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将所有boost fusion映射键收集到std::tuple中

boost-fusion template-meta-programming c++

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Juergen · 技术社区 · 3 年前

#include <boost/fusion/container/map.hpp>
#include <boost/fusion/sequence/intrinsic/at_key.hpp>
#include <boost/fusion/sequence/intrinsic/value_at_key.hpp>
#include <tuple>

struct MyEvents {
    struct EventA;
    struct EventB;
    using EventMap = boost::fusion::map<boost::fusion::pair<EventA, int>,
                                        boost::fusion::pair<EventB, double>>;
};

template <typename T>
struct GetTypes;

template <typename T>
struct GetTypes<boost::fusion::map<...>> {
    using type = std::tuple<...>;
};

int main() {
    using Map = typename MyEvents::EventMap;
    using AllKeys = GetTypes<Map>::type;

    return 0;
}

Demo

我想从一个数据库中收集所有密钥类型 boost::fusion::map std::tuple .

MyEvents::EventA 和 MyEvents::EventB AllKeys = std::tuple<MyEvents::EventA, MyEvents::EventB> .

如何使用模板专门化实现这一点?我需要某种递归调用吗?

1 回复 | 直到 3 年前

1

2

åº·æ¡ç 3 年前

boost::fusion::pair 利用 T::first_type .

template <typename T>
struct GetTypes;

template <typename... Pairs>
struct GetTypes<boost::fusion::map<Pairs...>> {
    using type = std::tuple<typename Pairs::first_type...>;
};

Demo.