CPP 元编程：包含元组函数

Question

我想实现一个 contains 函数，它检查元组中是否包含特定类型。

特别是我想尝试 std::condition 模板来完成这项工作。但是编译时出现以下错误：

In file included from /Users/harald/Desktop/Meta/main.cpp:2:
/Users/harald/Desktop/Meta/type_set.h:19:70: error: no member named 'value' in 'type_set_contains<float, type_set<int, float, double> >'
constexpr bool type_set_contains_v = type_set_contains<Type, Tuple>::value;
                                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
/Users/harald/Desktop/Meta/main.cpp:6:18: note: in instantiation of variable template specialization 'type_set_contains_v<float, type_set<int, float, double> >' requested here
    std::cout << type_set_contains_v<float, type_set<int, float, double>>;

到目前为止我的代码是：

#include <type_traits>

template <typename... Types>
class type_set {};

template<typename Type, typename Tuple>
struct type_set_contains: public std::false_type{};

template<typename Type, typename Head, typename... Rest>
struct type_set_contains<Type, type_set<Head, Rest...>> :
        public std::conditional<std::is_same<Type, Head>::value, std::true_type,
                type_set_contains<Type, type_set<Rest...>>>{};

template<typename Type, typename Tuple>
constexpr bool type_set_contains_v = type_set_contains<Type, Tuple>::value;

我尝试通过调用此行来执行函数：

type_set_contains_v<float, type_set<int, float, double>>;

所以我的错误是什么？

Answer 1

您继承自 std::conditional 而不是 std::conditional_t。错误消息 error: no member named 'value' 是正确的，因为 std::conditional 没有 value 成员。

此外，您提到了元组，但您使用的是自定义 type_set 类型。如果您想普遍接受 std::tuple、type_set 或其他可变参数模板类型，则需要使用 template template parameter.

template<typename T, typename TypeSet>
struct type_set_contains : public std::false_type {};

template<typename T, template<typename...> class TypeSet, typename Head, typename ...Ts>
struct type_set_contains<T, TypeSet<Head, Ts...>> :
    std::conditional_t<std::is_same<Head, T>::value, std::true_type, type_set_contains<T, TypeSet<Ts...>>> {};

template<typename T, typename TypeSet>
constexpr bool const type_set_contains_v = type_set_contains<T, TypeSet>::value;

Live demo

CPP 元编程：包含元组函数

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