...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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/phoenix/core.hpp> #include <boost/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 |
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 Boost.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
Boost.Phoenix
that n
is a mutable reference.
_1
is a Boost.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 :-) ?