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

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;
}