Sum - adding numbers

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#### Sum - adding numbers

Here's a parser that sums a comma-separated list of numbers.

Ok we've glossed over some details in our previous examples. First, our includes:

    #include <boost/spirit/include/qi.hpp>
    #include <boost/spirit/include/phoenix_core.hpp>
    #include <boost/spirit/include/phoenix_operator.hpp>
    #include <iostream>
    #include <string>

Then some using directives:

    namespace qi = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;
    namespace phoenix = boost::phoenix;

    using qi::double_;
    using qi::_1;
    using ascii::space;
    using phoenix::ref;

Namespace | Description
--- | ---
boost::phoenix | All of phoenix
boost::spirit | All of spirit
boost::spirit::qi | All of spirit.qi
boost::spirit::ascii | ASCII version of `char_` and all char related parsers. Other encodings are also provided (e.g. also an ISO8859.1)
boost::spirit::arg_names | Special phoenix placeholders for spirit

| Note
If you feel uneasy with using whole namespaces, feel free to qualify your code, use namespace aliases, etc. For the purpose of this tutorial, we will be presenting unqualified names for both Spirit and Phoenix. No worries, we will always present the full working code, so you won't get lost. In fact, all examples in this tutorial have a corresponding cpp file that QuickBook (the documentation tool we are using) imports in here as code snippets.

Now the actual parser:

    template <typename Iterator>
    bool adder(Iterator first, Iterator last, double& n)
    {
        bool r = qi::phrase_parse(first, last,

            //  Begin grammar
            (
                double_[ref(n) = _1] >> *(',' >> double_[ref(n) += _1])
            )
            ,
            //  End grammar

            space);

        if (first != last) // fail if we did not get a full match
            return false;
        return r;
    }

The full cpp file for this example can be found here: ../../example/qi/sum.cpp

This is almost like our original numbers list example. We're incrementally building on top of our examples. This time though, like in the complex number example, we'll be adding the smarts. There's an accumulator (`double& n`) that adds the numbers parsed. On a successful parse, this number is the sum of all the parsed numbers.

The first `double_` parser attaches this action:

    ref(n) = _1

This assigns the parsed result (actually, the attribute of `double_`) to `n`. `ref(n)` tells Phoenix that `n` is a mutable reference. `_1` is a Phoenix placeholder for the parsed result attribute.

The second `double_` parser attaches this action:

    ref(n) += _1

So, subsequent numbers add into `n`.

That wasn't too bad, was it :-) ?