如何在 std::tuple 中合并 std::unordered_map？

Question

我想创建一个类，其中包含一个std::tuple其中包含std::unordered_map 。 我想创建一个方法来合并元组内的映射。

#include <tuple>
#include <unordered_map>

template <typename ... Ts, std::size_t ... Is>
std::tuple<Ts...> merge_map (std::tuple<Ts...>& t1, std::tuple<Ts...> & t2, std::index_sequence<Is...> const &) { 
    return { 
        (std::get<Is>(t1).merge( std::get<Is>(t2)))... 
    }; 
}

template <class ...Ts>
struct MyContainer
{
    std::tuple<Ts...> data;

    void merge(MyContainer<Ts...>& other){
        data = merge_map(data, other.data, std::make_index_sequence<sizeof...(Ts)>{});
    }

};

int main()
{
    using namespace std;
    unordered_map<string, int> f = {{"zero", 0}, {"one", 1}};
    MyContainer <unordered_map<string, int>> container1 = {.data = f};

    unordered_map<string, int> s = {{"two", 2}, {"three", 3}};
    MyContainer <unordered_map<string, int>> container2 = {.data = s};

    container1.merge(container2);
}

但我无法编译这段代码。 我试图用 un std::tuple of int创建一个简单的例子，并对它们求和，它起作用了。 但我坚持这个更复杂的例子。 谢谢你的任何建议。

Answer 1

我看到的更大的问题是std::unorderd_map<something...>::merge()返回void 。

所以肯定是错的

return { 
    (std::get<Is>(t1).merge( std::get<Is>(t2)))... 
}; 

我建议修改merge_map() ，使用模板折叠，如下

template <typename ... Ts, std::size_t ... Is>
std::tuple<Ts...> merge_map (std::tuple<Ts...> & t1,
                             std::tuple<Ts...> & t2,
                             std::index_sequence<Is...> const &) { 
   (std::get<Is>(t1).merge(std::get<Is>(t2)), ...); 

   return t1;
}

但也要记住包含<string>并且{.data = f}初始化语法不是 C++17（如果我没记错的话，将从 C++20 开始可用）。

Answer 2

尝试这个：

#include <tuple>
#include <unordered_map>
#include <iostream>

template <typename T>
void merge_map_helper (T& t1, T& t2) { }
template <typename T, std::size_t I, std::size_t ... Is>
void merge_map_helper (T& t1, T& t2) { 
    std::get<I>(t1).merge( std::get<I>(t2));
    merge_map_helper<T, Is...>(t1, t2);
}

template <typename ... Ts, std::size_t ... Is>
void merge_map (std::tuple<Ts...>& t1, std::tuple<Ts...> & t2, std::index_sequence<Is...> const &) { 
    merge_map_helper<std::tuple<Ts...>, Is...>(t1, t2);
}

template <class ...Ts>
struct MyContainer
{
    std::tuple<Ts...> data;

    MyContainer(std::tuple<Ts...> data) : data(std::move(data)) { }

    void merge(MyContainer<Ts...>& other){
        merge_map(data, other.data, std::make_index_sequence<sizeof...(Ts)>{});
    }
};

int main()
{
    using namespace std;
    unordered_map<string, int> f1 = {{"zero1", 0}, {"one", 1}};
    unordered_map<string, int> f2 = {{"zero2", 0}, {"one", 1}};
    MyContainer <unordered_map<string, int>, unordered_map<string, int>> container1 = 
        { { f1, f2 } };

    unordered_map<string, int> s1 = {{"two1", 2}, {"three", 3}};
    unordered_map<string, int> s2 = {{"two2", 2}, {"three", 3}};
    MyContainer <unordered_map<string, int>, unordered_map<string, int>> container2 = 
        { { s1, s2} };

    container1.merge(container2);

    for(auto & k :std::get<0>(container1.data)) {
        std::cout << k.first << " " << k.second << "\n";
    }

    for(auto & k :std::get<1>(container1.data)) {
        std::cout << k.first << " " << k.second << "\n";
    }
}

请注意， merge_map不返回 map，它修改第一个。 所以我更新了参数并返回值来反映它。

Answer 3

我就是这样解决的。 我还让它对 c++14 友好 :)

#include <tuple>
#include <unordered_map>



template <typename ... Ts, std::size_t ... Is>
auto merge_map (std::tuple<Ts...>& t1, std::tuple<Ts...> & t2, std::index_sequence<Is...> const &) { 
    return std::make_tuple( 
        std::get<Is>(t1)..., std::get<Is>(t2)...
    ); 
}

template <class ...Ts>
struct MyContainer
{
    std::tuple<Ts...> data;

};

template <class ...Ts>
auto merge(MyContainer<Ts...>& c1, MyContainer<Ts...>& c2){
   return merge_map(c1.data, c2.data, std::make_index_sequence<sizeof...(Ts)>{});
}

int main()
{
    using namespace std;
    unordered_map<string, int> f = {{"zero", 0}, {"one", 1}};
    MyContainer <unordered_map<string, int>> container1 = {.data = std::make_tuple(f)};

    unordered_map<string, int> s = {{"two", 2}, {"three", 3}};
    MyContainer <unordered_map<string, int>> container2 = {.data = std::make_tuple(s)};

    MyContainer<decltype(f), decltype(s)> cc;
    cc.data = merge(container1, container2);

}