How to use spirit X3 parse into a class with constructor containing parameters？ - c++14

Question

I am a new man on using spirit x3, I read some document from official site or from other github repositories. But, I can not find how to parse into a class with parameters. I referred to the former question: Boost-Spirit (X3) parsing without default constructors
I wrote a sample to test it, I will present my codes in the following area. My pain is how to use x3::_attr, and how to pass parsed parameters to the class constructor?
#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <vector>
struct MyPair {
MyPair(int x, int y) : mx(x), my(y) {};
int mx;
int my;
};
class MyDu {
public:
MyDu() {};
MyDu(int x, int y) : mx(x), my(y) {};
int mx;
int my;
};
int main()
{
namespace x3 = boost::spirit::x3;
using x3::int_;
std::vector<MyPair> pairs;
MyDu myDu;
char const *first = "11:22", *last = first + std::strlen(first);
//auto pair = x3::rule<struct pair_, std::vector<MyPair> >{}
// = (int_ >> ':' >> int_)
// [([&](auto& ctx) {
// auto& attr = x3::_attr(ctx);
// using boost::fusion::at_c;
// return x3::_val(ctx).emplace_back(at_c<0>(attr), at_c<1>(attr));
// })]
//;
auto pair = x3::rule<class MyDu_, MyDu >{}
= (int_ >> ':' >> int_)
[([&](auto& ctx) {
auto& attr = x3::_attr(ctx);
using boost::fusion::at_c;
//return x3::_val(ctx)(at_c<0>(attr), at_c<1>(attr));
ctx = MyDu(at_c<0>(attr), at_c<1>(attr));
return x3::_val(ctx);
})]
;
//bool parsed_some = parse(first, last, pair % ',', pairs);
bool parsed_some = parse(first, last, pair, myDu);
if (parsed_some) {
std::cout << "Parsed the following pairs" << std::endl;
//for (auto& p : pairs) {
// std::cout << p.mx << ":" << p.my << std::endl;
//}
std::cout<<myDu.mx<<","<<myDu.my<<std::endl;
}
system("pause");
}
Any one who can fix my error, and parse into a class in my code ? Thanks!

Answer 1

Perhaps you were missing the way to assign to the rule's value using _val:
Live On Coliru
#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <vector>
struct MyDu {
MyDu(int x, int y) : mx(x), my(y){};
int mx;
int my;
};
int main() {
namespace x3 = boost::spirit::x3;
using x3::int_;
MyDu myDu{1,2};
std::string const s = "11:22";
auto assign = [](auto& ctx) {
using boost::fusion::at_c;
auto& attr = x3::_attr(ctx);
x3::_val(ctx) = MyDu(at_c<0>(attr), at_c<1>(attr));
};
auto pair = x3::rule<class MyDu_, MyDu>{} = (int_ >> ':' >> int_)[assign];
if (parse(begin(s), end(s), pair, myDu)) {
std::cout << "Parsed: " << myDu.mx << ", " << myDu.my << "\n";
}
}
Prints
Parsed: 11, 22

Answer 2

Oh, fantastic! Many thanks, sehe, you help me solve the problem bothering me for some while.
In fact I can not find document on spirit how to use attr, i only find a doc from "Ruben-Van-Boxem-Parsing-CSS-in-C-with-Boost-Spirit-X3",
_val :A reference to the attribute of the innermost rule invoking _where :the parser Iterator range to the input stream
_attr : A reference to the a˛ribute of the parser
_pass: A reference to a bool flag that can be used to force the parser to fail
could you share some info on these parameters. Many thanks again!