什么是std :: false_type或std :: true_type？ [英] What is std::false_type or std::true_type?

问题描述

我看到它的用法如下

template <typename T>
struct DependentFalse : std::false_type
{};

然后，在这里使用

template <typename T>
class RadarSensor
{
    static_assert(DependentFalse<T>::value, "RadarSensor must be created using Identifier template");
};

我不知道它的用途是什么？

I do not have idea what is it used for?

什么是DependentFalse结构？

What is DependentFalse structure?

推荐答案

std :: false_type 用作traits类型的构建块，并定义为 std :: integral_constant< bool，false> （我将在这里跳过）。它的定义可以归结为以下形式（简化）：

std::false_type is used as a building block in type traits and is defined as std::integral_constant<bool, false> (which I will skip over here). It's definition boils down to something like this (simplified):

struct false_type {
    static constexpr bool value = false;
    constexpr operator bool() const noexcept { return value; }
    // There is more here, but it doesn't really matter for your question
};

类似地：

struct true_type {
    static constexpr bool value = true;
    constexpr operator bool() const noexcept { return value; }
    // There is more here, but it doesn't really matter for your question
};

用于表示值 false 和 true 作为类型。这在类型特征中很有用，您可以让类模板从 std :: false_type 或 std :: true_type 继承对于不同的（部分）专业化，这取决于template参数满足的某些条件。这样做可以测试给定类型是否满足类型特征的条件，并通过访问静态 value value 。 c $ c>成员，它继承自 std :: false_type 或 std :: true_type 或通过转换

It is used to represent the values false and true as types. This is useful in type traits where you let a class template inherit from either std::false_type or std::true_type for different (partial) specializations, depending on some condition met by the template argument. Doing so allows one to test whether a given type satisfies the condition of the type trait and to obtain a compile time constant value indicating the result through access to the static value member which is inherited from either std::false_type or std::true_type or alternative through conversion of an instance of the type trait using the conversion operator.

这里显示的是一个简单的类型特征，它始终（对于所有 T ）的值为 std :: false_type 。它在 static_asserts 中使用，当实例化它们所在的模板时，它应该总是失败。这是必要的，因为不依赖模板参数的 static_assert 已经在定义点而不是实例化点被触发，因此使每个程序都包含某些内容。像 static_assert（false）; 格式错误。

What you are showing here is a simple type trait which always (for all T) evaluates to std::false_type. It is used in static_asserts that should always fail when the template they are located in is instantiated. This is necessary, because a static_assert that does not dependent on a template parameter is triggered already at the point of definition, rather than the point of instantiation, therefore making every program containing something like static_assert(false); ill-formed.

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