模板化别名声明的部分专业化

问题描述

在这个问题中，我引出了一个特殊的解决方案，其中涉及对模板化别名声明的部分专门化.通用案例在此答案中进行了描述.假设我有一个模板类

In this question I am led to a particular solution which involves partial specializations of templatized alias declarations. The generic case is described in this answer. Suppose I have a template class

template<typename T, ...>
class X {
    // ....
};

我不是在让T腾空并专门化其他模板参数的情况下，而是在其他参数取决于T且仅取决于T的情况下.作为一个非常具体的示例(比另一个问题中的示例更易于管理)，请考虑模板类

Rather than leaving T free and specializing the other template parameters I am in a situation in which the other arguments depend on T, and on T alone. As a very concrete example (more manageable than the example in the other question) consider a template class

template<typename T, T absVal(T)>
class Number_impl {
 private:
  T _t;
 public:
  Number_impl(T t): _t(t) {}
  T abs() const {return absVal(_t);}
};

可能的专业是

 Number_impl<int, std::abs>;

和

Number_impl<double, std::fabs>;

(我知道有重载的Abs版本，这只是出于说明的目的.如果需要，请参见我的其他示例).

(I know there are overloaded abs versions, this is just for the sake of illustration. See my other example if you want).

理想情况下，我想根据单个参数(类型)定义模板类Number，以使Number< int>等于

Ideally I would like to define a template class Number depending on a single argument, the type, so that Number<int> is equal to

Number_impl<int, std::abs>;

和数字< double>等于

and Number<double> is equal to

Number_impl<double, std::fabs>;

类似以下内容(无效):

Something like the following (which doesn't work):

template<typename T>
using Number = Number_impl<T, nullptr>;

template<>
using Number<int> = Number_impl<int, std::abs>;

template<>
using Number<double> = Number_impl<double, std::fabs>;

有人知道这种方法是否可行，如何实现，或者如何以不同的方式实现吗?

Does anyone know if and how this can be made to work, or how the same can be achieved in a different way?

推荐答案

执行此类操作的正常方法与标准库的处理方法相同-具有可以专门研究的traits类:

The normal way to do this kind of thing is the same way the standard library does it - with a traits class that you can specialise:

#include <iostream>
#include <cmath>

template<typename T> struct NumberTraits;

template<typename T, class Traits = NumberTraits<T>>
class Number {
private:
    T _t;
public:
    Number(T t): _t(t) {}
    T abs() const {
        return Traits::abs(_t);
    }
};

template<> struct NumberTraits<int>
{
    static int abs(int i) {
        return std::abs(i);
    }
};

template<> struct NumberTraits<double>
{
    static double abs(double i) {
        return std::fabs(i);
    }
};


using namespace std;

auto main() -> int
{
    Number<int> a(-6);
    Number<double> b(-8.4);
    cout << a.abs() << ", " << b.abs() << endl;
    return 0;
}

预期输出:

6, 8.4

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