Boost Spirit semantic actions on non-Fusion adapted structs

Question

Say if I have a simple struct like:

struct huh { char xxx; };  

Without going to Boost Fusion adapted structs, I'd like to find a simpler proxy for operating on a member variable for in a Spirit rule's semantic action. Now this, of course, doesn't work:

_val.xxx = _1

but I'd like to get a little cleaner than binding with phoenix:

bind(&huh::xxx, _val) =  _1 

Here is a working example of the simple picture that compiles on a clang and a g++:

#define BOOST_SPIRIT_USE_PHOENIX_V3 1
#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/phoenix/bind/bind_member_variable.hpp>

struct huh { char xxx; };

int main() {
    namespace qi = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;
    namespace phx   = boost::phoenix;

    qi::rule<std::string::const_iterator, huh(), ascii::space_type> start;

    start =  qi::char_[
        phx::bind(&huh::xxx, qi::_val) =  qi::_1 
    ];

    return 0;
}

Any ideas?

Answer 1

Binding with fusion is exactly the glue to make it "a little cleaner than binding with phoenix".

So, there's your solution. Alternatively, factor your expression templates, maybe:

auto _xxx = phx::bind(&huh::xxx, _val);

So you can write:

start =  char_[ _xxx = _1 ];

Alternatively, use a function or a customization point:

• boost spirit semantic action parameters

Customization point

Adding a trait

namespace boost { namespace spirit { namespace traits {

    template <> struct assign_to_attribute_from_value<huh, char, void> {
        static void call(char c, huh& attr) { attr.xxx = c; }
    };

} } }

You can now simply write

qi::rule<std::string::const_iterator, huh(), ascii::space_type> start;

start = qi::char_;

Live On Coliru

#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>

struct huh { char xxx; };

namespace boost { namespace spirit { namespace traits {

    template <> struct assign_to_attribute_from_value<huh, char, void> {
        static void call(char c, huh& attr) { attr.xxx = c; }
    };

} } }

int main() {
    namespace qi    = boost::spirit::qi;
    namespace ascii = boost::spirit::ascii;

    qi::rule<std::string::const_iterator, huh(), ascii::space_type> start;

    start = qi::char_;
}