我可以对继承的东西使用 BOOST_FUSION_ADAPT_STRUCT 吗？

Question

假设我有

struct cat
{
    int tail;
    int head;
};

struct bird
{
    int wing;
    int bursa;
};

如果我这样做......

struct wat : public cat, public bird
{

};

BOOST_FUSION_ADAPT_STRUCT(cat,tail,head)
BOOST_FUSION_ADAPT_STRUCT(bird, wing, bursa)
BOOST_FUSION_ADAPT_STRUCT(wat, wat::cat, wat::bird)

...我无法获得构建，但如果我像下面这样明确引用继承的对象，它是完全有效的。

#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
namespace qi = boost::spirit::qi;
struct wat
{
public:
    cat z;
    bird q;
};

BOOST_FUSION_ADAPT_STRUCT(cat,tail,head)
BOOST_FUSION_ADAPT_STRUCT(bird, wing, bursa)
BOOST_FUSION_ADAPT_STRUCT(wat, z, q)

有没有办法让第一个版本工作，这样我就可以适应一个继承的公共成员的结构？我绝对不想这样做BOOST_FUSION_ADAPT_STRUCT(wat,tail,head,wing,bursa)，但这似乎是我能找到的继承成员的唯一途径。

Answer 1

这里有类似的问题：c++/boost fusion handle parent class

简短的问题：不，这不是功能。

您可以使用聚合而不是继承，

struct wat {
    cat _cat;
    bird _bird;
};

BOOST_FUSION_ADAPT_STRUCT(wat, _cat, _bird)

但你不会神奇地得到一个扁平的序列。

您可能希望在处理融合序列的代码中执行对已知基类的支持的编码并处理它们以及适应的序列成员。

聚合演示

Live On Coliru

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

struct cat {
    int tail;
    int head;
};

struct bird {
    int wing;
    int cloaca;
};

struct wat {
    cat _cat;
    bird _bird;
};

BOOST_FUSION_ADAPT_STRUCT(cat, tail, head)
BOOST_FUSION_ADAPT_STRUCT(bird, wing, cloaca)
BOOST_FUSION_ADAPT_STRUCT(wat, _cat, _bird)

#include <iostream>
#include <boost/fusion/include/at_c.hpp>

template <typename T, int N = 0> void print(T const& obj) {
    namespace fus = boost::fusion;
    if constexpr (fus::traits::is_sequence<T>::value) {
        if (N==0)
            std::cout << "{";

        if constexpr (N < fus::size(obj).value) {
            auto name = boost::fusion::extension::struct_member_name<T, N>::call();
            std::cout << ' ' << name << '=';
            print(fus::at_c<N>(obj));
            std::cout << ';';
            print<T, N+1>(obj);
        } else {
            std::cout << " }";
        }
    } else {
        std::cout << obj;
    }
}

int main() {
    print(wat { {1,2}, {3,4} });
}

打印

{ _cat={ tail=1; head=2; }; _bird={ wing=3; cloaca=4; }; }

带有硬编码碱基列表的演示

Live On Coliru

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

struct cat {
    int tail;
    int head;
};

struct bird {
    int wing;
    int cloaca;
};

struct wat : cat, bird {
    int something;
    int extra;

    wat(int tail, int head, int wing, int cloaca, int something, int extra)
        : cat{tail, head}, bird{wing, cloaca}, something(something), extra(extra)
    { }
};

BOOST_FUSION_ADAPT_STRUCT(cat, tail, head)
BOOST_FUSION_ADAPT_STRUCT(bird, wing, cloaca)
BOOST_FUSION_ADAPT_STRUCT(wat, something, extra)

#include <iostream>
#include <boost/fusion/include/at_c.hpp>

template <typename... KnownBases>
struct Demo {
    template <typename T, int N = 0> static void print(T const& obj, bool outer_sequence_braces = true) {
        namespace fus = boost::fusion;
        if constexpr (fus::traits::is_sequence<T>::value) {
            if (N==0)
            {
                if (outer_sequence_braces) std::cout << "{";
                print_bases<KnownBases...>(obj);
            }

            if constexpr (N < fus::size(obj).value) {
                auto name = boost::fusion::extension::struct_member_name<T, N>::call();
                std::cout << ' ' << name << '=';
                print(fus::at_c<N>(obj), true/*inner sequences get braces*/);
                std::cout << ';';
                print<T, N+1>(obj, outer_sequence_braces);
            } else {
                if (outer_sequence_braces) std::cout << " }";
            }
        } else {
            std::cout << obj;
        }
    }

    template <typename Base, typename T> static bool print_base(T const& obj) {
        if constexpr (not std::is_same<Base, T>() && std::is_base_of<Base, T>())
            print(static_cast<Base const&>(obj), false);
        return true;
    }

    template <typename... Bases, typename T> static void print_bases(T const& obj) {
        bool discard[] = { print_base<Bases>(obj)... };
        (void) discard;
    }
};

int main() {
    Demo<cat, bird>::print(wat { 1, 2, 3, 4, 5, 6 });
}

打印

{ tail=1; head=2; wing=3; cloaca=4; something=5; extra=6; }