Why can I not access the value in a semantic action?

Question

I'm trying to write a parser to create an AST using boost::spirit. As a first step I'm trying to wrap numerical values in an AST node. This is the code I'm using:

AST_NodePtr make_AST_NodePtr(const int& i) {
    return std::make_shared<AST_Node>(i);
}

namespace qi = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;
namespace l = qi::labels;

 template<typename Iterator>
    struct test_grammar : qi::grammar<Iterator, AST_NodePtr(), ascii::space_type> {
        test_grammar() : test_grammar::base_type(test) {

            test = qi::int_ [qi::_val = make_AST_NodePtr(qi::_1)];
        }
        qi::rule<Iterator, AST_NodePtr(), ascii::space_type> test;

    }; 

As far as I understood it from the documentation q::_1 should contain the value parsed by qi::int_, but the above code always gives me an error along the lines

invalid initialization of reference of type ‘const int&’ from expression of type ‘const _1_type {aka const boost::phoenix::actor<boost::spirit::argument<0> >}

Why does this not work even though qi::_1 is supposed to hold the parsed valued? How else would I parse the input into a custom AST?

Answer 1

You're using a regular function inside the semantic action.

This means that in the contructor the compiler will try to invoke that make_AST_NodePtr function with the argument supplied: qi::_1.

Q. Why does this not work even though qi::_1 is supposed to hold the parsed valued?

A. qi::_1 does not hold the parsed value. It represents (is-a-placeholder-for) the first unbound argument in the function call

This can /obviously/ never work. The function expects an integer.

So what gives?

---

You need to make a "lazy" or "deferred" function for use in the semantic action. Using only pre-supplied Boost Phoenix functors, you could spell it out:

test  = qi::int_ [ qi::_val = px::construct<AST_NodePtr>(px::new_<AST_Node>(qi::_1)) ];

You don't need the helper function this way. But the result is both ugly and suboptimal. So, let's do better!

Using a Phoenix Function wrapper

struct make_shared_f {
    std::shared_ptr<AST_Node> operator()(int v) const {
        return std::make_shared<AST_Node>(v);
    }
};
px::function<make_shared_f> make_shared_;

With this defined, you can simplify the semantic action to:

test  = qi::int_ [ qi::_val = make_shared_(qi::_1) ];

Actually, if you make it generic you can reuse it for many types:

template <typename T>
    struct make_shared_f {
        template <typename... Args>
            std::shared_ptr<T> operator()(Args&&... args) const {
                return std::make_shared<T>(std::forward<Args>(args)...);
            }
    };
px::function<make_shared_f<AST_Node> > make_shared_;

DEMO

Here's a self-contained example showing some style fixes in the process:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <memory>

struct AST_Node {
    AST_Node(int v) : _value(v) {}
    int value() const { return _value; }
  private:
    int _value;
};

using AST_NodePtr = std::shared_ptr<AST_Node>;

AST_NodePtr make_AST_NodePtr(const int& i) {
    return std::make_shared<AST_Node>(i);
}

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

template<typename Iterator>
struct test_grammar : qi::grammar<Iterator, AST_NodePtr()> {
    test_grammar() : test_grammar::base_type(start) {
        using boost::spirit::ascii::space;

        start = qi::skip(space) [ test ];

        test  = qi::int_ [ qi::_val = make_shared_(qi::_1) ];
    }
  private:
    struct make_shared_f {
        std::shared_ptr<AST_Node> operator()(int v) const {
            return std::make_shared<AST_Node>(v);
        }
    };
    px::function<make_shared_f> make_shared_;
    //
    qi::rule<Iterator, AST_NodePtr()> start;
    qi::rule<Iterator, AST_NodePtr(), boost::spirit::ascii::space_type> test;
}; 

int main() {
AST_NodePtr parsed;

    std::string const input ("42");
    auto f = input.begin(), l = input.end();
    test_grammar<std::string::const_iterator> g;
    bool ok = qi::parse(f, l, g, parsed);

    if (ok) {
        std::cout << "Parsed: " << (parsed? std::to_string(parsed->value()) : "nullptr") << "\n";
    } else {
        std::cout << "Failed\n";
    }

    if (f!=l)
    {
        std::cout << "Remaining input: '" << std::string(f, l) << "'\n";
    }
}

Prints

Parsed: 42

BONUS: Alternative using BOOST_PHOENIX_ADAPT_FUNCTION

You can actually use your free function if you wish, and use it as follows:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <memory>

struct AST_Node {
    AST_Node(int v) : _value(v) {}
    int value() const { return _value; }
  private:
    int _value;
};

using AST_NodePtr = std::shared_ptr<AST_Node>;

AST_NodePtr make_AST_NodePtr(int i) {
    return std::make_shared<AST_Node>(i);
}

BOOST_PHOENIX_ADAPT_FUNCTION(AST_NodePtr, make_AST_NodePtr_, make_AST_NodePtr, 1)

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

template<typename Iterator>
struct test_grammar : qi::grammar<Iterator, AST_NodePtr()> {
    test_grammar() : test_grammar::base_type(start) {
        using boost::spirit::ascii::space;

        start = qi::skip(space) [ test                            ] ;
        test  = qi::int_        [ qi::_val = make_AST_NodePtr_(qi::_1) ] ;
    }
  private:
    qi::rule<Iterator, AST_NodePtr()> start;
    qi::rule<Iterator, AST_NodePtr(), boost::spirit::ascii::space_type> test;
}; 

int main() {
AST_NodePtr parsed;

    std::string const input ("42");
    auto f = input.begin(), l = input.end();
    test_grammar<std::string::const_iterator> g;
    bool ok = qi::parse(f, l, g, parsed);

    if (ok) {
        std::cout << "Parsed: " << (parsed? std::to_string(parsed->value()) : "nullptr") << "\n";
    } else {
        std::cout << "Failed\n";
    }

    if (f!=l)
    {
        std::cout << "Remaining input: '" << std::string(f, l) << "'\n";
    }
}