of Spirit's Attribute Handling]

[heading Why do we need Attribute Customization Points]

[important Before you read on please be aware that the interfaces described in
           this section are not finalized and may change in the future without 
           attempting to be backwards compatible. We document the customization
           point interfaces anyways as we think they are important. 
           Understanding customization points helps understanding Spirit. 
           Additionally they prove to be powerful tools enabling full 
           integration of the user's data structures with /Qi's/ parsers and
           /Karma's/ generators.]

__spirit__ has been written with extensibility in mind. It provides many 
different attribute customization points allowing to integrate custom data 
types with the process of parsing in __qi__ or output generation with 
__karma__. All attribute customization points are exposed using a similar 
technique: full or partial template specialization. __spirit__ generally 
implements the main template, providing a default implementation. You as the 
user have to provide a partial or full specialization of this template for the 
data types you want to integrate with the library. In fact, the library uses 
these customization points itself for instance to handle the magic of the 
__unused_type__ attribute type. 

Here is an example showing the __customize_container_value__ customization point 
used by different parsers (such as __qi_kleene__, __qi_plus__, etc.) to find 
the attribute type to be stored in a supplied STL container:

[import ../../../../boost/spirit/home/support/container.hpp]

[customization_container_value_default]

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__:

[customization_container_value_unused]

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

All attribute customization points follow the same scheme. The last template 
parameter is always `typename Enable = void` allowing to apply SFINAE for
fine grained control over the template specialization process. But most of the
time you can safely forget about its existence.

The following sections will describe all customization points, together with a 
description which needs to be specialized for what purpose.

[heading The Usage of Customization Points]

The different customizations points are used by different parts of the library.
Part of the customizations points are used by both, __qi__ and __karma__, 
whereas others are specialized to be applied for one of the sub-libraries only.
We will explain when a specific customization point needs to be implemented and,
equally important, which customization points need to be implemented at the 
same time. Often it is not sufficient to provide a specialization for one 
single customization point only, in this case you as the user have to provide
all necessary customizations for your data type you want to integrate with the
library.

[/////////////////////////////////////////////////////////////////////////////]
[section:is_container Determine if a Type Should be Treated as a Container (Qi and Karma)]

[heading is_container]

The template `is_container` is a template meta-function used as an attribute 
customization point. It is invoked by the /Qi/ __qi_sequence__ (`>>`) and 
/Karma/ __karma_sequence__ operators in order to determine whether a supplied
attribute can potentially be treated as a container.

[heading Header]

    #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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Enable>
    struct is_container
    {
        <unspecified>;
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]                                 [Default]]
    [[`Container`]          [The type, `Container` which needs to 
                             be tested whether it has to be treated
                             as a container]                              [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `is_container` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`C`]          [A type to be tested whether it needs to be treated
                     as a container.]]
    [[`T1`, `T2`, ...]  [Arbitrary types]]
]

[heading Expression Semantics]

[table
    [[Expression]                 [Semantics]]
    [[`is_container<C>::type`]    [Result of the metafunction that evaluates to 
                                   `mpl::true_` if a given type, `C`, is to be 
                                   treated as a container, `mpl::false_` otherwise 
                                   Generally, any implementation of `is_container` 
                                   needs to behave as if if was a __mpl_boolean_constant__..]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
conditions for which the corresponding specializations will evaluate to 
`mpl::true_` (see __mpl_boolean_constant__):

[table
    [[Template Parameters]     [Semantics]]
    [[`T`]                    [Returns `mpl::true_` if `T` has the following 
                               embedded types defined: `value_type`, 
                               `iterator`, `size_type`, and`reference`.
                               Otherwise it will return `mpl::false_`.]]
    [[`boost::optional<T>`]   [Returns `is_container<T>::type`]]
    [[`boost::variant<T1, T2, ...>`]
                              [Returns `mpl::true_` if at least one of the 
                              `is_container<TN>::type` returns `mpl::true_` 
                              (where `TN` is `T1`, `T2`, ...).
                              Otherwise it will return `mpl::false_`.]]
    [[__unused_type__]        [Returns `mpl::false_`.]]
]

[heading 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 (__qi__) and generators (__karma__).
As a rule of thumb: it has to be implemented whenever a certain type 
is to be passed as an attribute to a parser or a generator normally exposing a 
STL container, `C` and if the type does not expose the interface of a STL container
(i.e. `is_container<C>::type` would normally return `mpl::false_`). 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`.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_container_value__]      [Needs to be implemented whenever `is_container` is implemented.]]
    [[__customize_push_back_container__]  [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_clear_value__]          [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
]

[heading Example]

For examples of how to use the customization point `is_container` please
see here: __customize_embedded_container_example__, 
__customize_use_as_container_example__, and __customize_counter_example__.

[endsect] [/ is_container]

[/////////////////////////////////////////////////////////////////////////////]
[section:is_string Determine if a Type Should be Treated as a String (Qi and Karma)]

[heading is_string]

The `is_string` customization point is a template meta-function. It is used by
/Qi/ [qi_lit_string String Literals] (`lit(str)`), /Qi/ [qi_lit_char Character Literals]
(`lit(c)`), /Karma/ [karma_lit_string String Literals] (`lit(str)`), /Karma/ [karma_lit_char Character Literals]
(`lit(c)`) and other Spirit components. It determines whether a supplied type
can be treated as a string.

[heading Module Headers]

    #include <boost/spirit/home/support/string_traits.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T>
    struct is_string
    {
        <unspecified>;
    };

[heading Template parameters]

[table
    [[Parameter] [Description]                                        [Default]]
    [[`T`]       [The type, `T` which needs to be tested as a string] [none]]
]

[variablelist Notation
    [[`T`]              [An arbitrary type.]]
    [[`N`]              [An arbitrary integral constant.]]
    [[`Char`]           [A character type.]]
    [[`Traits`]         [A character traits type.]]
    [[`Allocator`]      [A standard allocator type.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[`is_string<T>::type`] [Result of the metafunction that evalutes to mpl::true_
                             if a given type, `T`, is to be treated as a string and
                             mpl::false_ otherwise. Generally,
                             any implementation of `is_string` needs to behave as 
                             if if was a __mpl_boolean_constant__.]]
]

[heading Predefined Specializations]

[table
    [[Type]                                           [Semantics]]
    [[`T`]                                            [Returns `mpl::false_`.]]
    [[`T const`]                                      [Returns `is_string<T>`.]]
    [[`char const*`]                                  [Returns `mpl::true_`.]]
    [[`wchar_t const*`]                               [Returns `mpl::true_`.]]
    [[`char*`]                                        [Returns `mpl::true_`.]]
    [[`wchar_t*`]                                     [Returns `mpl::true_`.]]
    [[`char[N]`]                                      [Returns `mpl::true_`.]]
    [[`wchar_t[N]`]                                   [Returns `mpl::true_`.]]
    [[`char const[N]`]                                [Returns `mpl::true_`.]]
    [[`wchar_t const[N]`]                             [Returns `mpl::true_`.]]
    [[`char(&)[N]`]                                   [Returns `mpl::true_`.]]
    [[`wchar_t(&)[N]`]                                [Returns `mpl::true_`.]]
    [[`char const(&)[N]`]                             [Returns `mpl::true_`.]]
    [[`wchar_t const(&)[N]`]                          [Returns `mpl::true_`.]]
    [[`std::basic_string<Char, Traits, Allocator>`]   [Returns `mpl::true_`.]]
]

[heading When to implement]

This customization point needs to be implemented to use user-defined string classes
that do not correspond to std::string syntax and semantics.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_is_char__]   [For string types whose underlying character type
                                is not `char` or `wchar_t`, `is_char` must be 
                                implemented.]]
    [[__customize_char_type_of__] [Whenever `is_string` is implemented.]]
    [[__customize_extract_c_string__] [Whenever `is_string` is implemented.]]
]

[/ TODO: examples ]

[endsect] [/ is_string]


[/////////////////////////////////////////////////////////////////////////////]
[section:handles_container Determine Whether a Component Handles Container Attributes (Qi and Karma)]

[heading handles_container]

The template `handles_container` is a template meta-function used as an attribute 
customization point. It is invoked by the /Qi/ __qi_sequence__ (`>>`) and 
/Karma/ __karma_sequence__ operators in order to determine whether a sequence 
element (component) handles container attributes directly. This customization
point is invoked for container attributes only, and only if the sequence is
compatible with the supplied container attribute.

If a component, which is part of a sequence is able to handle a container 
attribute directly, the sequence passes the attribute to the component without
any additional action. In __qi__ the component uses the attribute to directly
store all matched attributes. In __karma__ the generator component extracts
the attributes needed for output generation directly from this attribute.

If a component, which is part of a sequence is not able to handle container
attributes, in __qi__ the sequence passes a new instance of the container 
attributes' `value_type` to the parser component, inserting the result into
the attribute on behalf of the parser component. In __karma__ the sequence 
extracts the next container element on behalf of the generator component and 
passing it the extracted value.

[heading Header]

    #include <boost/spirit/home/support/handles_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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <
        typename Component, typename Attribute, typename Context, 
        typename Iterator, typename Enable>
    struct handles_container
    {
        <unspecified>;
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]                                 [Default]]
    [[`Component`]          [The component type `Component` which needs to 
                             be tested whether it handles container attributes 
                             directly.]                              [none]]
    [[`Attribute`]          [The attribute type `Attribute` as passed to the 
                             sequence operator.]                     [none]]
    [[`Context`]            [This is the type of the current component execution 
                             context.]                               [`unused_type`]]
    [[`Iterator`]           [The type, `Iterator` is the type of the iterators
                             used to invoke the component.]          [`unused_type`]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `is_container` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Component`]  [A component type to be tested whether it directly handles 
                     container attributes in the context of sequences.]]
    [[`Attribute`]  [A container attribute type as passed to the sequence.]]
    [[`T1`, `T2`, ...]  [Arbitrary types]]
]

[heading Expression Semantics]

[table
    [[Expression]                   [Semantics]]
    [[`handles_container<Component, Attribute>::type`] 
                                    [Result of the metafunction that evaluates to 
                                     `mpl::true_` if a given component type `Component`, 
                                     handles container attributes directly, 
                                     `mpl::false_` otherwise. Generally,
                                    any implementation of `handles_container` needs to 
                                    behave as if if was a __mpl_boolean_constant__.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
conditions for which the corresponding specializations will evaluate to 
`mpl::true_` (see __mpl_boolean_constant__):

[table
    [[Template Parameters]     [Semantics]]
    [[`Component`, `Attribute`] [Always returns `mpl::false_` (the default).]]
    [[`rule<Iterator, T1, T2, T3, T4>`, `Attribute`]
                                [Returns `is_container<A>`, where `A` is the 
                                 attribute exposed by the rule (__qi__ and 
                                 __karma__).]]
    [[`grammar<Iterator, T1, T2, T3, T4>`, `Attribute`]
                                [Returns `is_container<A>`, where `A` is the 
                                 attribute exposed by the grammar (__qi__ and 
                                 __karma__).]]
]

[heading When to implement]

The customization point `handles_container` needs to be implemented for a 
specific type whenever this type directly handles container attributes. 
It is applicable for parsers (__qi__) and generators (__karma__). It will have
to be implemented under rare circumstances only.

[/ TODO: examples ]

[endsect] [/ handles_container]

[/////////////////////////////////////////////////////////////////////////////]
[section:transform Transform an Attribute to a Different Type (Qi and Karma)]

[heading transform_attribute]

The template `transform_attribute` is a type used as an attribute customization 
point. It is invoked by /Qi/ `rule`, semantic action and `attr_cast`, and /Karma/ 
`rule`, semantic action and [karma_attr_cast `attr_cast`]. It is used to 
automatically transform the user 
provided attribute to the attribute type expected by the right hand side 
component (for `rule`), the semantic action, or the embedded component 
(for `attr_cast`).

[note The interface of this customization point has been changed with Boost 
      V1.44. We added the `Domain` template parameter to allow for more fine 
      grained specializations for __qi__ and __karma__.]

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Exposed, typename Transformed, typename Domain, typename Enable>
    struct transform_attribute
    {
        typedef <unspecified> type;

        static type pre(Exposed& val);
        static void post(Exposed& val, type attr);    // Qi only
        static void fail(Exposed&);                   // Qi only
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Exposed`]            [The attribute type supplied to the component
                             which needs to be transformed.]    [none]]
    [[`Transformed`]        [The attribute type expected by the component
                             to be provided as the result of the transformation.] [none]]
    [[`Domain`]             [The domain of the sub library the template is 
                             instantiated in. Typically this is either `qi::domain`
                             or `karma::domain`.]        [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `transform_attribute` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Exposed`]          [The type, `Exposed` is the type of the attribute as
                           passed in by the user.]]
    [[`Transformed`]      [The type, `Transformed` is the type of the attribute 
                           as passed along to the right hand side of the `rule`
                           (embedded component of `attr_cast`).]]
    [[`Domain`]             [The domain of the sub library the template is 
                             instantiated in. Typically this is either `qi::domain`
                             or `karma::domain`.]]
    [[`exposed`]          [An instance of type `Exposed`.]]
    [[`transformed`]      [An instance of type `Transformed`.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[`transform_attribute<Exposed, Transformed, Domain>::type`] 
                        [Evaluates to the type to be used as the result of the 
                         transformation (to be passed to the right hand side of
                         the `rule` or to the embedded component of the 
                         `attr_cast`. Most of the time this is equal to 
                         `Transformed`, but in other cases this might evaluate to
                         `Transformed&` instead avoiding superfluous object
                         creation.]]
    [[
``type transform_attribute<Exposed, Transformed, Domain>::pre(exposed)``]
                        [Do `pre`-transformation before invoking the right hand
                         side component for `rule` (or the embedded component 
                         for `attr_cast`). This takes the attribute supplied as by
                         the user (of type `Exposed`) and returns the attribute
                         to be passed down the component hierarchy (of the type 
                         as exposed by the metafunction `type`). This function
                         will be called in /Qi/ and for /Karma/.]]
    [[
``void transform_attribute<Exposed, Transformed, Domain>::post(exposed, transformed)``]
                        [Do `post`-transformation after the invocation of the 
                         right hand side component for `rule` (or the embedded 
                         component for `attr_cast`). This takes the original 
                         attribute as supplied by the user and the attribute 
                         as returned from the right hand side (embedded) 
                         component and is expected to propagate the result back 
                         into the supplied attribute instance. This function 
                         will be called in /Qi/ only.]]
    [[
``void transform_attribute<Exposed, Transformed, Domain>::fail(exposed)``]
                        [Handling failing parse operations of the
                         right hand side component for `rule` (or the embedded 
                         component for `attr_cast`). This function 
                         will be called in /Qi/ only.]]
]

[heading Predefined Specializations]

[table
    [[Template parameters]        [Semantics]]
    [[`Exposed`, `Transformed`]   [`type` evaluates to `Transformed`, 
                                   `pre()` returns a new instance of `Transformed`
                                   constructed from the argument of type `Exposed`, 
                                   `post()` assigns `transformed` to `exposed`.]]
    [[`optional<Exposed>`, `Transformed`, 
      `typename disable_if<is_same<optional<Exposed>, Transformed> >::type`]   
                                  [`type` evaluates to `Transformed&`, 
                                   `pre()` returns a reference to the instance of `Transformed`
                                   stored in the passed optional (the argument of type `optional<Exposed>`), 
                                   the optional instance is initialized, if needed.
                                   `post()` does nothing, `fail()` resets the
                                   optional (its parameter) instance to the non-initialized state.]]
    [[`Exposed&`, `Transformed`]  [`type` evaluates to `Transformed`, 
                                   `pre()` returns a new instance of `Transformed`
                                   constructed from the argument of type `Exposed`, 
                                   `post()` assigns `transformed` to `exposed`.]]
    [[`Attrib&`, `Attrib`]        [`type` evaluates to `Attrib&`, 
                                   `pre()` returns it's argument, `post()` does
                                   nothing.]]
    [[`Exposed const`, `Transformed`] [(usind in /Karma/ only) `type` evaluates to 
                                   `Transformed`, `pre()` returns it's argument, 
                                   `post()` is not implemented.]]
    [[`Attrib const&`, `Attrib`]  [(usind in /Karma/ only) `type` evaluates to `Attrib const&`, 
                                   `pre()` returns it's argument, `post()` is not 
                                   implemented.]]
    [[`Attrib const`, `Attrib`]   [(usind in /Karma/ only) `type` evaluates to `Attrib const&`, 
                                   `pre()` returns it's argument, `post()` is not 
                                   implemented.]]
    [[__unused_type__, `Attrib`]  [`type` evaluates to __unused_type__, `pre()` 
                                   and `post()` do nothing.]]
    [[`Attrib`, __unused_type__]  [`type` evaluates to __unused_type__, `pre()` 
                                   and `post()` do nothing.]]
]

[heading When to implement]

The customization point `transform_attribute` needs to be implemented for a 
specific pair of types whenever the attribute type supplied to a `rule` or 
`attr_cast` cannot automatically transformed to the attribute type expected by
the right hand side of the `rule` (embedded component of the `attr_cast`) 
because the default implementation as shown above is not applicable. Examples 
for this could be that the type `Transformed` is not constructible from 
the type `Exposed`.

[/ TODO: examples ]

[endsect] [/ transform]

[/////////////////////////////////////////////////////////////////////////////]
[/ section:optional Handling of Optional Attributes (Qi and Karma)]

[/ optional_attribute]

[/ endsect] [/ optional]

[/////////////////////////////////////////////////////////////////////////////]
[section:assign_to Store a Parsed Attribute Value (Qi)]

After parsing input and generating an attribute value this value needs to 
assigned to the attribute instance provided by the user. The customization
points `assign_to_attribute_from_iterators` and `assign_to_attribute_from_value`
are utilized to adapt this assignment to the concrete type to be assigned.
This section describes both.

[section:assign_to_attribute_from_iterators Store an Attribute after a Parser Produced a Pair of Iterators (Qi)]

[heading assign_to_attribute_from_iterators]

The template `assign_to_attribute_from_iterators` is a type used as an attribute 
customization point. It is invoked by the those /Qi/ parsers not producing any
attribute value but returning a pair of iterators pointing to the matched input
sequence. It is used to either store the iterator pair into the attribute 
instance provided by the user or to convert the iterator pair into an attribute 
as provided by the user.

[heading Module Headers]

    #include <boost/spirit/home/qi/detail/assign_to.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Attrib, typename Iterator, typename Enable>
    struct assign_to_attribute_from_iterators
    {
        static void call(Iterator const& first, Iterator const& last, Attrib& attr);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Attrib`]             [The type, `Attrib` is the type of the attribute as
                             passed in by the user.]                 [none]]
    [[`Iterator`]           [The type, `Iterator` is the type of the iterators
                             as produced by the parser.]             [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `assign_to_attribute_from_value` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Attrib`]     [A type to be used as the target to store the attribute value in.]]
    [[`attr`]       [An attribute instance of type `Attrib`.]]
    [[`Iterator`]   [The iterator type used by the parser. This type usually
                     corresponds to the iterators as passed in by the user.]]
    [[`begin`, `end`] [Iterator instances of type `Iterator` pointing to the 
                     begin and the end of the matched input sequence.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[
``assign_to_attribute_from_iterators<Attrib, Iterator>::call(b, e, attr)``] 
                        [Use the iterators `begin` and `end` to initialize
                         the attribute `attr`.]]
]

[heading Predefined Specializations]

[table
    [[Template Parameters]    [Semantics]]
    [[`Attrib`, `Iterator`]   [Execute an assignment `attr = Attrib(begin, end)`.]]
    [[__unused_type__, `T`]   [Do nothing.]]
]

[heading When to implement]

The customization point `assign_to_attribute_from_iterators` 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 `Attrib` is
not constructible from the pair of iterators.

[/ TODO: examples ]

[endsect] [/ assign_to_attribute_from_iterators]

[section:assign_to_attribute_from_value Store an Attribute Value after a Parser Produced a Value (Qi)]

[heading assign_to_attribute_from_value]

The template `assign_to_attribute_from_value` is a type used as an attribute 
customization point. It is invoked by all primitive /Qi/ parsers in order 
to store a parsed attribute value into the attribute instance provided by the 
user, if this attribute is not a container type (`is_container<T>::type` 
evaluates to `mpl::false_`, where `T` is the attribute type).

[heading Module Headers]

    #include <boost/spirit/home/qi/detail/assign_to.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Attrib, typename T, typename Enable>
    struct assign_to_attribute_from_value
    {
        static void call(T const& val, Attrib& attr);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Attrib`]             [The type, `Attrib` is the type of the attribute as
                             passed in by the user. This type is not a container
                             type (`is_container<Attrib>::type` evaluates to 
                             `mpl::false_`).]                 [none]]
    [[`T`]                  [The type, `T` is the type of the attribute instance
                             as produced by the parser.]             [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `assign_to_attribute_from_value` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Attrib`]     [A type to be used as the target to store the attribute 
                     value in. This type is guaranteed not to be a container 
                     type (`is_container<Attrib>::type` evaluates to 
                     `mpl::false_`).]]
    [[`attr`]       [An attribute instance of type `Attrib`.]]
    [[`T`]          [A type as produced by the parser. The parser temporarily stores 
                     its parsed values using this type.]]
    [[`t`]          [An attribute instance of type `T`.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[
``assign_to_attribute_from_value<Attrib, T>::call(t, attr)``] 
                        [Copy (assign) the value `t` to the attribute `attr`.]]
]

[heading Predefined Specializations]

[table
    [[Template Parameters]    [Semantics]]
    [[`Attrib`, `T`]          [Assign the argument `t` to `attr`.]]
    [[__unused_type__, `T`]   [Do nothing.]]
]

[heading When to implement]

The customization point `assign_to_attribute_from_value` 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 `Attrib` is
not copy constructible.

[/ TODO: examples ]

[endsect] [/ assign_to_attribute_from_value]

[section:assign_to_container_from_value Store an Attribute Value into a Container after a Parser Produced a Value (Qi)]

[heading assign_to_container_from_value]

The template `assign_to_container_from_value` is a type used as an attribute 
customization point. It is invoked by all primitive /Qi/ parsers in order 
to store a parsed attribute value into the attribute instance provided by the 
user, if this attribute is a container type (`is_container<T>::type` evaluates 
to `mpl::true_`, where `T` is the attribute type).

[heading Module Headers]

    #include <boost/spirit/home/qi/detail/assign_to.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Attrib, typename T, typename Enable>
    struct assign_to_container_from_value
    {
        static void call(T const& val, Attrib& attr);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Attrib`]             [The type, `Attrib` is the type of the attribute as
                             passed in by the user. This type is a container
                             type (`is_container<Attrib>::type` evaluates to 
                             `mpl::true_`).]                 [none]]
    [[`T`]                  [The type, `T` is the type of the attribute instance
                             as produced by the parser.]             [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `assign_to_container_from_value` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Attrib`]     [A type to be used as the target to store the attribute 
                     value in. This type is guaranteed to be a container 
                     type (`is_container<Attrib>::type` evaluates to 
                     `mpl::true_`).]]
    [[`attr`]       [An attribute instance of type `Attrib`.]]
    [[`T`]          [A type as produced by the parser. The parser temporarily stores 
                     its parsed values using this type.]]
    [[`t`]          [An attribute instance of type `T`.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[
``assign_to_container_from_value<Attrib, T>::call(t, attr)``] 
                        [Add the value `t` to the container attribute `attr`.]]
]

[heading Predefined Specializations]

[table
    [[Template Parameters]    [Semantics]]
    [[`Attrib`, `T`]          [Add the argument `t` to `attr`.]]
    [[__unused_type__, `T`]   [Do nothing.]]
]

[heading When to implement]

The customization point `assign_to_container_from_value` 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 `Attrib` is
not copy constructible.

[/ TODO: examples ]

[endsect] [/ assign_to_container_from_value]

[endsect] [/ assign_to]

[/////////////////////////////////////////////////////////////////////////////]
[section:store_value Store Parsed Attribute Values into a Container (Qi)]

In order to customize Spirit to accept a given data type as a container for 
elements parsed by any of the repetitive parsers (__qi_kleene__, __qi_plus__,
__qi_list__, and [qi_repeat Repeat]) two attribute customization points have to be 
specialized: __customize_container_value__ and __customize_push_back_container__. 
This section describes both.

[section:container_value Determine the Type to be Stored in a Container (Qi)]

[heading 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
(__qi_kleene__, __qi_plus__, __qi_list__, and [qi_repeat Repeat]) to determine the 
type to store in a container.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Enable>
    struct container_value
    {
        typedef <unspecified> type;
    };

[heading Template parameters]

[table
    [[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`]]
]

[variablelist 
    [[`C`]          [A type to be tested whether it needs to be treated
                     as a container.]]
    [[`T1`, `T2`, ...]  [Arbitrary types]]
]

[heading Expression Semantics]

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

[heading 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:

[table
    [[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__.]]
]

[heading 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 (__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`.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_push_back_container__]  [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
    [[__customize_clear_value__]  [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
]

[heading Example]

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

[customization_container_value_default]

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__:

[customization_container_value_unused]

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

[/ TODO: examples ]

[endsect] [/ container_value]

[section:push_back Store a Parsed Attribute Value into a Container (Qi)]

[heading push_back_container]

The template `push_back_container` is a type used as an attribute customization 
point. It is invoked by the /Qi/ repetitive parsers (__qi_kleene__, __qi_plus__, 
__qi_list__, and [qi_repeat Repeat]) to store a parsed attribute value into a 
container.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Attrib, typename Enable>
    struct push_back_container
    {
        static bool call(Container& c, Attrib const& val);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Container`]          [The type, `Container` needs to 
                             be tested whether it has to be treated
                             as a container]                              [none]]
    [[`Attrib`]             [The type, `Attrib` is the one returned from the 
                             customization point __customize_container_value__
                             and represents the attribute value to be stored in 
                             the container of type `Container`.]          [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `push_back_container` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`C`]          [A type to be used as a container to store attribute values in.]]
    [[`c`]          [A container instance of type `C`.]
    [[`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]]
]

[heading Expression Semantics]

[table
    [[Expression]                     [Semantics]]
    [[
``push_back_container<C, Attrib>::call(c, attr)``] 
                                      [Static function that is invoked whenever an 
                                       attribute value, `attr` needs to be stored 
                                       into the container instance `c`. This function
                                       should return `true` on success and `false`
                                       otherwise. Returning `false` causes the 
                                       corresponding parser to fail.]]
]

[heading 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:

[table
    [[Template Parameters]     [Semantics]]
    [[`C`, `Attrib`]           [Store the provided attribute instance `attr` into 
                                the given container `c` using the function call
                                `c.insert(c.end(), attr)`.]]
    [[`boost::optional<C>`, `Attrib`] 
                               [If the provided instance of `boost::optional<>` is not
                                initialized, invoke the appropriate initialization
                                and afterwards apply the customization point
                                `push_back_container<C, Attrib>`, treating the
                                instance held by the optional (of type `C`) as 
                                the container to store the attribute in.]]
    [[`boost::variant<T1, T2, ...>`, `Attrib`]
                               [If the instance of the variant currently holds a
                                value with a type, `TN`, for which `is_container<TN>::type`
                                evaluates to `mpl::true_`, this customization
                                point specialization will apply 
                                `push_back_container<TN, Attrib>`, treating the
                                instance held by the variant (of type `TN`) as 
                                the container to store the attribute in. Otherwise
                                it will raise an assertion.]]
    [[__unused_type__]         [Do nothing.]]
]

[heading When to Implement]

The customization point `push_back_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 (__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 function being equivalent to 
`c.insert(c.end(), attr)`. 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`.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_container_value__]  [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
    [[__customize_clear_value__]  [Qi: __qi_list__, __qi_kleene__, __qi_plus__, [qi_repeat Repeat].]]
]

[heading Example]

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

[customization_push_back_default]

This template is instantiated by the library at the appropriate places while 
using the supplied container and element types as the template arguments. The 
member function `call()` will be called whenever an element has to be added to 
the supplied container

The following example shows the predefined specialization for __unused_type__:

[customization_push_back_unused]

which defines an empty member function `call()`.

[/ TODO: examples ]

[endsect] [/ push_back]

[endsect] [/ store_value]

[/////////////////////////////////////////////////////////////////////////////]
[section:clear_value Re-Initialize an Attribute Value before Parsing (Qi)]

[heading clear_value]

The template `clear_value` is a type used as an attribute customization point. 
It is invoked by the /Qi/ repetitive parsers (__qi_kleene__, __qi_plus__, 
__qi_list__, and [qi_repeat 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.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Attrib, typename Enable>
    struct clear_value
    {
        static void call(Attrib& val);
    };

[heading Template parameters]

[table
    [[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`]]
]

[variablelist 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]]
]

[heading Expression Semantics]

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

[heading 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:

[table
    [[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.]]
]

[heading 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()`.

[/ TODO: examples ]

[endsect] [/ clear_value]

[/////////////////////////////////////////////////////////////////////////////]
[section:extract_from Extract an Attribute Value to Generate Output (Karma)]

[heading 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 __karma__ attribute 
      transformations.]

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading 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);
    };

[heading Template parameters]

[table
    [[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.]]
]

[variablelist 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`.]]
]

[heading Expression Semantics]

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

[heading 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:

[table
    [[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__.]]
]

[heading 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.

[/ TODO: examples ]

[endsect] [/ extract_from]

[/////////////////////////////////////////////////////////////////////////////]
[section:extract_from_container Extract From a Container Attribute Value to Generate Output (Karma)]

[heading extract_from_container]

Before generating output for a value this value needs to extracted from the 
attribute instance provided by the user. The customization point 
`extract_from_container` 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 __karma__ attribute 
      transformations.]

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Exposed, typename Attrib, typename Enable>
    struct extract_from_container
    {
        typedef <unspecified> type;

        template <typename Context>
        static type call(Attrib const& attr, Context& context);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Exposed`]            [The type, `Exposed` of the attribute natively
                             exposed by the component the `extract_from_container` is
                             invoked from.]             [none]]
    [[`Attrib`]             [The type, `Attrib` is the container 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.]]
]

[heading Notation]

[variablelist Notation
    [[`Exposed`]    [A type exposed as the native attribute of a component.]]
    [[`Attrib`]     [A container type to be used to generate output from.]]
    [[`attr`]       [An attribute instance of type `Attrib`.]]
    [[`ctx`]        [An instance of type `Context`.]]
]

[heading Expression Semantics]

[table
    [[Expression]       [Semantics]]
    [[
``extract_from_container<Exposed, Attrib>::call(attr, ctx)``] 
                        [Extract the value to generate 
                         output from the contaner given by `attr` and return 
                         it to the caller.]]
]

[heading 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:

[table
    [[Template Parameters]     [Value]]
    [[`Attrib`]                [The exposed typedef `type` is defined to 
                                `Attrib const&`. The function `call()` returns 
                                the 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__.]]
]

[heading When to implement]

The customization point `extract_from_container` needs to be implemented for a 
specific container 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.

[heading Example]

TBD

[endsect] [/ extract_from]

[/////////////////////////////////////////////////////////////////////////////]
[section:iterate Extract Attribute Values to Generate Output from a Container (Karma)]

[section:container_iterator Determine the Type of the Iterator of a Container]

[heading container_iterator]

The template `container_iterator` is a template meta-function used as an attribute 
customization point. It is invoked by the /Karma/ repetitive generators (such 
as __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, and
[karma_repeat Repeat]) in order to determine the type of the iterator to use to
iterate over the items to be exposed as the elements of a container.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Enable>
    struct container_iterator
    {
        typedef <unspecified> type;
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Container`]          [The type, `Container` for which the 
                             iterator type has to be returned]            [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `container_iterator` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`C`]              [A container type the iterator type needs to be evaluated for.]]
]

[heading Expression Semantics]

[table
    [[Expression]                     [Semantics]]
    [[`container_iterator<C>::type`]  [Result of the metafunction that evaluates 
                                       the type to be used as the iterator for
                                       accessing all elements of a container, `C`.]]
]

The returned type conceptually needs to be equivalent to a standard forward
iterator. But it does not have to expose the standardized interface. If this
customization point is implemented for a certain container type, all related
customization points need to be implemented as well (see 
[link spirit.advanced.customize.iterate.container_iterator.related_attribute_customization_points Related Attribute Customization Points] 
below). This encapsulates the specific iterator interface required for a 
given type. The minimal requirements for a type to be exposed as an iterator in 
this context are:

* it needs to be comparable for equality (see __customize_compare_iterators__),
* it needs to be incrementable (see __customize_next_iterator__),
* it needs to be dereferencible (see __customize_deref_iterator__).

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`C`]                     [Returns `C::iterator`.]]
    [[`C const`]               [Returns `C::const_iterator`.]]
    [[__unused_type__]         [Returns __unused_type__` const*`.]]
]

[heading When to implement]

The customization point `container_iterator` 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 generators (__karma__) only. As a rule of thumb: 
it has to be implemented whenever a certain type is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the type does not expose 
the interface of a STL container (i.e. `is_container<C>::type` would normally 
return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

Here are the header files needed to make the example code below compile:

[customize_karma_embedded_container_includes]

The example (for the full source code please see here:
[@../../example/karma/customize_embedded_container.cpp customize_embedded_container.cpp]) 
uses the data structure

[customize_karma_embedded_container_data]

as a direct container attribute to the __karma_list__ generator. In order to 
make this data structure compatible we need to specialize a couple of attribute 
customization points: __customize_is_container__, __customize_container_iterator__, 
__customize_begin_container__, and __customize_end_container__. As you can see
the specializations simply expose the embedded `std::vector<int>` as the 
container to use. We don't need to specialize the customization points related 
to iterators (__customize_deref_iterator__, __customize_next_iterator__,
and __customize_compare_iterators__) as we expose a standard iterator and the 
default implementation of these customizations handles standard iterators out 
of the box. 

[customize_karma_embedded_container_traits]

The last code snippet shows an example using an instance of the data structure 
`client::embedded_container` to generate output from a __karma_list__ 
generator:

[customize_karma_embedded_container]

As you can see, the specializations for the customization points as defined 
above enable the seamless integration of the custom data structure without 
having to modify the output format or the generator itself.

For other examples of how to use the customization point `container_iterator` 
please see here: __customize_use_as_container_example__ and
__customize_counter_example__.

[endsect] [/ container_iterator]


[section:begin_container Get the Iterator pointing to the Begin of a Container Attribute]

[heading begin_container]

The template `begin_container` is a type used as an attribute customization point. 
It is invoked by the /Karma/ repetitive generators (such as __karma_list__, 
[karma_kleene Kleene (unary `*`)], __karma_plus__, and [karma_repeat Repeat]) 
in order to get an iterator pointing to the first element of the container 
holding the attributes to generate output from.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Enable>
    struct begin_container
    {
        static typename container_iterator<Container>::type 
        call(Container& c);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Container`]          [The type, `Container` for which the iterator pointing 
                             to the first element has to be returned]    [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `begin_container` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`C`]              [A container type the begin iterator needs to be returned for.]]
    [[`c`]              [An instance of a container, `C`.]]
]

[heading Expression Semantics]

[table
    [[Expression]                     [Semantics]]
    [[`begin_container<C>::call(c)`]  [Return the iterator usable to dereference 
                                       the first element of the given container, 
                                       `c`. The type of the returned iterator is 
                                       expected to be the same as the type returned 
                                       by the customization point 
                                       __customize_container_iterator__.]]
]

The returned instance conceptually needs to be equivalent to a standard forward
iterator. But it does not have to expose the standardized interface. If this
customization point is implemented for a certain container type, all related
customization points need to be implemented as well (see 
[link spirit.advanced.customize.iterate.begin_container.related_attribute_customization_points Related Attribute Customization Points] 
below). This encapsulates the specific iterator interface required for a 
given type. The minimal requirements for a type to be exposed as an iterator in 
this context are:

* it needs to be comparable for equality (see __customize_compare_iterators__),
* it needs to be incrementable (see __customize_next_iterator__),
* it needs to be dereferencible (see __customize_deref_iterator__).

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`C`]                     [Returns `c.begin()`.]]
    [[`C const`]               [Returns `c.begin()`.]]
    [[__unused_type__]         [Returns `&unused`.]]
]

[heading When to implement]

The customization point `begin_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 generators (__karma__) only. As a rule of thumb: 
it has to be implemented whenever a certain type is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the type does not expose 
the interface of a STL container (i.e. `is_container<C>::type` would normally 
return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

For examples of how to use the customization point `begin_container` please
see here: __customize_embedded_container_example__,
__customize_use_as_container_example__, and __customize_counter_example__.

[endsect] [/ begin_container]


[section:end_container Get the Iterator pointing to the End of a Container Attribute]

[heading end_container]

The template `end_container` is a type used as an attribute customization point. 
It is invoked by the /Karma/ repetitive generators (such as __karma_list__, 
[karma_kleene Kleene (unary `*`)], __karma_plus__, and [karma_repeat Repeat]) 
in order to get an iterator pointing to the end of the container 
holding the attributes to generate output from.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Container, typename Enable>
    struct end_container
    {
        static typename container_iterator<Container>::type 
        call(Container& c);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Container`]          [The type, `Container` for which the iterator pointing 
                             to the first element has to be returned]    [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `end_container` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`C`]              [A container type the end iterator needs to be returned for.]]
    [[`c`]              [An instance of a container, `C`.]]
]

[heading Expression Semantics]

[table
    [[Expression]                     [Semantics]]
    [[`end_container<C>::call(c)`]    [Return the iterator usable to compare a
                                       different iterator with in order to detect
                                       whether the other iterator reached the end 
                                       of the given container, `c`. The type of 
                                       the returned iterator is expected to be the 
                                       same as the type returned by the 
                                       customization point __customize_container_iterator__.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`C`]                     [Returns `c.end()`.]]
    [[`C const`]               [Returns `c.end()`.]]
    [[__unused_type__]         [Returns `&unused`.]]
]

[heading When to implement]

The customization point `end_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 generators (__karma__) only. As a rule of thumb: 
it has to be implemented whenever a certain type is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the type does not expose 
the interface of a STL container (i.e. `is_container<C>::type` would normally 
return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

For examples of how to use the customization point `end_container` please
see here: __customize_embedded_container_example__,
__customize_use_as_container_example__, and __customize_counter_example__.

[endsect] [/ end_container]


[section:next_iterator Increment the Iterator pointing into a Container Attribute]

[heading next_iterator]

The template `next_iterator` is a type used as an attribute customization point. 
It is invoked by the /Karma/ repetitive generators (such as __karma_list__, 
[karma_kleene Kleene (unary `*`)], __karma_plus__, and [karma_repeat Repeat]) 
in order to get an iterator pointing to the next element of a container 
holding the attributes to generate output from.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Iterator, typename Enable>
    struct next_iterator
    {
        static void call(Iterator& it);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Iterator`]           [The type, `Iterator` of the iterator to increment.
                             This is the same as the type returned by the 
                             customization point __customize_container_iterator__.]    [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `next_iterator` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`Iterator`]       [An iterator type.]]
    [[`it`]             [An instance of an iterator of type `Iterator`.]]
    [[`C`]              [A container type whose iterator type is `Iterator`.]]
]

[heading Expression Semantics]

[table
    [[Expression]                     [Semantics]]
    [[`next_iterator<Iterator>::call(it)`]    [Increment the iterator pointing so that 
                                       it is pointing to the next element.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`Iterator`]              [Executes `++it`.]]
    [[__unused_type__` const*`][Does nothing.]]
]

[heading When to implement]

The customization point `next_iterator` needs to be implemented for a specific 
iterator type whenever the container this iterator belongs to is to be used as 
an attribute in place of a STL container. It is applicable for generators 
(__karma__) only. As a rule of thumb: it has to be implemented whenever a certain 
iterator type belongs to a container which is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the container type 
does not expose the interface of a STL container (i.e. `is_container<C>::type` 
would normally return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

Here are the header files needed to make the example code below compile:

[customize_karma_use_as_container_includes]

The example (for the full source code please see here:
[@../../example/karma/customize_use_as_container.cpp customize_use_as_container.cpp]) 
uses the data structure

[customize_karma_use_as_container_data]

as a direct attribute to the __karma_list__ generator. This type does not 
expose any of the interfaces of an STL container. It does not even expose the 
usual semantics of a container. The purpose of this artificial example is to 
demonstrate how the customization points can be used to expose independent data
elements as a single container. The example shows how to enable its use as an 
attribute to /Karma's/ repetitive generators.

In order to make this data structure compatible we need to specialize a couple 
of attribute customization points: __customize_is_container__, 
__customize_container_iterator__, __customize_begin_container__, and
__customize_end_container__. In addition, we specialize all of the 
iterator related customization points as well: __customize_deref_iterator__,
__customize_next_iterator__, and __customize_compare_iterators__.

[customize_karma_use_as_container_traits]
[customize_karma_use_as_container_iterator_traits]

The last code snippet shows an example using an instance of the data structure 
`client::use_as_container` to generate output from a __karma_list__ generator:

[customize_karma_use_as_container]

As you can see, the specializations for the customization points as defined 
above enable the seamless integration of the custom data structure without 
having to modify the output format or the generator itself.

[endsect] [/ next_iterator]


[section:deref_iterator Dereference the Iterator pointing into a Container Attribute]

[heading deref_iterator]

The template `deref_iterator` is a type used as an attribute customization point. 
It is invoked by the /Karma/ repetitive generators (such as __karma_list__, 
[karma_kleene Kleene (unary `*`)], __karma_plus__, and [karma_repeat Repeat]) 
in order to dereference an iterator pointing to an element of a container 
holding the attributes to generate output from.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Iterator, typename Enable>
    struct deref_iterator
    {
        typedef <unspecified> type;
        static type call(Iterator& it);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Iterator`]           [The type, `Iterator` of the iterator to dereference.
                             This is the same as the type returned by the 
                             customization point __customize_container_iterator__.]    [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `deref_iterator` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`Iterator`]       [An iterator type.]]
    [[`it`]             [An instance of an iterator of type `Iterator`.]]
    [[`C`]              [A container type whose iterator type is `Iterator`.]]
]

[heading Expression Semantics]

[table
    [[Expression]                             [Semantics]]
    [[`deref_iterator<Iterator>::type`]       [Metafunction result evaluating to the 
                                      type returned by dereferencing the iterator.]]
    [[`deref_iterator<Iterator>::call(it)`]   [Return the element in the container
                                      referred to by the iterator. The type of the
                                      returned value is the same as returned by the 
                                      metafunction result `type`.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`Iterator`]              [The metafunction result `type` evaluates to 
              `std::iterator_traits<Iterator>::reference` and the 
              function `call()` returns `*it`.]]
    [[__unused_type__` const*`][The metafunction result `type` evaluates to 
              __unused_type__ and the function `call()` returns `unused`.]]
]

[heading When to implement]

The customization point `deref_iterator` needs to be implemented for a specific 
iterator type whenever the container this iterator belongs to is to be used as 
an attribute in place of a STL container. It is applicable for generators 
(__karma__) only. As a rule of thumb: it has to be implemented whenever a certain 
iterator type belongs to a container which is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the container type 
does not expose the interface of a STL container (i.e. `is_container<C>::type` 
would normally return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

Here are the header files needed to make the example code below compile:

[customize_karma_counter_includes]

The example (for the full source code please see here:
[@../../example/karma/customize_counter.cpp customize_counter.cpp]) 
uses the data structure

[customize_karma_counter_data]

as a direct attribute to the __karma_list__ generator. This type does not 
expose any of the interfaces of an STL container. It does not even expose the 
usual semantics of a container. The presented customization points build a 
counter instance which is incremented each time it is accessed. The examples 
shows how to enable its use as an attribute to /Karma's/ repetitive generators.

In order to make this data structure compatible we need to specialize a couple 
of attribute customization points: __customize_is_container__, 
__customize_container_iterator__, __customize_begin_container__, and
__customize_end_container__. In addition, we specialize one of the 
iterator related customization points as well: __customize_deref_iterator__.

[customize_karma_counter_traits]
[customize_karma_counter_iterator_traits]

The last code snippet shows an example using an instance of the data structure 
`client::counter` to generate output from a __karma_list__ generator:

[customize_karma_counter]

As you can see, the specializations for the customization points as defined 
above enable the seamless integration of the custom data structure without 
having to modify the output format or the generator itself.

For other examples of how to use the customization point `deref_iterator` 
please see here: __customize_use_as_container_example__.

[endsect] [/ deref_iterator]


[section:compare_iterators Compare two Iterator pointing into a Container Attribute for Equality]

[heading compare_iterators]

The template `compare_iterators` is a type used as an attribute customization 
point. It is invoked by the /Karma/ repetitive generators (such as __karma_list__, 
[karma_kleene Kleene (unary `*`)], __karma_plus__, and [karma_repeat Repeat]) 
in order to compare the current iterator (returned either from 
__customize_begin_container__ or from __customize_next_iterator__) with the end 
iterator (returned from __customize_end_container__) in order to find the end 
of the element sequence to generate output for.

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename Iterator, typename Enable>
    struct compare_iterators
    {
        static bool call(Iterator const& it1, Iterator const& it2);
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`Iterator`]           [The type, `Iterator` of the iterator to dereference.
                             This is the same as the type returned by the 
                             customization point __customize_container_iterator__.]    [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `compare_iterators` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation
    [[`Iterator`]       [An iterator type.]]
    [[`it1`, `it2`]     [Instances of an iterator of type `Iterator`.]]
    [[`C`]              [A container type whose iterator type is `Iterator`.]]
]

[heading Expression Semantics]

[table
    [[Expression]                             [Semantics]]
    [[`compare_iterators<Iterator>::call(it1, it2)`]   
                                              [Returns whether the iterators `it1`
                                               `it2` are to be treated as being 
                                               equal.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. The following table lists those types together with the 
types returned by the embedded typedef `type`:

[table
    [[Template Parameters]     [Semantics]]
    [[`Iterator`]              [The function `call()` returns it1 == it2.]]
    [[__unused_type__` const*`][The function `call()` always returns false.]]
]

[heading When to implement]

The customization point `compare_iterators` needs to be implemented for a specific 
iterator type whenever the container this iterator belongs to is to be used as 
an attribute in place of a STL container. It is applicable for generators 
(__karma__) only. As a rule of thumb: it has to be implemented whenever a certain 
iterator type belongs to a container which is to be passed as an attribute 
to a generator normally exposing a STL container, `C` and if the container type 
does not expose the interface of a STL container (i.e. `is_container<C>::type` 
would normally return `mpl::false_`). 

[heading Related Attribute Customization Points]

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

[table
    [[Name]                               [When to implement]]
    [[__customize_is_container__]         [Needs to be implemented whenever a type is to be used as a container attribute in /Karma/.]]
    [[__customize_container_iterator__]   [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_begin_container__]      [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_end_container__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_deref_iterator__]       [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_next_iterator__]        [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
    [[__customize_compare_iterators__]    [Karma: __karma_list__, [karma_kleene Kleene (unary `*`)], __karma_plus__, [karma_repeat Repeat].]]
]

[heading Example]

For an example of how to use the customization point `compare_iterators` 
please see here: __customize_use_as_container_example__.

[endsect] [/ compare_iterators]

[endsect] [/ iterate]

[/////////////////////////////////////////////////////////////////////////////]
[section:string_traits Extract a C-Style String to Generate Output from a String Type (Karma)]

[section:is_char Determine if a Type is a Character]

[heading is_char]

`is_char` is a metafunction that detects if a given type is a character.

[heading Module Headers]

    #include <boost/spirit/home/support/string_traits.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T>
    struct is_char 
    {
        <unspecified>;
    };

[heading Template parameters]

[table
    [[Parameter] [Description]         [Default]]
    [[`T`]       [The type to detect.] [none]]
]

[variablelist Notation
    [[`T`]     [An arbitrary type]]
]

[heading Expression Semantics]

[table
    [[Expression]         [Semantics]]
    [[`is_char<T>::type`] [`mpl::true_` if T should be treated as a character
                           type, and `mpl::false_` otherwise. Generally,
                           any implementation of `is_char` needs to behave as 
                           if if was a __mpl_boolean_constant__.]]
]

[heading Predefined Specializations]

[table
    [[Type]      [Semantics]]
    [[`T`]       [`mpl::false_`]]
    [[`T const`] [`is_char<T>`]]
    [[`char`]    [`mpl::true_`]]
    [[`wchar_t`] [`mpl::true_`]]
]

[heading When to implement]

This customization point needs to be implemented for any strings that use a
type other than `char` or `wchar_t to store character data.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_is_string__] [Whenever `is_char` is implemented.]]
    [[__customize_char_type_of__] [Whenever `is_char` is implemented.]]
]

[/ TODO: examples ]

[endsect] [/ is_char]

[section:char_type_of Determine the Character Type of a String]

[heading char_type_of]

This customization point is an MPL metafunction which returns the character
type of a given string type. `char_type_of` handles user-defined types such as
std::string, as well as C-style strings.

[heading Module Headers]

    #include <boost/spirit/home/support/string_traits.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T>
    struct char_type_of
    {
        typedef <unspecified> type;
    };

[heading Template parameters]

[table
    [[Parameter] [Description]    [Default]]
    [[`T`]       [A string type.] [none]]
]

[variablelist Notation
    [[`T`]              [An arbitrary type.]]
    [[`N`]              [An arbitrary integral constant.]]
    [[`Char`]           [A character type.]]
    [[`Traits`]         [A character traits type.]]
    [[`Allocator`]      [A standard allocator type.]]
]

[heading Expression Semantics]

[table
    [[Expression]               [Semantics]]
    [[`char_type_of<T>::type`]  [The character type of the string type `T`.]]
]

[heading Predefined Specializations]

[table
    [[Type]  [Semantics]]
    [[`T const`]                                      [Returns `char_type_of<T>`.]]
    [[`char`]                                         [Returns `char`.]]
    [[`wchar_t`]                                      [Returns `wchar_t`.]]
    [[`char const*`]                                  [Returns `char const`.]]
    [[`wchar_t const*`]                               [Returns `wchar_t const`.]]
    [[`char*`]                                        [Returns `char`.]]
    [[`wchar_t*`]                                     [Returns `wchar_t`.]]
    [[`char[N]`]                                      [Returns `char`.]]
    [[`wchar_t[N]`]                                   [Returns `wchar_t`.]]
    [[`char const[N]`]                                [Returns `char const`.]]
    [[`wchar_t const[N]`]                             [Returns `wchar_t const`.]]
    [[`char(&)[N]`]                                   [Returns `char`.]]
    [[`wchar_t(&)[N]`]                                [Returns `wchar_t`.]]
    [[`char const(&)[N]`]                             [Returns `char const`.]]
    [[`wchar_t const(&)[N]`]                          [Returns `wchar_t const`.]]
    [[`std::basic_string<Char, Traits, Allocator>`]   [Returns `Char`.]]
]

[heading When to implement]

This customization point needs to be implemented whenever __customize_is_string__
is implemented.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_is_char__]   [For string types whose underlying character type
                                is not `char` or `wchar_t`, `is_char` must be 
                                implemented.]]
    [[__customize_is_string__] [Whenever `char_type_of` is implemented.]]
    [[__customize_extract_c_string__] [Whenever `char_type_of` is implemented.]]
]

[/ TODO: examples ]

[endsect] [/ char_type_of]

[section:extract_c_string Get a C-style String from a String Type]

[heading extract_c_string]

`extract_c_string` returns a pointer to an array of elements of a const character
type. It is invoked through a static method `call`. This customization point is
responsible for handling it's own garbage collecting; the lifetime of the returned
C-string must be no shorter than the lifetime of the string instance passed to
the `call` method. 

[heading Module Headers]

    #include <boost/spirit/home/support/string_traits.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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename String>
    struct extract_c_string
    {
        typedef <unspecified> char_type;

        static char_type const* call (String const&);
    };

[heading Template parameters]

[table
    [[Parameter] [Description]    [Default]]
    [[`String`]  [A string type.] [none]]
]

[variablelist Notation
    [[`T`]              [An arbitrary type.]]
    [[`Char`]           [A character type.]]
    [[`Traits`]         [A character traits type.]]
    [[`Allocator`]      [A standard allocator type.]]
    [[`str`]            [A string instance.]]
]

[heading Expression Semantics]

[table
    [[Expression]                       [Semantics]]
    [[`extract_c_string<T>::char_type`] [The return type of `call`.]]
    [[`extract_c_string<T>::call(str)`] [Extract a c-string of type `char_type`
                                         from `str`.]]
]

[heading Predefined Specializations]

[table
    [[Type]  [Semantics]]
    [[`T`]   [`call` takes a parameter of type `T const*`, and returns it without
              modification. An overload of `call` takes a parameter of type `T*`
              and casts it to `T const*`, returning the result. `char_type` is
              `char_type_of<T>::type`.]]
    [[`T const`]   [`call` takes a parameter `str` of type `T const` and returns
                    `extract_c_string<T>::call(str)`. `char_type` is 
                    `char_type_of<T>::type`.]]
    [[`T&`]        [`call` takes a parameter `str` of type `T&` and returns
                    `extract_c_string<T>::call(str)`. `char_type` is 
                    `char_type_of<T>::type`.]]
    [[`T const&`]  [`call` takes a parameter `str` of type `T const&` and returns
                    `extract_c_string<T>::call(str)`. `char_type` is 
                    `char_type_of<T>::type`.]]
    [[`std::basic_string<Char, Traits, Allocator>`]
     [`call` takes a parameter `str` and returns `str.c_str()`. `char_type` is
      `Char`.]]
]

[heading When to implement]

This customization point needs to be implemented whenever __customize_is_string__
is implemented.

[heading Related Attribute Customization Points]

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

[table
    [[Name]                   [When to implement]]
    [[__customize_is_char__]   [For string types whose underlying character type
                                is not `char` or `wchar_t`, `is_char` must be 
                                implemented.]]
    [[__customize_is_string__] [Whenever `extract_c_string` is implemented.]]
    [[__customize_char_type_of__] [Whenever `extract_c_string` is implemented.]]
]

[/ TODO: examples ]

[endsect] [/ string]

[endsect] [/ string_traits]

[/////////////////////////////////////////////////////////////////////////////]
[section:attribute_as Atomically Extract an Attribute Value from a Container (Karma)]

[heading attribute_as]

`attribute_as` atomically extracts an instance of a type from another type.
This customization point is used by the __karma_as__ directive. 

[heading 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.]

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T, typename Attribute, typename Enable = void>
    struct attribute_as;

[heading Template parameters]

[table
    [[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`]]
]

[variablelist Notation
    [[`attr`]       [An instance of type `Attrib`.]]
]

[heading Expression Semantics]

[table
    [[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.]]
]

[heading 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:

[table
    [[Template Parameters]     [Semantics]]
    [[__unused_type__]         [The exposed typedef `type` is defined to 
                                __unused_type__. The function `call()` returns
                                an instance of __unused_type__.]]
]

[heading When to implement]

This customization point may need to be implemented when using the __karma_as__
directive.

[/ TODO: examples ]

[endsect] [/ attribute_as]

[/////////////////////////////////////////////////////////////////////////////]
[section:auto Create Components from Attributes (Qi and Karma)]

[def __auto_parser_requirements__ [link spirit.qi.reference.auto.additional_requirements Additional Attribute Requirements for Parsers]]
[def __auto_generator_requirements__ [link spirit.karma.reference.auto.additional_requirements Additional Attribute Requirements for Generators]]
[def __auto_parser_example__ [link spirit.qi.reference.auto.example Example for Using the `qi::auto_` Parser]]
[def __auto_generator_example__ [link spirit.karma.reference.auto.example Example for Using the `karma::auto_` Generator]]

__spirit__ supports the creation of a default parser or a default generator from
a given attribute type. It implements a minimal set of predefined mappings from 
different attribute types to parsers and generators (for a description of the
predefined mappings see __auto_parser_requirements__ and 
__auto_generator_requirements__). 
The customization points described in this section (__customize_create_parser__ 
and __customize_create_generator__) can be specialized to define additional 
mappings for custom data types.

[section:create_parser Define a Custom Attribute Mapping for a Parser]

[heading create_parser]

The template `create_parser` is a type used as an customization point. It is 
invoked by the /Qi/ __create_parser__ API function in order to create
a custom mapping of the given data type to a parser expression. This 
parser expression will be returned from __create_parser__ whenever the
given data type is encountered.

[heading Module Headers]

    // forwards to <boost/spirit/home/qi/auto.hpp>
    #include <boost/spirit/include/qi_auto.hpp>

Also, see __include_structure__.

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T, typename Enable>
    struct create_parser
    {
        typedef <unspecified> type; 
        static type const& call();
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`T`]                  [The type, `T` for which a custom mapping to a 
                             parser should be established.]            [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `create_generator` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`T`]              [An arbitrary type.]]
]

[heading Expression Semantics]

[table
    [[Expression]                             [Semantics]]
    [[`create_parser<T>::type`]               [Defines the type of the parser 
                                               expression returned from `call`.]]
    [[`create_parser<T>::call()`]             [Returns a parser expression (usually
                                               this is a proto::expression) to be used
                                               as the default parser for the given 
                                               type, `T`.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. All predefined mappings are listed here: __auto_parser_requirements__.

[note It is possible to overload the predefined mappings for the listed types 
      by providing your own specialization of the `create_parser` customization
      point for the type to modify.]

[heading When to implement]

The customization point `create_parser` needs to be implemented for a specific 
type whenever this type should be usable with the API function __create_parser__ 
(which includes using the `qi::auto_` parser and the special API functions
based on the automatic creation of the matching parser type). 

[heading Example]

For an example of how to use the customization point `create_parser` 
please see here: __auto_parser_example__.

[endsect]

[section:create_generator Define a Custom Attribute Mapping for a Generator]

[heading create_generator]

The template `create_generator` is a type used as an customization point. It is 
invoked by the /Karma/ __create_generator__ API function in order to create
a custom mapping of the given data type to a generator expression. This 
generator expression will be returned from __create_generator__ whenever the
given data type is encountered.

[heading Module Headers]

    // forwards to <boost/spirit/home/karma/auto.hpp>
    #include <boost/spirit/include/karma_auto.hpp>

Also, see __include_structure__.

[heading Namespace]

[table
    [[Name]]
    [[`boost::spirit::traits`]]
]

[heading Synopsis]

    template <typename T, typename Enable>
    struct create_generator
    {
        typedef <unspecified> type; 
        static type const& call();
    };

[heading Template parameters]

[table
    [[Parameter]            [Description]               [Default]]
    [[`T`]                  [The type, `T` for which a custom mapping to a 
                             generator should be established.]            [none]]
    [[`Enable`]             [Helper template parameter usable to selectively
                             enable or disable certain specializations
                             of `create_generator` utilizing SFINAE (i.e.
                             `boost::enable_if` or `boost::disable_if`).] [`void`]]
]

[variablelist Notation 
    [[`T`]              [An arbitrary type.]]
]

[heading Expression Semantics]

[table
    [[Expression]                             [Semantics]]
    [[`create_generator<T>::type`]            [Defines the type of the generator 
                                               expression returned from `call`.]]
    [[`create_generator<T>::call()`]          [Returns a generator expression (usually
                                               this is a proto::expression) to be used
                                               as the default generator for the given 
                                               type, `T`.]]
]

[heading Predefined Specializations]

__spirit__ predefines specializations of this customization point for  
several types. All predefined mappings are listed here: __auto_generator_requirements__.

[note It is possible to overload the predefined mappings for the listed types 
      by providing your own specialization of the `create_generator` customization
      point for the type to modify.]

[heading When to implement]

The customization point `create_generator` needs to be implemented for a specific 
type whenever this type should be usable with the API function __create_generator__ 
(which includes using the `karma::auto_` generator and the special API functions
based on the automatic creation of the matching generator type). 

[heading Example]

For an example of how to use the customization point `create_generator` 
please see here: __auto_generator_example__.

[endsect]

[endsect]

[endsect] [/ customize]