c++ struct reference types

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*);
 开发者_开发知识库       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?

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.

Change your function declaration to take the argument as a pointer rather than a reference:

void Uartcom::req_doSomething(IAckHandler * ackHandler)
                                         ^^^

You have a few issues with your req_doSomething function:

1. ackHandler is a reference which cannot be NULL.
2. You are trying to assign a reference to a pointer (req.ackHandler = &ackHandler; is what you want).

However, it's probably a better idea to do something like this:

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);
}

It describes exactly what you should do.

just past the object's address to the pointer:

req.ackHandler = &ackHandler;

Look carefully. The member in the struct is an "IAckHandler*" while the arg received in the setter function in a reference to IAckHandler. What you may want to do is:

req.ackHandler = &ackHandler;

regards,

Yati Sagade

It looks as if you must have declared ackHandler as an IAckHandler* in struct reqmsg. In C++, reference types (like IAckHandler&), although implemented using pointers, are treated syntactically as if they were embedded objects - basically you need to work with an IAckHandler& as if it were an IAckHandler, not an IAckHandler*.

Therefore, you should be able to fix your code like this:

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);
}