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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/home/x3.hpp> #include <iostream> #include <string>
Then some using directives:
namespace x3 = boost::spirit::x3; namespace ascii = boost::spirit::x3::ascii; using x3::double_; using ascii::space; using x3::_attr;
Now the actual parser:
template <typename Iterator> bool adder(Iterator first, Iterator last, double& n) { auto assign = [&](auto& ctx){ n = _attr(ctx); }; auto add = [&](auto& ctx){ n += _attr(ctx); }; bool r = x3::phrase_parse(first, last, // Begin grammar ( double_[assign] >> *(',' >> double_[add]) ) , // 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: 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:
[&](auto& ctx){ n = _attr(ctx); }
This assigns the parsed result (actually, the attribute of double_
) to n
.
The second double_
parser
attaches this action:
[&](auto& ctx){ n += _attr(ctx); }
So, subsequent numbers add into n
.
That wasn't too bad, was it :-) ?