类的类型模板参数的限制范围 [英] Limit range of type template arguments for class

问题描述

在没有任意typedef的情况下如何实现这种效果？

How can I have this effect without the arbitrary typedefs?

#include <type_traits>
#include <iostream>

typedef int Primary;
typedef float Secondary;

template<Class C, std::enable_if<std::is_same<Class, Primary>::value || std::is_same<Class, Secondary>::value> = 0>
class Entity {
public:
    template<std::enable_if<std::is_same<Class, Secondary>::value>::type = 0>
    void onlyLegalForSecondaryEntities() {
        std::cout << "Works" << std::endl;  
    }
};

int main() {
    Entity<Secondary> e;
    e.onlyLegalForSecondaryEntities();
    return 0;
}

是否有更优雅的方式来生成此内容，从而使实体只能使用 Primary 或 Secondary 作为模板参数来实例化吗？

Is there a more elegant way to produce this so that Entity can only be instantiated with Primary or Secondary as template arguments?

推荐答案

更正代码中的错误后：

在C ++ 1z中您可以使用 std :: disjunction ：

In C++1z you can easily roll a trait is_any with std::disjunction:

template<typename T, typename... Others>
struct is_any : std::disjunction<std::is_same<T, Others>...>
{
};

在C ++ 11中，您可以实现分离 as

In C++11, you can implement disjuncation as

template<class...> struct disjunction : std::false_type { };
template<class B1> struct disjunction<B1> : B1 { };
template<class B1, class... Bn>
struct disjunction<B1, Bn...> 
    : std::conditional<B1::value != false, B1, disjunction<Bn...>>::type { };

然后将您的班级模板定义为

Then define your class template as

template<class C, typename std::enable_if<is_any<C, Primary, Secondary>::value>::type* = nullptr>
class Entity {
public:
    template<typename std::enable_if<std::is_same<C, Secondary>::value>::type* = nullptr>
    void onlyLegalForSecondaryEntities() {
        std::cout << "Works" << std::endl;
    }
};

演示

您可以更进一步，并创建 enable_if_any 别名以解决到 void （如果可能）：

You can take this further and make enable_if_any alias that would resolve to void if possible:

template<typename This, typename... Elems>
using enable_if_is_any = typename std::enable_if<is_any<This, Elems...>::value>::type;

template<class C, enable_if_is_any<C, Primary, Secondary>* = nullptr>
class Entity {
public:
    template<typename std::enable_if<std::is_same<C, Secondary>::value>::type* = nullptr>
    void onlyLegalForSecondaryEntities() {
        std::cout << "Works" << std::endl;
    }
};

演示

这篇关于类的类型模板参数的限制范围的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！