c++ - 在类模板中创建映射元组

Question

struct VectorOfMaps {
  std::tuple<std::map<std::string,double>,
              std::map<std::string,int>,
              std::map<std::string,short>
            > tuple;
};

我想将它概括为一个模板，所以上面的类将由类似的东西生成

VectorOfMaps<3, std::string,double,
                std::string,int,
                std::string,short>

可以这样做或类似的事情吗？

我知道我可以将 map 类型直接放入模板中，但是这样重复“std::map”会不会有点难看？你怎么看？

(我想过我是否可以在模板中指定 std::pair 类型，但我不确定是否可以直接使用它们来创建 std::map 类型)..

Answer 1

由于您提到将对的特化传递给类模板:

template<typename... Pairs>
struct VectorOfMaps {
    std::tuple<std::map<
        typename Pairs::first_type
        , typename Pairs::second_type
    >...> tuple;
};

这有效地使用了 std::pair作为类型列表——你可以使用类似 template<typename T, typename U> struct pair { using first_type = T; using second_type = U; }; 的东西也一样。

也可以在没有一对的情况下传递类型，但这需要一些元计算。一个可能的解决方案:

// First argument is accumulator, supposed to be an empty tuple
template<typename Acc, typename... T> struct compute_tuple
{
    // Only triggered when the primary template is instantiated, which should
    // only happen if sizeof...(T) is odd -- it's possible to assert on that
    // instead, too.
    static_assert( !sizeof(Acc), "An even number of arguments is required" );
};

// Recursive case
template<typename... Acc, typename First, typename Second, typename... Rest>
struct compute_tuple<std::tuple<Acc...>, First, Second, Rest...>
: compute_tuple<std::tuple<Acc..., std::map<First, Second>>, Rest...> {};

// Terminal case
template<typename Acc>
struct compute_tuple<Acc> { using type = Acc; };

template<typename... T>
struct VectorOfMaps {
    /* 
     * You can assert that sizeof...(T) is even here; it might be more
     * helpful than an error deeper inside compute_tuple.
     */
    using tuple_type = typename compute_tuple<std::tuple<>, T...>::type;
    tuple_type tuple;
};