多继承下的“模棱两可”问题

多继承下的“模棱两可”的问题

        对于多继承，如果继承的多个类有相同的方法，在调用时会出现错误，例如如下的代码：

        

#include <iostream>

using namespace std;

class Lottery
{
public:
    virtual int draw();
};

int Lottery::draw()
{
    cout << "Lottery draw" << endl;
}

class GraphicalObject
{
public:
    virtual int draw();
};

int GraphicalObject::draw()
{
    cout << "Lottery draw" << endl;
}

class LotterySimulation : public Lottery, public GraphicalObject
{
};

int main()
{

    LotterySimulation *pls = new LotterySimulation;
    pls->draw();
    pls->Lottery::draw();
    pls->GraphicalObject::draw();

    return 0;
}

        错误：

        E:\ProgramExcise\Code\Test\main.cpp   35   error: request for member `draw' is ambiguous

        错误的意思即：draw 具有二义性

        通过增加一对新的classes，就可以绕个弯解决这个问题：

        

#include <iostream>

using namespace std;

class Lottery
{
public:
    virtual int draw();
};

int Lottery::draw()
{
    cout << "Lottery draw" << endl;
}

class GraphicalObject
{
public:
    virtual int draw();
};

int GraphicalObject::draw()
{
    cout << "Lottery draw" << endl;
}

class AuxLottery : public Lottery
{
public:
    virtual int lotteryDraw() = 0;
    virtual int draw() { return lotteryDraw();}
};

class AuxGraphicalObject : public GraphicalObject
{
public:
    virtual int graphicalObjectDraw() = 0;
    virtual int draw(){ return graphicalObjectDraw();}
};

class LotterySimulation : public AuxLottery, public AuxGraphicalObject
{
public:
    virtual int lotteryDraw();
    virtual int graphicalObjectDraw();
};

int LotterySimulation::lotteryDraw()
{
    cout << "lottery" << endl;
}

int LotterySimulation::graphicalObjectDraw()
{
    cout << "graphicObjectDraw" << endl;
}


int main()
{

    LotterySimulation *pls = new LotterySimulation();

    Lottery *pl = pls;
    GraphicalObject *pgo = pls;

    pl->draw();
    pgo->draw();

    return 0;
}

        但是上述这种机制，虽然能够解决掉二义性的问题，但是它无疑增加了很大的随机性。

        其次一个二义性产生源于菱形继承的情况，这种情况可以通过虚继承来解决。如下这种继承情况：

                           A

                        /      \

                     B        C

                       \       /

                           D

        

------Andy  @ 2013-06-22