balas bellobas balas bellobas - 1 year ago 171
C++ Question

Boost fusion sequence type and name identification for structs and class

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 <typeinfo>
#include <string>
#include <iostream>
#include <boost/fusion/include/sequence.hpp>
#include <boost/fusion/include/algorithm.hpp>
#include <boost/fusion/include/vector.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/adapt_adt.hpp>

#include <boost/lexical_cast.hpp>

using namespace boost::fusion;

struct Foo
int integer_value;
bool boolean_value;

class Bar
int integer_value;
bool boolean_value;
Bar(int i_val, bool b_val):integer_value(i_val),boolean_value(b_val) {}
int get_integer_value() const { return integer_value; }
void set_integer_value(int i_val) { integer_value = i_val; }
bool get_boolean_value() const { return boolean_value; }
void set_boolean_value(bool b_val) { boolean_value = b_val; }

(int, integer_value)
(bool, boolean_value)

(int, int, obj.get_integer_value() , obj.set_integer_value(val))
(bool, bool, obj.get_boolean_value(), obj.set_boolean_value(val))

struct DisplayMembers

template <typename T>
void operator()(T& t) const {
std::cout << typeid(t).name() << " : " << boost::lexical_cast<std::string>(t) << std::endl;


int main(int argc, char *argv[])
struct Foo f = { 33, false};
for_each(f, DisplayMembers());

Bar b(34,true);
for_each(b, DisplayMembers());
return 0;

In the above example the result is
int : 33
bool : 0
struct boost::fusion::extension::adt_attribute_proxy : 34
struct boost::fusion::extension::adt_attribute_proxy : 1

I want the result as
int : integer_value : 33
bool : boolean_value : 0
int : integer_value : 34
bool : boolean_value : 1

Answer Source

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)
                        VisitorApplication<FusionVisitorConcept, current_t>::handle(visitor, boost::fusion::at<N>(s));
                        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)
                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)
                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) {

    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);
                    ; // perhaps some default action?

    template <typename FusionVisitorConcept, typename T>
        struct select_application
                //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()  { }

    std::stringstream ss;

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


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.


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:

    (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*

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download