C++ iterate into nested struct field with boost fusion adapt_struct

Question

Two stackoverflow answers suggest the approach using fusion adapt_struct to iterate over struct fields. The approach looks nice. However, how do you iterate into a field whi

Answer 1

I made an example of what you want that you can see at my blog site. In this is case it's a JSON serializer that works with nested structs. It uses a 'more Boost' solution since I saw it in the Boost.Serialization library. (See also below and live on Coliru.)

The solution uses Fusion Sequence adaptation of structs and a metafunction that walks object members (recursively) - using Boost.TypeTraits and different traits for specific types.

You can see a more complex example of the same solution at the site for googlecode corbasim project for creating an run-time reflexive API.

Code listing for the generic JSON serializer:

See it Live on Coliru

#ifndef JSON_SERIALIZER_HPP
#define JSON_SERIALIZER_HPP

#include <boost/type_traits.hpp> // is_array, is_class, remove_bounds

#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/next_prior.hpp>

#include <boost/fusion/mpl.hpp>
#include <boost/fusion/adapted.hpp> // BOOST_FUSION_ADAPT_STRUCT

// boost::fusion::result_of::value_at
#include <boost/fusion/sequence/intrinsic/value_at.hpp>
#include <boost/fusion/include/value_at.hpp>

// boost::fusion::result_of::size
#include <boost/fusion/sequence/intrinsic/size.hpp>
#include <boost/fusion/include/size.hpp>

// boost::fusion::at
#include <boost/fusion/sequence/intrinsic/at.hpp>
#include <boost/fusion/include/at.hpp>

namespace json
{

// Forward
template < typename T >
struct serializer;

namespace detail
{

namespace iterator
{

template < typename S, typename N >
struct Comma
{
    template < typename Ostream >
    static inline void comma(Ostream& os)
    {
        os << ", ";
    }
};

template < typename S >
struct Comma< S, typename boost::mpl::prior< typename boost::fusion::result_of::size< S >::type >::type >
{
    template < typename Ostream >
    static inline void comma(Ostream& os)
    {
    }
};

// Iteracion sobre una estructura
template < typename S, typename N >
struct StructImpl
{
    // Tipo del campo actual
    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;

    template < typename Ostream >
    static inline void serialize(Ostream& os, const S& s)
    {
        os << "\"" << name_t::call() << "\": ";
        ::json::serializer< current_t >::serialize(os, boost::fusion::at< N >(s));

        // Insert comma or not    
        Comma< S, N >::comma(os);

        StructImpl< S, next_t >::serialize(os, s);
    }
};

// Fin de la iteracion sobre estructuras.
template < typename S >
struct StructImpl< S, typename boost::fusion::result_of::size< S >::type >
{
    template < typename Ostream >
    static inline void serialize(Ostream& os, const S& s)
    {
        // Nada que hacer
    }
};

// Iterador sobre una estructura. Template fachada.
template < typename S >
struct Struct : StructImpl< S, boost::mpl::int_< 0 > > {};

} // iterator

template < typename T >
struct array_serializer 
{
    typedef array_serializer< T > type;

    typedef typename boost::remove_bounds< T >::type slice_t;

    static const size_t size = sizeof(T) / sizeof(slice_t);

    template < typename Ostream >
    static inline void serialize(Ostream& os, const T& t)
    {
        os << "[";
        for(size_t idx=0; idx<size; idx++)
        {
            ::json::serializer< slice_t >::serialize(os, t[idx]);
            if (idx != size-1)
                os << ", ";
        }
        os << "]";
    }

};

template < typename T >
struct struct_serializer 
{
    typedef struct_serializer< T > type;

    template < typename Ostream >
    static inline void serialize(Ostream& os, const T& t)
    {
        os << "{";
        iterator::Struct< T >::serialize(os, t);
        os << "}";
    }
};

template < typename T >
struct arithmetic_serializer 
{
    typedef arithmetic_serializer< T > type;

    template < typename Ostream >
    static inline void serialize(Ostream& os, const T& t)
    {
        os << t;
    }
};

template < typename T >
struct calculate_serializer
{
    typedef
        typename boost::mpl::eval_if< boost::is_array< T >,
            boost::mpl::identity< array_serializer < T > >,
        //else
        typename boost::mpl::eval_if< boost::is_class< T >,
            boost::mpl::identity< struct_serializer < T > >,
        //else
            boost::mpl::identity< arithmetic_serializer < T > >
        >
        >::type type;

};

} // detail

template < typename T >
struct serializer : public detail::calculate_serializer < T >::type
{
};


} // json

#endif // JSON_SERIALIZER_HPP

//#include "json.hpp"
#include <iostream>

struct my_other_struct
{
    int my_other_integer;
};

struct my_struct
{
    int my_integer;

    typedef int my_array_t[2];
    my_array_t my_array;

    typedef my_other_struct my_other_structs_t[3];
    my_other_structs_t my_other_structs;
};

BOOST_FUSION_ADAPT_STRUCT(my_struct, (int, my_integer) (my_struct::my_array_t, my_array) (my_struct::my_other_structs_t, my_other_structs))
BOOST_FUSION_ADAPT_STRUCT(my_other_struct, (int, my_other_integer))


int main(int argc, char *argv[])
{
    my_struct s1 = my_struct { 1, { 42, -42 }, { { 11 }, { 22 }, { 33 } } };

    json::serializer< my_struct >::serialize(std::cout, s1);

    std::cout << std::endl;
}

Answer 2

Andres gives an excellent answer. The problem in my original code is that "for_each" takes only sequence types. When compiler evaluates the T for an int, it passes to "for_each" an int argument thus it fails. The idea behind Adries' solution is to hide "for_each" in a sequence-specific class (DecImplSeq_s below), and provide an alternative class (DecImplVoid_s) for non-sequence fields. Then create a facade class to divide the decoding of sequence and non-sequence fields (DecCalc_s).

The common header goes with the first example below to show Adres' idea.

/* compile with g++ 4.4.6: g++ -I boost_1_35_0 test.cpp */
#include <typeinfo>
#include <string>
#include <boost/fusion/include/sequence.hpp>
#include <boost/fusion/include/algorithm.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/is_sequence.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/lexical_cast.hpp>
#include <cxxabi.h>
#include <stdio.h>
using namespace boost::fusion;

The common code of the solution derived directly from Adres' sample:

template <typename T2> struct Dec_s;
struct AppendToTextBox {
  template <typename T> void operator()(T& t) const {
        //decode T and t as the original code here...
        Dec_s<T>::decode(t);
  }
};
template <typename T2> struct DecImplSeq_s {
  typedef DecImplSeq_s<T2> type;
  static void decode(T2   & f) { for_each(f, AppendToTextBox()); };
};
template <typename T2> struct DecImplVoid_s {
  typedef DecImplVoid_s<T2> type;
  static void decode(T2   & f) { };
};

template <typename T2> struct DecCalc_s {
  typedef typename
    boost::mpl::eval_if< traits::is_sequence<T2>, DecImplSeq_s<T2>, DecImplVoid_s<T2> >
  ::type type;
};

template <typename T2> struct Dec_s : public DecCalc_s<T2>::type { };

Here is how you can use the common code above:

struct Foo_s { int i; char k[100]; };
struct Bar_s { int v; Foo_s w; };

BOOST_FUSION_ADAPT_STRUCT( Foo_s,  (int, i)  (char, k[100]) )
BOOST_FUSION_ADAPT_STRUCT( Bar_s, (int, v)  (Foo_s, w) )

int main(int argc, char *argv[]) {
  Bar_s f = { 2, { 3, "abcd" } };
  Dec_s<Bar_s>::decode(f);
  return 0;
}

---

Another solution that is more straightforward without using advanced boost tricks, you can implement a specialized decoder class for each primitive types, without using "eval_if". To use this solution, you need to do a specialization for each primitive type in your structs.

struct Foo_s { int i; char k[100]; };
BOOST_FUSION_ADAPT_STRUCT( Foo_s,  (int, i)  (char, k[100]) )

struct Bar_s { int v; Foo_s w; };
BOOST_FUSION_ADAPT_STRUCT( Bar_s, (int, v)  (Foo_s, w) )

template <typename T2> struct Dec_s {  static void decode(T2   & f); };
struct AppendToTextBox {
    template <typename T>
    void operator()(T& t) const {
        //decode T and t as the original code here...
        Dec_s<T>::decode(t);
    }
};

template <typename T2> void Dec_s<T2>::decode(T2 & f) {
    for_each(f, AppendToTextBox());
};
template<> void Dec_s<int >::decode(int  & f) {};
template<> void Dec_s<char>::decode(char & f) {};

int main(int argc, char *argv[]) {
  Bar_s f = { 2, { 3, "abcd" } };
  Dec_s<Bar_s>::decode(f);
  return 0;
}

---

After some progressive exploration, here is a complete example. It uses more recent boost features, but does not build with early boost versions like 1.35.0. It works well with boost 1.47.0 and 1.51.0.

The common header part:

#include <typeinfo>
#include <string>
#include <boost/fusion/include/sequence.hpp>
#include <boost/fusion/include/algorithm.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/is_sequence.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/type_traits.hpp> // is_array, is_class, remove_bounds
#include <boost/lexical_cast.hpp>
#include <cxxabi.h>
#include <stdio.h>

extern int dec_indents; /* 0, 4, 8, ... */
struct NL {
    static void print() { printf("\n");
        for (int i=0; i<dec_indents; i++) printf(" ");
    }
};

using namespace boost::fusion;

Then the common decoder with output formatting:

template <typename T2> struct Dec_s;

template <typename S, typename N> struct Comma {
  static inline void comma() { printf(" , "); }
};
template <typename S> struct Comma<S, typename
 boost::mpl::prior<typename boost::fusion::result_of::size<S>::type >::type> {
   static inline void comma() {}
};

template <typename S, typename N> struct DecImplSeqItr_s {
  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 decode(S& s) {
    printf(" \"%s\" = ", name_t::call() );
    Dec_s<current_t>::decode(boost::fusion::at<N>(s));
    Comma<S, N>::comma();  // Insert comma or not
    DecImplSeqItr_s<S, next_t>::decode(s);
  }
};
template <typename S>
struct DecImplSeqItr_s<S, typename boost::fusion::result_of::size<S>::type > {
    static inline void decode(S& s) { }
};
template <typename S>
struct DecImplSeqStart_s:DecImplSeqItr_s<S, boost::mpl::int_<0> > {};

template <typename S> struct DecImplSeq_s {
  typedef DecImplSeq_s<S> type;
  static void decode(S & s) {
    printf("  struct  start --- { --- ");
    dec_indents += 4;
    NL::print();
    DecImplSeqStart_s<S>::decode(s);
    dec_indents -= 4;
    NL::print();
    printf("  struct  done  --- } --- ");
    NL::print();
  };
};

template <typename T2> struct DecImplArray_s {
  typedef DecImplArray_s<T2> type;
  typedef typename boost::remove_bounds<T2>::type slice_t;
  static const size_t size = sizeof(T2) / sizeof(slice_t);
  static inline void decode(T2 & t) {
    printf("  array start --- [ --- ");
    dec_indents += 4;
    NL::print();
    for(size_t idx=0; idx<size; idx++) {
        Dec_s<slice_t>::decode(t[idx]);
        if (idx < size-1) {
            NL::print(); printf(" , ");
        }
    }
    dec_indents -= 4;
    NL::print();
    printf("  array done  --- ] --- \n");
    NL::print();
  }
};

template <typename T2> struct DecImplVoid_s {
  typedef DecImplVoid_s<T2> type;
  static void decode(T2   & t) {
    int status = 0;
    const char *realname = abi::__cxa_demangle(typeid(t).name(),0,0,&status);
    printf(" type %s", realname);
    NL::print();
  };
};

template <typename T2> struct DecCalc_s {
  typedef
    typename boost::mpl::eval_if< traits::is_sequence<T2>, DecImplSeq_s<T2>,
    typename boost::mpl::eval_if< boost::is_array<T2>,
                                 boost::mpl::identity< DecImplArray_s<T2> >,
    DecImplVoid_s<T2>   > >
  ::type type;
};

template <typename T2> struct Dec_s : public DecCalc_s<T2>::type { };

To use this common decoder, you can put it into a .h file, and use the following .c code:

/* compile with g++ 4.5.1: g++ -I boost_1_47_0 test.cpp */

#include "common_decoder.h"

using namespace boost::fusion;

int dec_indents=0;

struct Foo_s { int i; typedef char j_t[10]; Foo_s::j_t j; };
BOOST_FUSION_ADAPT_STRUCT( Foo_s, (int, i) (Foo_s::j_t, j) )

struct Bar_s { int v; typedef Foo_s w_t[2]; Bar_s::w_t w; };
BOOST_FUSION_ADAPT_STRUCT( Bar_s, (int, v) (Bar_s::w_t, w) )

int main(int argc, char *argv[]) {
  Bar_s f = { 2, {{ 3, "abcd" },{ 4, "defg" }} };
  Dec_s<Bar_s>::decode(f);
  return 0;
}