Resolving ambiguous this pointer in C++
- by Paul Tevis
I'm trying to derive a new class from an old one. The base class declaration looks like this:
class Driver : public Plugin, public CmdObject
{
protected:
Driver();
public:
static Driver* GetInstance();
virtual Engine& GetEngine();
public:
// Plugin methods...
virtual bool InitPlugin (Mgr* pMgr);
virtual bool Open();
virtual bool Close();
// CmdObject
virtual bool ExecObjCmd(uint16 cmdID, uint16 nbParams, CommandParam *pParams, CmdChannelError& error);
Mgr *m_pMgr;
protected:
Services *m_pServices;
Engine m_Engine;
};
Its constructor looks like this:
Driver::Driver() :
YCmdObject("Driver", (CmdObjectType)100, true),
m_Engine("MyEngine")
{
Services *m_pServices = NULL;
Mgr *m_pMgr = NULL;
}
So when I created my derived class, I first tried to simply inherit from the base class:
class NewDriver : public Driver
and copy the constructor:
NewDriver::NewDriver() :
CmdObject("NewDriver", (EYCmdObjectType)100, true),
m_Engine("MyNewEngine")
{
Services *m_pServices = NULL;
Mgr *m_pMgr = NULL;
}
The compiler (VisualDSP++ 5.0 from Analog Devices) didn't like this:
".\NewDriver.cpp", line 10: cc0293: error: indirect nonvirtual base
class is not allowed
CmdObject("NewDriver", (EYCmdObjectType)100, true),
That made sense, so I decided to directly inherit from Plugin and CmdObject. To avoid multiple inheritance ambiguity problems (so I thought), I used virtual inheritance:
class NewDriver : public Driver, public virtual Plugin, public virtual CmdObject
But then, in the implementation of a virtual method in NewDriver, I tried to call the Mgr::RegisterPlugin method that takes a Plugin*, and I got this:
".\NewDriver.cpp", line 89: cc0286: error: base class "Plugin" is
ambiguous
if (!m_pMgr->RegisterPlugin(this))
How is the this pointer ambiguous, and how do I resolve it?
Thanks,
--Paul
