Boost fusion sequence type and name identification for structs and class

Question

I am trying to use boost fusion for one of my projects and I an figuring out how to get type names and variable names for structures and classes.

#include &         


        

Answer 1

I distilled the answer by sehe into something much simpler, provided you are using C++14

#include <iostream>

#include <boost/fusion/include/algorithm.hpp>
#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/mpl/range_c.hpp>

struct MyStruct {
    std::string foo;
    double bar;
};

BOOST_FUSION_ADAPT_STRUCT(MyStruct,
                          foo,
                          bar)

namespace fuz = boost::fusion;
namespace mpl = boost::mpl;

int main(int argc, char* argv[]) {
  MyStruct dummy{"yo",3.14};

  fuz::for_each(mpl::range_c<
                unsigned, 0, fuz::result_of::size<MyStruct>::value>(),
                [&](auto index){
      std::cout << "Name: "
                << fuz::extension::struct_member_name<MyStruct,index>::call()
                << " Value: "
                << fuz::at_c<index>(dummy) << std::endl; 
    });

}

Outputs:

Name: foo Value: yo
Name: bar Value: 3.14

See it live on coliru

Answer 2

There's boost::fusion::extension::struct_member_name<S, N::value> to access the names.

Here's a generic fusion object visitor that I use:

namespace visitor {

    template <typename Flavour, typename T> struct VisitorApplication;

    namespace detail
    {
        template <typename V, typename Enable = void>
        struct is_vector : boost::mpl::false_ { };

        template <typename T>
        struct is_vector<std::vector<T>, void> : boost::mpl::true_ { };

        namespace iteration
        {
            // Iteration over a sequence
            template <typename FusionVisitorConcept, typename S, typename N>
                struct members_impl
                {
                    // Type of the current member
                    typedef typename boost::fusion::result_of::value_at<S, N>::type   current_t;
                    typedef typename boost::mpl::next<N>::type                        next_t;
                    typedef boost::fusion::extension::struct_member_name<S, N::value> name_t;

                    static inline void handle(FusionVisitorConcept& visitor, const S& s)
                    {
                        visitor.start_member(name_t::call());
                        VisitorApplication<FusionVisitorConcept, current_t>::handle(visitor, boost::fusion::at<N>(s));
                        visitor.finish_member(name_t::call());
                        members_impl<FusionVisitorConcept, S, next_t>::handle(visitor, s);
                    }
                };

            // End condition of sequence iteration
            template <typename FusionVisitorConcept, typename S>
                struct members_impl<FusionVisitorConcept, S, typename boost::fusion::result_of::size<S>::type>
                {
                    static inline void handle(FusionVisitorConcept const&, const S&) { /*Nothing to do*/ }
                };

            // Iterate over struct/sequence. Base template
            template <typename FusionVisitorConcept, typename S>
                struct Struct : members_impl<FusionVisitorConcept, S, boost::mpl::int_<0>> {};

    } // iteration

    template <typename FusionVisitorConcept, typename T>
        struct array_application
        {
            typedef array_application<FusionVisitorConcept, T> type;

            typedef typename T::value_type value_type;

            static inline void handle(FusionVisitorConcept& visitor, const T& t)
            {
                visitor.empty_array();
                for (auto& el : t)
                    VisitorApplication<FusionVisitorConcept, value_type>::handle(visitor, el);
            }
        };

    template <typename FusionVisitorConcept, typename T>
        struct struct_application
        {
            typedef struct_application<FusionVisitorConcept, T> type;

            static inline void handle(FusionVisitorConcept& visitor, const T& t)
            {
                visitor.empty_object();
                iteration::Struct<FusionVisitorConcept, T>::handle(visitor, t);
            }
        };

    template <typename FusionVisitorConcept, typename T, typename Enable = void>
        struct value_application
        {
            typedef value_application<FusionVisitorConcept, T> type;

            static inline void handle(FusionVisitorConcept& visitor, const T& t) {
                visitor.value(t);
            }
        };

    template <typename FusionVisitorConcept, typename T>
        struct value_application<FusionVisitorConcept, boost::optional<T> >
        {
            typedef value_application<FusionVisitorConcept, boost::optional<T> > type;

            static inline void handle(FusionVisitorConcept& visitor, const boost::optional<T>& t) {
                if (t)
                    VisitorApplication<FusionVisitorConcept, T>::handle(visitor, *t);
                else
                    ; // perhaps some default action?
            }
        };

    template <typename FusionVisitorConcept, typename T>
        struct select_application
        {
            typedef
                //typename boost::mpl::eval_if<boost::is_array<T>,                  boost::mpl::identity<array_application<FusionVisitorConcept, T>>,
                typename boost::mpl::eval_if<detail::is_vector<T>,                  boost::mpl::identity<array_application <FusionVisitorConcept, T>>,
                typename boost::mpl::eval_if<boost::fusion::traits::is_sequence<T>, boost::mpl::identity<struct_application<FusionVisitorConcept, T>>,
                boost::mpl::identity<value_application<FusionVisitorConcept, T>>
                > >::type type;
        };

    } // detail

    template <typename FusionVisitorConcept, typename T>
        struct VisitorApplication : public detail::select_application<FusionVisitorConcept, T>::type
    {
    };
}

template <typename FusionVisitorConcept, typename T>
void apply_fusion_visitor(FusionVisitorConcept& visitor, T const& o)
{
    visitor::VisitorApplication<FusionVisitorConcept, T>::handle(visitor, o);
}

You can use it by supplying a visitor, e.g. for xml-like output:

struct DisplayMemberVisitor {
    typedef std::string result_type;

    DisplayMemberVisitor() { ss << std::boolalpha; }

    std::string complete() { return ss.str(); }

    void start_member (const char* name) { 
        ss << "<" << name << ">";
    }
    void finish_member(const char* name) { 
        ss << "</" << name << ">";
    }

    template <typename T> void value(T const& value) {
        ss << value;
    }

    void empty_object() { }
    void empty_array()  { }

private:
    std::stringstream ss;
};

See it Live On Coliru where (including some debug output) it prints:

<integer_value>33</integer_value><boolean_value>false</boolean_value><integer_value>34</integer_value><boolean_value>true</boolean_value>

Note that the ADT adaptation macro doesn't include a name (because none is available). You can probably quite easily make a macro FUSION_ADAPT_KEYD_ADT that also accepts a name and generates the relevant specializations of boost::fusion::extension::struct_member_name.

BONUS MATERIAL

Adding member name traits to ADT adapted members

Here's a simplistic approach that shows what little amount of work needs to be done.

#define MY_ADT_MEMBER_NAME(CLASSNAME, IDX, MEMBERNAME)                                                                                   \
        namespace boost { namespace fusion { namespace extension {                                                                       \
            template <> struct struct_member_name<CLASSNAME, IDX> { typedef char const *type; static type call() { return #MEMBERNAME; } \
        }; } } }

MY_ADT_MEMBER_NAME(Bar, 0, integer_value)
MY_ADT_MEMBER_NAME(Bar, 1, boolean_value)

This defines a macro to avoid most of the repetition. If you are a BOOST_PP whizkid you could somehow weave this into an adt_ex.hpp¹ header of sorts, so you could instead say:

BOOST_FUSION_ADAPT_ADT(Bar, // NOTE THIS PSEUDO-CODE
    (integer_value, int,  int,  obj.get_integer_value(), obj.set_integer_value(val))
    (boolean_value, bool, bool, obj.get_boolean_value(), obj.set_boolean_value(val)))

For now here's the ADT adapted trick Live On Coliru

¹ in case you're interested, here's a tarball of a prepared adt_ex tree (drop in alongsize adt.hpp): adt_ex.tgz as a starting point. It's just adt* but with macros and header guards renamed to adt_ex*