我可以将 BOOST_FUSION_ADAPT_STRUCT 用于继承的东西吗？

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)

...我无法构建，但如果我像下面这样显式引用继承的对象，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)

有什么方法可以让第一个版本正常工作，这样我就可以调整继承的 public 成员的结构吗？我绝对不想做 BOOST_FUSION_ADAPT_STRUCT(wat,tail,head,wing,bursa) 但这似乎是我能找到的继承成员到达那里的唯一方法。

Answer 1

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

简短的问题：不，这不是一项功能。

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。

您可能想在处理融合序列的代码中做的是编写对已知碱基类的支持的代码，并在适应的序列成员之外处理它们。

带聚合的演示

Live On Coliru

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

struct cat {
    int tail;
    int head;
};

struct bird {
    int wing;
    int cloaca;
};

struct wat {
    cat _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} });
}

版画

{ _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 });
}

版画

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

我可以将 BOOST_FUSION_ADAPT_STRUCT 用于继承的东西吗？

Can I use BOOST_FUSION_ADAPT_STRUCT with inherited stuff?

c++

inheritance

boost

boost-spirit

boost-fusion

带聚合的演示

带有硬编码碱基列表的演示