Using lambdas or external functions as spirit.qi semantic actions

Question

Iʼm playing with creating Boost.Spirit.Qi-based parsing. Having an example like calc_utree, I'm trying to extend what to use as semantic action.

It's trivial to reuse the same approach with an alone assignment as semantic action, for example

        term =
            factor [_val = _1]

as literally in the example. But, when I try to pass both into a function (method) external to the rule definition, or even write it as lambda, for example

        term =
            factor [([&] {_val = _1; })]

it causes silent misassignment in that place: _val remains unchanged (without any error or warning from compiler). The same if I change it to something like

        term =
            factor [do_term_factor(_val, _1)]
<...>
template <typename D, typename S>
D& do_term_factor(D& d, S& s) {
    d = s;
}

Seems Iʼve fallen into principal misconception with Qi and Phoenix. The questions are (principally they are different forms of the same one):

• What is specific with Phoenix variables that they donʼt work in C++ lambdas?
• How to make it working with such external action call?

Alternatively, how _val can be achieved without Phoenix? Spirit documentation seems quite obscure in this.

Environment details: Boost 1.58.0, gcc 5.4.0 or clang 4.0 (all of Ubuntu 16.04).

Answer 1

The links provided by @llonesmiz are excellent.

In short: you need to adapt functions to be lazy actors. _val = _1 is that too, but "magically" generated using expression templates.

For "regular" calleables you have

• boost::phoenix::function
• boost::phoenix::bind
• BOOST_FUNCTION_ADAPT_*

Here's a small parade

Live On Coliru

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

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

//////////////////// bindables
struct GenericDoubler {
    template <typename T>
    auto operator()(T const& v) const { return v * 2; }
};
static const px::function<GenericDoubler> s_genericDoubler;

template <typename T>
T freeGenericDouble(T const& v) { return v * 2; }

BOOST_PHOENIX_ADAPT_FUNCTION(int, genericDouble_, freeGenericDouble, 1)

/////////////////// raw actors
int main() {
    using It = std::string::const_iterator;
    std::string const input = "42";

    using namespace qi::labels;

    for (auto rule : std::vector<qi::rule<It, int()> > { 
                // binds
                qi::int_ [ _val = 2*_1 ],
                qi::int_ [ _val = px::bind([](int i) { return 2*i; }, _1) ],
                qi::int_ [ _val = px::bind(GenericDoubler(), _1) ],
                qi::int_ [ _val = px::bind(&freeGenericDouble<int>, _1) ],
                qi::int_ [ _val = genericDouble_(_1) ],
                qi::int_ [ _val = s_genericDoubler(_1) ],
                // actors
                qi::int_ [ ([](int const& /*attribute*/, auto& /*context*/, bool& pass) { 
                        // context is like boost::spirit::context<boost::fusion::cons<int&, boost::fusion::nil_>, boost::fusion::vector<> >
                        pass = false;
                    }) ],
                qi::int_ [ ([](int& attribute, auto& context, bool& pass) { 
                        int& exposed = boost::fusion::at_c<0>(context.attributes);
                        exposed = 2*attribute;
                        pass = true;
                    }) ],
            }) 
    {
        It f = begin(input), l = end(input);
        int data = 99;
        if (parse(f, l, rule, data))
            std::cout << "Parsed: " << data << " ";
        else
            std::cout << "Parse failed at '" << std::string(f,l) << "' ";

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

Prints

Parsed: 84 
Parsed: 84 
Parsed: 84 
Parsed: 84 
Parsed: 84 
Parsed: 84 
Parse failed at '42' Remaining: '42'
Parsed: 84