覆盖可变参数类模板中的多个虚函数

Question

让我举一个代码示例.

#include <iostream>
#include <utility>
#include <tuple>

template <typename Service>
struct SubscriberImpl {
    virtual void handleService(Service const&) = 0;
};

template <typename...ServiceType>
struct Subscriber : SubscriberImpl<ServiceType>... {

};

struct IntService {};
struct FloatService {};

template <typename StatusUpdatePolicy, typename... ServiceType>
struct StatusUpdater : Subscriber<ServiceType...>
{
    StatusUpdater(StatusUpdatePolicy const& statusUpdater)
    : m_statusUpdater{statusUpdater}
    {}
    // wont work
    void handleService(IntService const& service) override {
        m_statusUpdater.updateService(service);
    }

    void handleService(FloatService const& service) override {
        m_statusUpdater.updateService(service);
    }

    StatusUpdatePolicy m_statusUpdater;
}; 

struct DummyPolicy {
    void updateService(IntService const& service) {
        m_i = 42;
        std::cout << m_i << "\n";
    }

    void updateService(FloatService const& service) {
        m_f = 3.14f;
        std::cout << m_f << "\n";
    }

    int m_i;
    float m_f;
};
int main() {

    StatusUpdater<DummyPolicy, IntService, FloatService> su(DummyPolicy{}); 

    su.handleService(IntService{});
    su.handleService(FloatService{});
}

这里Subscriber有一个handleService(ServiceType const)包含每个模板参数的纯虚函数ServiceType....所以我必须覆盖每一个StatusUpdater.在这里,我提供了我需要手工对那些IntService和FloatService,知道我将只需要在那些本小例子.但是我希望能够为包中的任何内容提供覆盖ServiceType....updateService无论如何,他们都将调用给定策略的方法.

请注意,Subscriber它来自外部库,我无法修改其定义.

Answer 1

您不能将此类实现直接放入类中，而必须继承它们（类似于如何Subscriber从多个SubscriberImpl实例化继承）。但是，要覆盖它们，并仍然使您的类可以作为多态使用Subscriber，则必须“顺序”继承它们，而不是“并行”继承它们。此外，好奇地重复使用的模板模式可用于为所有实现提供对最终重写对象的访问权限：

template <class Self, class SubscriberClass, class... ServiceTypes>
struct StatusUpdaterOverride;


template <class Self, class SubscriberClass, class ThisType, class... RemainingTypes>
struct StatusUpdaterOverride<Self, SubscriberClass, ThisType, RemainingTypes...> : StatusUpdaterOverride<Self, SubscriberClass, RemainingTypes...>
{
  void handleService(ThisType const& service) override
  {
    static_cast<Self*>(this)->m_statusUpdater.updateService(service);
  }
  using StatusUpdaterOverride<Self, SubscriberClass, RemainingTypes...>::handleService;
};


template <class Self, class SubscriberClass, class ThisType>
struct StatusUpdaterOverride<Self, SubscriberClass, ThisType> : SubscriberClass
{
  void handleService(ThisType const& service) override
  {
    static_cast<Self*>(this)->m_statusUpdater.updateService(service);
  }
};


template <class StatusUpdatePolicy, class... ServiceType>
struct StatusUpdater : StatusUpdaterOverride<StatusUpdater<StatusUpdatePolicy, ServiceType...>, Subscriber<ServiceType...>, ServiceType...>
{
    StatusUpdater(StatusUpdatePolicy const& statusUpdater)
    : m_statusUpdater{statusUpdater}
    {}

    StatusUpdatePolicy m_statusUpdater;
};

[现场示例]