如何使 is_arithmetic<myClass>::value 为真? [英] How to make is_arithmetic<myClass>::value to be true?

问题描述

这个想法是我有一个对输入进行算术运算的函数，所以可能是这样的:

The idea is that I have a function that does something arithmetic to the input, so maybe something like:

#include <type_traits>
#include <vector>

using namespace std;

template<typename T> 
double mean(const vector<T>& vec)
{
    static_assert(is_arithmetic<T>::value, "Arithmetic not possible on this type");
    //compute mean (average)
}//mean

这很好用，并计算我输入的所有数字类型的平均值.但是假设我创建了一个新类:

This works great, and computes the mean for all the number types that I put in. But lets say I then create a new class:

class foo
{
    // class that has arithmetic operations created
};// foo

在这个类的定义中，我定义了所需的运算符 + 和/，因此它们可以处理预期的输入.现在我想在我的新类中使用我的平均函数，但由于 static_assert，它显然不会编译.那么我如何告诉编译器我的新类应该满足 is_arithmetic<foo>::value ?

And in the definition of this class, I defined the needed operators, + and /, so they work with expected inputs. Now I want to use my mean function with my new class, but it obviously won't compile due to the static_assert. So how do I tell the compiler that my new class should satisfy is_arithmetic<foo>::value ?

如果我在创建类时可以给它一个满足 is_arithmetic 的类型，那就太好了，但这似乎可能会以某种方式导致 type_traits 出现问题?

It would be great if when I create the class I could give it a type that satisfies is_arithmetic, but this seems like it might cause a problem with type_traits somehow?

或者我需要创建一个新的测试，检查以查看

Or would I need to create a new test, that checks to see

is_arithmetic<T>::value || type(T,foo)

或者类似的东西?

如果可能的话，我宁愿只调整我的类，而不是调整函数，但我很好奇解决方案.

I'd prefer to only have to adapt my class, rather than the function if possible, but I'm curious for a solution.

推荐答案

标准库类型特征，例如 std::is_arithmetic，有一个例外(std::common_type代码>)，是一成不变的".尝试专门化它们会导致未定义的行为.is_arithmetic 测试类型是否为标准定义的算术类型；用户定义的类型绝不是算术类型.

The standard library type traits, such as std::is_arithmetic, with one exception (std::common_type), are "set in stone". Attempting to specialize them causes undefined behavior. is_arithmetic tests if the type is an arithmetic type as defined by the standard; user-defined types are never arithmetic types.

您可以编写自己的特征来测试对算术运算符的支持:

You can write your own trait that tests for support for the arithmetic operators:

template<class...> struct voidify { using type = void; };
template<class... Ts> using void_t = typename voidify<Ts...>::type;

template<class T, class = void>
struct supports_arithmetic_operations : std::false_type {};

template<class T>
struct supports_arithmetic_operations<T, 
           void_t<decltype(std::declval<T>() + std::declval<T>()),
                  decltype(std::declval<T>() - std::declval<T>()),
                  decltype(std::declval<T>() * std::declval<T>()),
                  decltype(std::declval<T>() / std::declval<T>())>> 
       : std::true_type {};

仅当所有四个表达式都是格式良好的(即 T 支持运算符 +、-、*、/)时，部分特化才会匹配.

The partial specialization will match only if all four expressions are well-formed (i.e., that T supports operators +, -, *, /).

演示.

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