参数包识别std :: is_base_of()

时间:2012-11-26 10:24:18

标签: c++ templates variadic-templates typetraits

是否有可能有一个静态断言,提供为模板参数的类型是否实现了参数包中列出的所有类型,即。一个参数包知道std :: is_base_of()?

template <typename Type, typename... Requirements>
class CommonBase
{
    static_assert(is_base_of<Requirements..., Type>::value, "Invalid.");
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            parameter pack aware version of std::is_base_of()
public:
    template <typename T> T* as()
    {
        static_assert(std::is_base_of<Requirements..., T>::value, "Invalid.");
        return reinterpret_cast<T*>(this);
    }
};

2 个答案:

答案 0 :(得分:16)

更新C ++ 17: 使用C ++ 17的折叠表达式,这几乎变得微不足道了:

template <typename Type, typename... Requirements>
class CommonBase
{
    static_assert((std::is_base_of_v<Type, Requirements> && ...), "Invalid.");
};

原始答案(C ++ 11/14): 您可以使用包扩展和std::all_of的某些静态版本:

template <bool... b> struct static_all_of;

//implementation: recurse, if the first argument is true
template <bool... tail> 
struct static_all_of<true, tail...> : static_all_of<tail...> {};

//end recursion if first argument is false - 
template <bool... tail> 
struct static_all_of<false, tail...> : std::false_type {};

// - or if no more arguments
template <> struct static_all_of<> : std::true_type {};

template <typename Type, typename... Requirements>
class CommonBase
{
    static_assert(static_all_of<std::is_base_of<Type, Requirements>::value...>::value, "Invalid.");
    //                                               pack expansion:      ^^^
};

struct Base {};
struct Derived1 : Base {};
struct Derived2 : Base {};
struct NotDerived {};

int main()
{
  CommonBase <Base, Derived1, Derived2> ok;
  CommonBase <Base, Derived1, NotDerived, Derived2> error;
}

通过在Requirements...中为问号添加std::is_base_of<Type, ?>::value中的每个类型,包扩展将扩展为您获得的值列表,即对于main中的第一行,它将扩展为{{1对于第二行,它将是static_all_of<true, true>

答案 1 :(得分:3)

仅供将来参考,因为我刚遇到这个问题,使用C ++ 17,您现在可以使用这样的折叠表达式:

template<typename Base, typename... Args>
constexpr auto all_base_of()
{
    return (std::is_base_of<Base, Args>::value && ...);
}

static_assert(all_base_of<Base, A, B, C>());