error C2614: 'CYYYRegister' : illegal member initialization:

Hello

I am trying to understand an issue which I know how to fix but don't
know why it works.

Getting this error:
error C2614: 'CYYYRegister' : illegal member initialization:
'CRequest' is not a base or member

Code is below.  If I change class CXXXRequest slightly as below:

class CXXXRequest : public virtual CRequest

Then the error goes away.

Why?  Why do I need the virtual keyword for compiler to realise that
CRequest is a base?


#include <string>
#include <iostream>


struct message_t
{
	long id;
	std::string strItem;
};

class TmMessage
{
public:
	TmMessage(message_t *_msg) : msg(_msg) {}
	message_t* GetMsg() const { return msg; }

protected:
	message_t* msg;
};

class CRequest : public TmMessage
{
public:
	CRequest::CRequest(int clientId, message_t *_msg)
  : TmMessage(_msg)
	{
		std::cout << "CRequest::CRequest(int clientId, message_t *_msg)" <<
std::endl;
	}
	void CRequest::Options(unsigned idx, ...)
	{
		std::cout << "CRequest::Options" << std::endl;
	}
};

class CXXXRequest : public /*virtual*/ CRequest
{
public:
	CXXXRequest::CXXXRequest(int clientId, message_t *msg)
		: CRequest(clientId, msg){}

	int CXXXRequest::Invoke()
	{
		std::cout << "CXXXRequest::Invoke" << std::endl;
		return 0;
	}
};

class CYYYRegister : public CXXXRequest
{
public:
  CYYYRegister(int clientId, message_t *msg) :
    CRequest(clientId, msg),
    CXXXRequest(clientId, msg)
	{
		std::cout << "CYYYRegister(int clientId, message_t *msg)" <<
std::endl;
	}
	int CYYYRegister::Invoke()
	{
		std::cout << "CYYYRegister::Invoke()" << std::endl;
	}
};



int main()
{
	int clientid = 1;

	message_t mymsg;
	mymsg.id = 7;
	mymsg.strItem = "msg item";
	CYYYRegister y(clientid, &mymsg);

	CXXXRequest x(clientid, &mymsg);

	message_t* pMsg = y.GetMsg();

	int ret = x.Invoke();


	return 0;
}