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Store a Parsed Attribute Value into a Container (Qi)
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 (Kleene, Plus, List, and Repeat) to store a parsed attribute value into a container.

Module Headers
#include <boost/spirit/home/support/container.hpp>

Also, see Include Structure.

[Note] Note

This header file does not need to be included directly by any user program as it is normally included by other Spirit header files relying on its content.

Namespace

Name

boost::spirit::traits

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

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 traits::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

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

Expression Semantics

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.

Predefined Specializations

Spirit predefines specializations of this customization point for several types. The following table lists those types together with the types exposed and the corresponding semantics:

Template Parameters

Semantics

C, 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.

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 (Spirit.Qi) only. As a rule of thumb: it has to be implemented whenever a certain type is to be passed as an attribute to a parser normally exposing a STL container and if the type does not expose the interface of a STL container (i.e. no 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.

Related Attribute Customization Points

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

Name

When to implement

traits::container_value

Qi: List, Kleene, Plus, Repeat.

traits::clear_value

Qi: List, Kleene, Plus, Repeat.

Example

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

template <typename Container, typename T, typename Enable/* = void*/>
struct push_back_container
{
    static bool call(Container& c, T const& val)
    {
        c.insert(c.end(), val);
        return true;
    }
};

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:

template <typename Container>
bool push_back(Container&, unused_type)
{
    return true;
}

which defines an empty member function call().


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