Question

I have a class called IAckHandler

class IAckHandler {
public:
    virtual ~IAckHandler();

    virtual void handleAck(long messageType, bool ackrcvd, uint8_t ackByte) = 0;
private:

};

The intended use is for other classes to inherit, so this is basically a “interface” class.

I am using message queues to submit requests to a polling thread(uart comm) and want to pass in an IAckHandler into the message queue message.
The struct is like so:

struct reqmsg
    {
        long int mtype;
        void (*reqHandler)(MasterRadioComm*, uint8_t*);
        IAckHandler* ackHandler;
        unsigned char mtext[NUM_INDICES];
    };

Here is the function to submit to message queue

void Uartcom::req_doSomething(IAckHandler& ackHandler)
{
    struct reqmsg req;
    if(ackHandler == NULL) req.ackHandler = this;
    else req.ackHandler = ackHandler;
    msgsnd(m_msgQueueKey, &req, sizeof(struct reqmsg) - sizeof(long), 0);
}

But when I pass in a reference to an IAckHandler, I get cannot convert ‘IAckHandler’ to ‘IAckHandler*’ in assignment.

Can I call req_doSomething(this) from another class that inherits IAckHandler?

Answer 1

You seem to be lacking some basic pointers and references knowledge. For the req_doSomething given in the example, &ref will give you a pointer to the referred object so it should be

else req.ackHandler = &ackHandler;

and to get a reference from a pointer, you derreference it with *ptr so the call would be

req_doSomething(*this);

…or just drop the reference and work entirely with pointers.

Update: As others have pointed out, a reference cannot be NULL.