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，但是如果我明确地引用继承的对象，如下所示，它就完全有效。

#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)

是否有某种方法可以使第一个版本起作用，这样我就可以调整一个固有公共成员的结构？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的唯一途径。

Answer 1

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

简短的问题：不，这不是一个功能。

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的操作是编码对已知基类的支持，并在适应的序列成员之外处理它们。

聚合演示

<强> 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; }; }

使用硬编码基础列表进行演示

<强> 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; }

我可以将BOOST_FUSION_ADAPT_STRUCT与继承的东西一起使用吗？

1 个答案:

聚合演示

使用硬编码基础列表进行演示