检查是否在可变参数模板参数包中传递了类型

问题描述

我在某处听说，使用新的 C++1z 语法，检查类型是否在可变参数模板参数包中传递非常容易 - 显然，您可以使用接近一行的代码来执行此操作.这是真的?这些相关功能是什么?(我尝试查看折叠表达式，但我不知道如何在该问题中使用它们...)

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...)

以下是我在 C++11 中解决问题的方法以供参考:

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

您正在寻找 std::disjunction.它在 N4564 中有规定[元.逻辑].

#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>());

现场演示

或者，适应一个 struct

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

<小时>

或者，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>{} || ...)>
{};

现场演示

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