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#### Semantic Actions with Generators

###### Description

Semantic actions may be attached to any point in the grammar specification. They allow to call a function or function object in order to provide the value output by the generator attached to the semantic action. Semantic actions are associated with a generator using the syntax `g[]`, where `g` is an arbitrary generator expression.

###### Header

    // forwards to <boost/spirit/home/karma/action.hpp>
    #include <boost/spirit/include/karma_action.hpp>

Also, see Include Structure.

###### Model of

> `UnaryGenerator`

Notation

`a`, `g`

  Instances of a generator, `G`

`A`

  Attribute type exposed by a generator, `a`

`fa`

  A (semantic action) function with signature `void(Attrib&, Context&, bool&)`. The third parameter is a boolean flag that can be set to false to force the generator to fail. Both `Context` and the boolean flag are optional. For more information see below.

`Attrib`

  The attribute to be used to generate output from.

`Context`

  The type of the generator execution context. For more information see below.

###### Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in `UnaryGenerator`.

Expression | Semantics
--- | ---
`g[fa]` | Call semantic action, `fa` before invoking `g`. The function or function object `fa` is expected to provide the value to generate output from to the generator `g`.

The possible signatures for functions to be used as semantic actions are:

    template <typename Attrib>
    void fa(Attrib& attr);

    template <typename Attrib, typename Context>
    void fa(Attrib& attr, Context& context);

    template <typename Attrib, typename Context>
    void fa(Attrib& attr, Context& context, bool& pass);

The function or function object is expected to return the value to generate output from by assigning it to the first parameter, `attr`. Here `Attrib` is the attribute type of the generator attached to the semantic action.

The type `Context` is the type of the generator execution context. This type is unspecified and depends on the context the generator is invoked in. The value `context` is used by semantic actions written using Boost.Phoenix to access various context dependent attributes and values. For more information about Boost.Phoenix placeholder expressions usable in semantic actions see `Nonterminal`.

The third parameter, `pass`, can be used by the semantic action to force the associated generator to fail. If pass is set to `false` the action generator will immediately return `false` as well, while not invoking `g` and not generating any output.

###### Attributes

Expression | Attribute
--- | ---
`a[fa]` | `a: A --> a[fa]: A`

###### Complexity

The complexity of the action generator is defined by the complexity of the generator the semantic action is attached to and the complexity of the function or function object used as the semantic action.

| Important
Please note that the use of semantic actions in Spirit.Karma generally forces the library to create a copy of the attribute, which might be a costly operation. Always consider using other means of associating a value with a particular generator first.

###### Example

| Note
The test harness for the example(s) below is presented in the Basics Examples section.

Some includes:

    #include <boost/spirit/include/karma.hpp>
    #include <boost/spirit/include/support_utree.hpp>
    #include <boost/spirit/include/phoenix_core.hpp>
    #include <boost/spirit/include/phoenix_operator.hpp>
    #include <boost/fusion/include/std_pair.hpp>
    #include <iostream>
    #include <string>

Some using declarations:

    using boost::spirit::karma::int_;
    using boost::spirit::karma::string;
    using boost::spirit::karma::_1;
    using boost::phoenix::ref;
    using boost::phoenix::val;

Some examples:

    int i = 42;
    test_generator("42", int_[_1 = ref(i)]);
    test_generator("abc", string[_1 = val("abc")]);

More examples for semantic actions can be found here: Examples of Semantic Actions.