BOOST_FUSION_ADAPT_STRUCT的替代实现

Question

来自link的BOOST_FUSION_ADAPT_STRUCT使用情况，即

namespace demo
{
    struct employee
    {
        std::string name;
        int age;
    };
}

BOOST_FUSION_ADAPT_STRUCT(
    demo::employee,
    (std::string, name)
    (int, age))

替换BOOST_FUSION_ADAPT_STRUCT宏的实际实现是什么？

Answer 1

我可能依赖于编译器/标志。关于我的：

namespace boost { namespace fusion { namespace traits { template< > struct tag_of<demo::employee > { typedef struct_tag type; }; template< > struct tag_of<demo::employee const> { typedef struct_tag type; }; } namespace extension { template< > struct access::struct_member< demo::employee , 0 > { typedef std::string attribute_type; typedef attribute_type type; template<typename Seq> struct apply { typedef typename add_reference< typename mpl::eval_if< is_const<Seq> , add_const<attribute_type> , mpl::identity<attribute_type> >::type >::type type; constexpr static type call(Seq& seq) { return seq. name; } }; }; template< > struct struct_member_name< demo::employee , 0 > { typedef char const* type; constexpr static type call() { return "name"; } }; template< > struct access::struct_member< demo::employee , 1 > { typedef int attribute_type; typedef attribute_type type; template<typename Seq> struct apply { typedef typename add_reference< typename mpl::eval_if< is_const<Seq> , add_const<attribute_type> , mpl::identity<attribute_type> >::type >::type type; constexpr static type call(Seq& seq) { return seq. age; } }; }; template< > struct struct_member_name< demo::employee , 1 > { typedef char const* type; constexpr static type call() { return "age"; } }; template< > struct struct_size<demo::employee> : mpl::int_<2> {}; template< > struct struct_is_view< demo::employee > : mpl::false_ {}; } } namespace mpl { template<typename> struct sequence_tag; template< > struct sequence_tag<demo::employee> { typedef fusion::fusion_sequence_tag type; }; template< > struct sequence_tag< demo::employee const > { typedef fusion::fusion_sequence_tag type; }; } }

只是为了可读性：

#include <string>
#include <boost/fusion/adapted/struct.hpp>

namespace demo {
    struct employee
    {
        std::string name;
        int age;
    };
}

namespace boost {
    namespace fusion {
        namespace traits {
            template <> struct tag_of<demo::employee> { typedef struct_tag type; };
            template <> struct tag_of<demo::employee const> { typedef struct_tag type; };
        } // namespace traits
        namespace extension {
            template <> struct access::struct_member<demo::employee, 0> {
                typedef std::string attribute_type;
                typedef attribute_type type;
                template <typename Seq> struct apply {
                    typedef typename add_reference<
                        typename mpl::eval_if<is_const<Seq>, add_const<attribute_type>,
                                 mpl::identity<attribute_type>>::type>::type
                                     type;
                    constexpr static type call(Seq& seq) { return seq.name; }
                };
            };
            template <> struct struct_member_name<demo::employee, 0> {
                typedef char const* type;
                constexpr static type call() { return "name"; }
            };
            template <> struct access::struct_member<demo::employee, 1> {
                typedef int attribute_type;
                typedef attribute_type type;
                template <typename Seq> struct apply {
                    typedef typename add_reference<
                        typename mpl::eval_if<is_const<Seq>, add_const<attribute_type>,
                                 mpl::identity<attribute_type>>::type>::type
                                     type;
                    constexpr static type call(Seq& seq) { return seq.age; }
                };
            };
            template <> struct struct_member_name<demo::employee, 1> {
                typedef char const* type;
                constexpr static type call() { return "age"; }
            };
            template <> struct struct_size<demo::employee> : mpl::int_<2> {};
            template <> struct struct_is_view<demo::employee> : mpl::false_ {};
        } // namespace extension
    } // namespace fusion
    namespace mpl {
        template <typename> struct sequence_tag;
        template <> struct sequence_tag<demo::employee> {
            typedef fusion::fusion_sequence_tag type;
        };
        template <> struct sequence_tag<demo::employee const> {
            typedef fusion::fusion_sequence_tag type;
        };
    } // namespace mpl
} // namespace boost

有关实际的替代方案，请参见

用于执行代码生成的

• Boost Hana (https://www.boost.org/doc/libs/1_61_0/libs/hana/doc/html/group__group-Struct.html)
• Ad-hoc宏，如Parsing Selector struct with alternating tokens using Boost Spirit X3