Assign default value to variable using boost spirit

Question

Suppose I have the following string to parse:

\"1.2, 2.0, 3.9\"

and when I apply the following parser for it:

struct DataStruct
{
    dou         


        

Answer 1

You might use the following code:

#include 
#include 
#include 
#include 
struct DataStruct
{
    double n1, n2, n3;
};

BOOST_FUSION_ADAPT_STRUCT(DataStruct, (double, n1)(double, n2)(double, n3))

int main() {
    using namespace boost::spirit;

    using Iterator = typename std::string::iterator;

    qi::rule reader_ = (qi::double_[_val = _1] | (lexeme["null"])[_val = 0.]);

    qi::rule data_ = reader_ >> ',' >> reader_ >> ',' >> reader_;

    try {
        DataStruct res;
        auto str = std::string("1.2,null,3.9");
        auto start = str.begin();
        auto end = str.end();

        bool ok = qi::parse(start, end, data_, res);
        if (ok) {
            std::cout << "parse completed" << std::endl;
        }
        else {
            std::cout << "parse failed" << std::endl;
        }
    }
    catch (const qi::expectation_failure& except) {
        std::cout << except.what();
    }

    std::cout << std::endl;
}