QueryInterface的2义性问题!

[yuantao]

#include<iostream>
using namespace std;
struct IUnknown
{
public:
virtual long QueryInterface(string& s, void ** ppv) = 0;
};

struct IA :public IUnknown
{
public:
virtual void Ax() = 0;
virtual void Ay() = 0;
};

struct IB:public IUnknown
{
public:
virtual void Bx() = 0;
virtual void By() = 0;
};

class CA :public IA,public IB
{
public:
void Ax(){cout<<"AX"<<endl;}
void Ay(){cout<<"AY"<<endl;}
void Bx(){cout<<"BX"<<endl;}
void By(){cout<<"BY"<<endl;}
};
long CA::QueryInterface(string &s,void **ppv)
{
if(s == "IA")
*ppv = static_cast<IA*>(this);

else if(s == "IB")
*ppv = static_cast<IB*>(this);

else if(s == "IUnknown")
*ppv = static_cast<IA*>(this);
else
{
*ppv =NULL;
return FALSE;
}
return TRUE;
}
CA *CreateInstance()
{
CA *pCA = new CA;
return static_cast<IA*>(pCA);
}
void Test(void **pt)
{
static_cast<CA*>(*pt);
}
int main(int argc, char* argv[])
{
IA *p = CreateInstance();
Test((void**)&p);
p->By();
return 0;
}

编译提示QueryInterface有2义性，但是为什么用com里自带的IUnknown就没有这个问题呢?

[Cline]

不是啦，是你的CA少QueryInterface()的申明
class CA :public IA,public IB
{
public:
void Ax(){cout<<"AX"<<endl;}
void Ay(){cout<<"AY"<<endl;}
void Bx(){cout<<"BX"<<endl;}
void By(){cout<<"BY"<<endl;}
／／virtual long QueryInterface(string& s, void ** ppv);
};

[yuantao]

#include <iostream>
using namespace std;
struct IUnknown
{
public:
virtual long QueryInterface(string& s, void ** ppv) = 0;
void Add(){return ;}
};

struct IA :public IUnknown
{
public:
virtual void Ax() = 0;
virtual void Ay() = 0;
};

struct IB:public IUnknown
{
public:
virtual void Bx() = 0;
virtual void By() = 0;
};

class CA :public IA,public IB
{
public:
void Ax(){cout<<"AX"<<endl;}
void Ay(){cout<<"AY"<<endl;}
void Bx(){cout<<"BX"<<endl;}
void By(){cout<<"BY"<<endl;}
virtual long QueryInterface(string& s, void ** ppv);
};

long CA::QueryInterface(string &s,void **ppv)
{
if(strcmp(s.c_str(), "IA") == 0)
*ppv = static_cast<IA*>(this);

else if(strcmp(s.c_str(), "IB") == 0)
*ppv = static_cast<IB*>(this);

else if(strcmp(s.c_str(), "IUnknown") == 0)
*ppv = static_cast<IA*>(this);
else
{
*ppv =NULL;
return 0;
}
return 1;
}
IUnknown *CreateInstance()
{
CA *pCA = new CA;
return static_cast<IA*>(pCA);
}

int main(int argc, char* argv[])
{
IUnknown *p = CreateInstance();

string s = "IB";
IB *pIB = NULL;
if(p->QueryInterface(s, (void**)&pIB))
pIB->By();
return 0;
}

已经搞定，但是有疑问
为什么把IUnknown改成1个非纯虚基类就可以了呢？？？

[captainwh]

应该是由于多重继承导致的问题吧

[yuantao]

是啊，谁能解释一下这其中的原理吗，肯定跟c++对多重继承的处理有关系