C ++ 1z - 检查是否在可变参数模板参数包中传递了类型 [英] C++1z - Check if a type is passed in variadic template parameter pack

问题描述

我听说过某个地方，使用新的C ++ 1z语法，很容易检查一个类型是否在variadic模板参数包中传递 - 显然你可以使用接近一行长的代码。这是真的？这些相关的功能是什么？ （我尝试查看折叠表达式，但我不能看到如何使用它们在该问题...）

I've heard somewhere, that using new C++1z syntax, it is really easy to check if a type is passed in variadic template parameter pack - apparently you can do this with code that is near one-line long. Is this true? What are those relevant features? (I tried looking through fold expressions but I can't see how to use them in that problem...)

这是我如何解决问题在C + + 11供参考：

Here's how I solved the problem in C++11 for reference:

#include <type_traits>


template<typename T, typename ...Ts>
struct contains;

template<typename T>
struct contains<T> {
    static constexpr bool value = false;
};

template<typename T1, typename T2, typename ...Ts>
struct contains<T1, T2, Ts...> {
    static constexpr bool value = std::is_same<T1, T2>::value ? true : contains<T1, Ts...>::value;
};

推荐答案

您正在查找 std :: disjunction 。它在 N4564 中指定 [meta.logical]

You're looking for std::disjunction. It's specified in N4564 [meta.logical].

#include <type_traits>

template<typename T, typename... Ts>
constexpr bool contains()
{ return std::disjunction_v<std::is_same<T, Ts>...>; }

static_assert(    contains<int,      bool, char, int, long>());
static_assert(    contains<bool,     bool, char, int, long>());
static_assert(    contains<long,     bool, char, int, long>());
static_assert(not contains<unsigned, bool, char, int, long>());

Live演示

或适用于 struct

template<typename T, typename... Ts>
struct contains : std::disjunction<std::is_same<T, Ts>...>
{};

---

或使用折叠表达式

---

Or, using fold expressions

template<typename T, typename... Ts>
struct contains : std::bool_constant<(std::is_same<T, Ts>{} || ...)>
{};

Live演示

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