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#### Generator Semantic Actions

In the previous section we mentioned a very important difference between parsers and generators. While parsers may be used without 'producing' any data, generators always need data to generate the output from. We mentioned one way of passing data to the generator by supplying it as a parameter to one of the main API functions (for instance `generate()` or `generate_delimited()`). But sometimes this is not possible or not desirable.

Very much like for Spirit.Qi we have semantic actions in Spirit.Karma as well. Semantic actions may be attached to any point in the grammar specification. These actions are C++ functions or function objects that are called whenever a part of the generator is about to be invoked. Say you have a generator `G`, and a C++ function `F`, you can make the generator call `F` just before it gets invoked by attaching `F`:

    G[F]

The expression above links `F` to the generator, `G`.

Semantic actions in Spirit.Qi are invoked after a parser successfully matches its input and the matched value is passed into the semantic action. In Spirit.Karma the opposite happens. Semantic actions are called before its associated generator is invoked. They may provide the data required by the generator.

The function/function object signature depends on the type of the generator to which it is attached. The generator `double_` expects the number to generate. Thus, if we were to attach a function `F` to `double_`, we need `F` to be declared as:

    void F(double& n);

where the function is expected to initialize the parameter `n` with the value to generate.

| Important

Generally, and more formally, the semantic action `F` attached to a generator `G` needs to take a reference to the generator's attribute type as its first parameter. For more information about generator attributes please see the section Generator Attributes.

In the example above the function F takes a `double&` as its first parameter as the attribute of the `double_` generator happens to be a `double`.

There are actually 2 more arguments being passed (the generator context and a reference to a boolean 'pass' parameter). We don't need these, for now, but we'll see more on these other arguments later. Spirit.Karma allows us to bind a single argument function, like above. The other arguments are simply ignored.

To sum up, the possible signatures for semantic actions are:

    void f(Attrib&);
    void f(Attrib&, Context&);
    void f(Attrib&, Context&, bool&);

###### Examples of Semantic Actions

In the following example we present various ways to attach semantic actions:

- Using a plain function pointer
- Using a simple function object
- Using Boost.Bind with a plain function
- Using Boost.Bind with a member function
- Using Boost.Lambda

Let's assume we have:

    namespace client
    {
        namespace karma = boost::spirit::karma;

        // A plain function
        void read_function(int& i)
        {
            i = 42;
        }

        // A member function
        struct reader
        {
            void print(int& i) const
            {
                i = 42;
            }
        };

        // A function object
        struct read_action
        {
            void operator()(int& i, unused_type, unused_type) const
            {
                i = 42;
            }
        };
    }

Take note that with function objects, we need to have an `operator()` with 3 arguments. Since we don't care about the other two, we can use `unused_type` for these. We'll see more of `unused_type` elsewhere. Get used to it. `unused_type` is a Spirit supplied support class. Most of the time it stands for 'I don't care, just use the appropriate default'.

All following examples generate outputs of the form:

    "{integer}"

An integer inside the curly braces.

The first example shows how to attach a plain function:

    generate(outiter, '{' << int_[&read_function] << '}');

What's new? Well `int_` is the sibling of `double_`. I'm sure you can guess what this generator does and what type of attribute it expects.

The next example shows how to attach a simple function object:

    generate(outiter, '{' << int_[read_action()] << '}');

We can use Boost.Bind to 'bind' member functions:

    reader r;
    generate(outiter, '{' << int_[boost::bind(&reader::print, &r, _1)] << '}');

Likewise, we can also use Boost.Bind to 'bind' plain functions:

    generate(outiter, '{' << int_[boost::bind(&read_function, _1)] << '}');

And last but not least, we can also use Boost.Lambda:

    std::stringstream strm("42");
    generate(outiter, '{' << int_[strm >> lambda::_1] << '}');

There are more ways to bind semantic action functions, but the examples above are the most common. Attaching semantic actions is the first hurdle one has to tackle when getting started with generating with Spirit. If you didn't do so yet, it is probably a good idea to familiarize yourself with the tools behind it such as Boost.Bind and Boost.Lambda.

The examples above can be found here: actions.cpp

###### Phoenix

Boost.Phoenix, a companion library bundled with Spirit, is specifically suited for binding semantic actions. It is like Boost.Lambda on steroids, with special custom features that make it easy to integrate semantic actions with Spirit. If your requirements go beyond simple to moderate generation, I suggest you use this library. Examples presented henceforth shall be using the Phoenix library exclusively.

| Important

There are different ways to write semantic actions for Spirit.Karma: using plain functions, Boost.Bind, Boost.Lambda, or Boost.Phoenix. The latter three allow you to use special placeholders to control parameter placement (`_1`, `_2`, etc.). Each of those libraries has it's own implementation of the placeholders, all in different namespaces. You have to make sure not to mix placeholders with a library they don't belong to and not to use different libraries while writing a semantic action.

Generally, for Boost.Bind, use `::_1`, `::_2`, etc. (yes, these placeholders are defined in the global namespace).

For Boost.Lambda use the placeholders defined in the namespace `boost::lambda`.

For semantic actions written using Boost.Phoenix use the placeholders defined in the namespace `boost::spirit`. Please note that all existing placeholders for your convenience are also available from the namespace `boost::spirit::karma`.