Question

我有3个类，一个是基类，其他是基类的继承类，这里是类的代码：

// Event Class
#ifndef EVENT_H
#define EVENT_H

#include <iostream>

namespace Engine
{
    namespace Data
    {
        // base class
        class Event
        {
            public:
                // Class Variable
                int Measure;
                int Beat;
                int Position;

                // This Class that was I mean
                class SampleEvent;
                class TimeEvent;

                // Constructor
                Event(int measure, int beat, int pos);
        };

        // Sample Event Class (inherit to Event Class)
        class Event::SampleEvent : public Event
        {
            public:
            // variable in SampleEvent Class
            int ID;
            float Pan;
            float Vol;

            // Constructor
            SampleEvent(int id, float pan, float vol, int measure, int beat, int pos);
        };

        // Time Event Class (inherit to Event class)
        class Event::TimeEvent : public Event
        {
            public:
            // variable in TimeEvent Class
            double Value;

            // Constructor
            TimeEvent(double value, int measure, int beat, int pos);
        };

        // Constructor of Event
        Event::Event(int measure, int beat, int pos)
        {
            Measure         = measure;
            Beat            = beat;
            Position        = pos;
        }

        // Constructor of Sample Event
        Event::SampleEvent::SampleEvent(int id, float pan, float vol, int measure, int beat, int pos) : Event(measure, beat, pos)
        {
            ID                      = id;
            Pan                     = pan;
            Vol                     = vol;
            Measure         = measure;
            Beat            = beat;
            Position        = pos;
        }

        // Constructor of Time Event
        Event::TimeEvent::TimeEvent(double value, int measure, int beat, int pos) : Event(measure, beat, pos)
        {
            Value                   = value;
            Measure         = measure;
            Beat            = beat;
            Position        = pos;
        }
    }      
}
#endif

假设我有2个变量，SE和TE，SE用于SampleEvent，TE用于TimeEvent，我只想将它们插入到vector中，并从向量中获取它们，这是我目前的代码：

Event::SampleEvent SE = Event::SampleEvent(1000, 0, 0, 10, 10, 10);
Event::TimeEvent TE = Event::TimeEvent(200, 20, 20, 20);
vector<Event> DataEvent;

// insert Event
DataEvent.push_back(SE);
DataEvent.push_back(TE);

// Now I just want to get it back
Event::SampleEvent RSE = DataEvent[0]; // -> Error no suitable user-defined conversion from "Engine::Data::Event" to "Engine::Data::Event::SampleEvent" exists
Event::TimeEvent RTE = DataEvent[0];   // -> Error no suitable user-defined conversion from "Engine::Data::Event" to "Engine::Data::Event::TimeEvent" exists

// And I don't know how to detecting the inheritance Class
// something like if (RSE == Event::SampleEvent) or if (RTE == Event::TimeEvent) @_@

Answer 1

我相信你需要施展才能取回它。因为虽然你可以隐式地将一个SampleEvent和TimeEvent强制转换为一个Event，但是你不能隐式地这样做。

您需要使用~~引用Event或~~指向Event的指针才能使其在转换时正常工作。

使用参考文献

*removed* you cannot make a vector reference.

使用指针

Event::SampleEvent SE = Event::SampleEvent(1000, 0, 0, 10, 10, 10);
Event::TimeEvent TE = Event::TimeEvent(200, 20, 20, 20);

std::vector<Event*> DataEvent;
// insert Event
DataEvent.push_back(&SE);
DataEvent.push_back(&TE);
// get the events back, note this can throw an exception if you cast incorrectly.
Event::SampleEvent* RSE = (Event::SampleEvent*)DataEvent[0]; 
Event::TimeEvent* RTE = (Event::TimeEvent*)DataEvent[1]; 
/// This also Works using static_cast
//Event::SampleEvent* RSE = static_cast<Event::SampleEvent*>(DataEvent[0]); 
//Event::TimeEvent* RTE = static_cast<Event::TimeEvent*>(DataEvent[1]);  
std::cout << RSE->ID << std::endl;
std::cout << RTE->Value << std::endl;

输出为：1000 200

有关投射的详情，请参阅此stackoverflow答案。

Answer 2

你有一个载体

vector<Event> DataEvent;

所以你应该这样使用它：

Event E = DataEvent[0];

如果键入Event::SampleEvent RSE = DataEvent[0];，那么您没有使用指向具有基类指针的子类的能力，而只是简单地转换对象。如果你想要这个演员阵容成功，你必须提供转换操作符，或者考虑使用指针向量：vector<Event* > DataEvent;然后如果你想获得特定事件，你可以使用dynamic_cast<>这将允许你动态获取子类的对象，只有它实际上是这个子类对象：记住你可以在你的向量中在公共基类下有许多不同的子类你也需要一些虚拟方法，否则类型不被视为多态，你不能使用dynamic_cast<>。它足以添加

virtual void f(){}

到事件类

Answer 3

不可能将子类对象itsef 强制转换为基类对象，而是可以将子类对象的引用（如指针）轻松地转换为基类的引用。

检测转换类并将其转换为基类

3 个答案:

使用参考文献

使用指针