返回指向子类的指针的好方法

尝试阅读有关访问者模式的信息，以避免降级。希望能有所帮助。

http://en.wikipedia.org/wiki/Visitor_pattern

处理这一问题的一种方法是使用消息传递系统，下面是游戏引擎如何通过消息传递而不是向调用函数投射指针来处理执行任务和请求数据的代码示例。

#include <iostream>
#include <stdio.h>
#include <list>
#include <map>
using namespace std;
struct Vector3
{
public:
    Vector3() : x(0.0f), y(0.0f), z(0.0f)
    {}
    float x, y, z;
};
enum eMessageType
{
    SetPosition,
    GetPosition,    
};
class BaseMessage
{
protected: // Abstract class, constructor is protected
    BaseMessage(int destinationObjectID, eMessageType messageTypeID) 
        : m_destObjectID(destinationObjectID)
        , m_messageTypeID(messageTypeID)
    {}
public: // Normally this isn't public, just doing it to keep code small
    int m_destObjectID;
    eMessageType m_messageTypeID;
};
class PositionMessage : public BaseMessage
{
protected: // Abstract class, constructor is protected
    PositionMessage(int destinationObjectID, eMessageType messageTypeID, 
                    float X = 0.0f, float Y = 0.0f, float Z = 0.0f)
        : BaseMessage(destinationObjectID, messageTypeID)
        , x(X)
        , y(Y)
        , z(Z)
    {
    }
public:
    float x, y, z;
};
class MsgSetPosition : public PositionMessage
{
public:
    MsgSetPosition(int destinationObjectID, float X, float Y, float Z)
        : PositionMessage(destinationObjectID, SetPosition, X, Y, Z)
    {}
};
class MsgGetPosition : public PositionMessage
{
public:
    MsgGetPosition(int destinationObjectID)
        : PositionMessage(destinationObjectID, GetPosition)
    {}
};
class BaseComponent
{
public:
    virtual bool SendMessage(BaseMessage* msg) { return false; }
};
class RenderComponent : public BaseComponent
{
public:
    /*override*/ bool SendMessage(BaseMessage* msg)
    {
        // Object has a switch for any messages it cares about
        switch(msg->m_messageTypeID)
        {
        case SetPosition:
            {                   
                // Update render mesh position/translation
                cout << "RenderComponent handling SetPositionn";
            }
            break;
        default:
            return BaseComponent::SendMessage(msg);
        }
        return true;
    }
};
class Object
{
public:
    Object(int uniqueID)
        : m_UniqueID(uniqueID)
    {
    }
    int GetObjectID() const { return m_UniqueID; }
    void AddComponent(BaseComponent* comp)
    {
        m_Components.push_back(comp);
    }
    bool SendMessage(BaseMessage* msg)
    {
        bool messageHandled = false;
        // Object has a switch for any messages it cares about
        switch(msg->m_messageTypeID)
        {
        case SetPosition:
            {               
                MsgSetPosition* msgSetPos = static_cast<MsgSetPosition*>(msg);
                m_Position.x = msgSetPos->x;
                m_Position.y = msgSetPos->y;
                m_Position.z = msgSetPos->z;
                messageHandled = true;
                cout << "Object handled SetPositionn";
            }
            break;
        case GetPosition:
            {
                MsgGetPosition* msgSetPos = static_cast<MsgGetPosition*>(msg);
                msgSetPos->x = m_Position.x;
                msgSetPos->y = m_Position.y;
                msgSetPos->z = m_Position.z;
                messageHandled = true;
                cout << "Object handling GetPositionn";
            }
            break;
        default:
            return PassMessageToComponents(msg);
        }
        // If the object didn't handle the message but the component
        // did, we return true to signify it was handled by something.
        messageHandled |= PassMessageToComponents(msg);
        return messageHandled;
    }
private: // Methods
    bool PassMessageToComponents(BaseMessage* msg)
    {
        bool messageHandled = false;
        std::list<BaseComponent*>::iterator compIt = m_Components.begin();
        for ( compIt; compIt != m_Components.end(); ++compIt )
        {
            messageHandled |= (*compIt)->SendMessage(msg);
        }
        return messageHandled;
    }
private: // Members
    int m_UniqueID;
    std::list<BaseComponent*> m_Components;
    Vector3 m_Position;
};
class SceneManager
{
public: 
    // Returns true if the object or any components handled the message
    bool SendMessage(BaseMessage* msg)
    {
        // We look for the object in the scene by its ID
        std::map<int, Object*>::iterator objIt = m_Objects.find(msg->m_destObjectID);       
        if ( objIt != m_Objects.end() )
        {           
            // Object was found, so send it the message
            return objIt->second->SendMessage(msg);
        }
        // Object with the specified ID wasn't found
        return false;
    }
    Object* CreateObject()
    {
        Object* newObj = new Object(nextObjectID++);
        m_Objects[newObj->GetObjectID()] = newObj;
        return newObj;
    }
private:
    std::map<int, Object*> m_Objects;
    static int nextObjectID;
};
// Initialize our static unique objectID generator
int SceneManager::nextObjectID = 0;
int main()
{
    // Create a scene manager
    SceneManager sceneMgr;
    // Have scene manager create an object for us, which
    // automatically puts the object into the scene as well
    Object* myObj = sceneMgr.CreateObject();
    // Create a render component
    RenderComponent* renderComp = new RenderComponent();
    // Attach render component to the object we made
    myObj->AddComponent(renderComp);
    // Set 'myObj' position to (1, 2, 3)
    MsgSetPosition msgSetPos(myObj->GetObjectID(), 1.0f, 2.0f, 3.0f);
    sceneMgr.SendMessage(&msgSetPos);
    cout << "Position set to (1, 2, 3) on object with ID: " << myObj->GetObjectID() << 'n';
    cout << "Retreiving position from object with ID: " << myObj->GetObjectID() << 'n';
    // Get 'myObj' position to verify it was set properly
    MsgGetPosition msgGetPos(myObj->GetObjectID());
    sceneMgr.SendMessage(&msgGetPos);
    cout << "X: " << msgGetPos.x << 'n';
    cout << "Y: " << msgGetPos.y << 'n';
    cout << "Z: " << msgGetPos.z << 'n';
}

在这种情况下不要使用工厂方法。你必须改变你的设计。看起来您需要其他接口而不是基类，或者根本不需要它。请更详细地描述你的问题，这样我们就可以为你的问题做出适当的设计。