检查是否在可变参数模板参数包中传递了类型

2021-12-13 00:00:00 templates c++ c++17 typetraits

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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