making boost::fusion::result_of::as_set<> 's instance doesn't call its elements' constructors

Question

#include <iostream>

#include <boost/mpl/front.hpp>
#include <boost/mpl/pop_front.hpp>
#include <boost/mpl/push_front.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/pop_back.hpp>
#include <boost/mpl/fold.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/set.hpp>
#include <boost/mpl/back_inserter.hpp>
#include <boost/fusion/include/set.hpp>
#include <boost/fusion/include/mpl.hpp>

/**********definition of nodes**************/
struct node_base
{};
struct node_a : public node_base
{
node_a(){std::cout << "node_a::Ctor"<< std::endl;}
};
struct node_b : public node_base
{
node_b(){std::cout << "node_b::Ctor"<< std::endl;}
};
struct node_c : public node_base
{
node_c(){std::cout << "node_c::Ctor"<< std::endl;}
};

struct empty_node {};

/***********definition of table*************/
struct my_table : ::boost::mpl::vector3<
::boost::mpl::vector3<node_a, node_b, node_c>
,::boost::mpl::vector3<node_b, node_c, node_a>
,::boost::mpl::vector3<node_c, node_a, node_b>
> {};

/*************meta-functions**************/
struct make_tag_vector_
{
template<class NODE1, class NODE2>
struct apply
{
    typedef typename ::boost::mpl::vector<NODE1>::type type;
};
};

struct fold_table
{
template<class LIST, class VECTOR>
struct apply
{
    typedef typename ::boost::mpl::front<VECTOR>::type parent;
    typedef typename ::boost::mpl::pop_front<VECTOR>::type children;
    typedef typename ::boost::mpl::pop_back<children>::type tmp;
    typedef typename ::boost::mpl::push_front<tmp, empty_node>::type right_shift_children;

    typedef typename ::boost::mpl::transform<
        children
        , right_shift_children
        , make_tag_vector_
        , ::boost::mpl::back_inserter<LIST>
    >::type type;
};
};

template<class TABLE>
struct create_table
{
typedef typename ::boost::mpl::fold< 
    TABLE
    , ::boost::mpl::vector0<>
    , fold_table
>::type type;
};

/**********process table**************/
typedef create_table<my_table>::type table_type;

typedef ::boost::mpl::reverse_fold<
table_type
, ::boost::mpl::set0<>
, ::boost::mpl::insert<
    ::boost::mpl::placeholders::_1
    , ::boost::mpl::front<::boost::mpl::placeholders::_2>
>
>::type node_set_type;

/**********result of node_set_type is :**************/
// struct node_c
// struct node_a
// struct node_b

/**********convert to fusion set type**************/
typedef ::boost::fusion::result_of::as_set<node_set_type> fusion_set_type;

/**********create fusion set instance**************/
fusion_set_type instance;

This code compiles. My question is that when we make instance of "fusion_set_type", all the types' constructors in that fusion set are supposed to get called. However, this "fusion_set_type" doesn't invoke any of its containing types' constructors. Maybe there is something incorrect in the nested fold/transform operations. The result of "node_set_type" is "struct node_c, struct node_a, struct node_b", and when I use a mpl::for_each plus a function object to loop through "node_set_type", all node's constructors are successfully getting called. Thanks!

score 1 · Accepted Answer · answered Jul 13 '11 at 12:55

1

Possibly typo? Constructors were called when I tested on ideone by adding ::type as the following:

/**********create fusion set instance**************/
fusion_set_type::type instance;

answered Jul 13 '11 at 12:55

Ise Wisteria

11,259
2
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26

Yeah, this works. Thank you very much! I think my original problem was that the number of elements in fusion set exceeds its maximum number which is 10. The FUSION_MAX_SET_SIZE needs to be defined explicitly. – DF24 Jul 13 '11 at 16:52

making boost::fusion::result_of::as_set<> 's instance doesn't call its elements' constructors

1 Answers1