Define a Custom Attribute Mapping for a Generator

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##### Define a Custom Attribute Mapping for a Generator

###### 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.

###### Module Headers

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

Also, see Include Structure.

###### Namespace

Name
`boost::spirit::traits`

###### Synopsis

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

###### Template parameters

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`

###### Notation

`T`

  An arbitrary type.

###### Expression Semantics

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`.

###### Predefined Specializations

Spirit predefines specializations of this customization point for several types. All predefined mappings are listed here: Additional Attribute Requirements for Generators.

| Note
It is possible to overload the predefined mappings for the listed types by providing your own specialization of the `create_generator` cutomization point for the type to modify.

###### 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).

###### Example

For an example of how to use the customization point `create_generator` please see here: Example for Using the `karma::auto_` Generator.