c++ - 加入std::variant <a，b，c>和std::variant <x，y，z>的类型

Question

有没有一种方法可以从包含两个变体类型的并集的两个(decltype-decuded)变体类型中声明第三个变体？

Answer 1

您可以使用此类帮助程序类型进行推导:

#include <variant>

template <class ...Args>
struct VariantUnionHelper;

template <class ...Args1, class ...Args2>
struct VariantUnionHelper<std::variant<Args1...>, std::variant<Args2...>> {
    using type = std::variant<Args1..., Args2...>;
};

连同方便的类型别名

template <class Variant1, class Variant2>
using VariantUnion = typename VariantUnionHelper<Variant1, Variant2>::type;

这允许以下用例

using Var1 = std::variant<int, long, short>;
using Var2 = std::variant<bool, char, double>;
VariantUnion<Var1, Var2> joined;

并通过以下编译时检查。

#include <type_traits>

static_assert(std::is_same_v<VariantUnion<Var1, Var2>,
     std::variant<int, long, short, bool, char, double>>);