callback in a class

I need to make a callback for a function in a class, when i just declare the function itself, the compiler complains when i want to pass the pointer to the function.

I know the anser has to do with a typedef (did something like this a long time ago), but I forgot how to declare the function with it....

here's something similar to what I have

Code:

---

class myclass
{
private:
    void CALLBACK myCallbackFunction(long num);
    void some_other_function(void);
}

void CALLBACK myclass::myCallbackFunction(long num)
{
}

void myclass::some_other_function(void)
{
      // compiler gives error on next line
      // cannot convert (function prototype here) to DWORD_PTR
     
      ret = API_function((DWORD_PTR)myCallbackFunction);
}

---

Anyone knows how to make it work ?

You have mispelled CALLBACK.

Your right, in this code I did (I edited to CALLBACK not COLLBACK as I had before)... but it is not mispelled in my real code...

If I did, the compiler would give me a sintax error...

And the code I wrote there is just a sample of what i'm doing... If I paste my real class, it will be too much code to figure out the problem...

How about trying this

Code:

---

class myclass
{
private:
    void CALLBACK myCallbackFunction(long num);
    void some_other_function(void);
}

void CALLBACK myclass::myCallbackFunction(long num)
{
}

void myclass::some_other_function(void)
{
      // compiler gives error on next line
      // cannot convert (function prototype here) to DWORD_PTR
     
      ret = API_function((DWORD_PTR)myclass::myCallbackFunction);
}

---

Now I get 2 errors at the same line...

error C2440: 'type cast' : cannot convert from 'void (__stdcall myclass::* )(long)' to 'DWORD_PTR'

What's that DWORD_PTR doing there?

Code:

---

ret = API_function((DWORD_PTR)myCallbackFunction);

---

All pointers whether it is pointing to char or short or integer are all 32 bit. Pointer is a special type.

What you are doing, is casting the function to a dword pointer?

This is the API function i'm trying to use:

MMRESULT acmStreamOpen(
LPHACMSTREAM phas,
HACMDRIVER had,
LPWAVEFORMATEX pwfxSrc,
LPWAVEFORMATEX pwfxDst,
LPWAVEFILTER pwfltr,
DWORD_PTR dwCallback,
DWORD_PTR dwInstance,
DWORD fdwOpen
);


If you look at dwCallback parameter, you'll see it's a DWORD_PTR

Quote:

---

Originally posted by CVMichael
Now I get 2 errors at the same line...

error C2440: 'type cast' : cannot convert from 'void (__stdcall myclass::* )(long)' to 'DWORD_PTR'

---

What's the other error?

I made a mistake there, I actually got one error... (the one I pasted)

But how to you do a callback in a class in general ?

Then I do not know what the error may be, though everything seems to be alright.

Quote:

---

Originally posted by CVMichael
But how to you do a callback in a class in general ?

---

The class method will have to be static.

I made my callback static, now I get this error...

error C2352: 'CACMClass::StreamEvent' : illegal call of non-static member function

I'm trying to call a method in the class from the callback function, how do I do it ?

From MSDN

Compiler Error C2352
'class::function' : illegal call of non-static member function

The specified nonstatic member function was called in a static member function.

The following is an example of this error:

class X
{
public:
static void func1();
void func2();
static void func3()
{
func1(); // OK, calls static func1
func2(); // error, calls nonstatic func2
}
};

==========================================
Your myCallbackFunction() uses non-static members, so we have to try the this pointer approach.

typedef (myclass::*PFN)();

PFN pfn=&myclass::myCallbackFunction;
........

API_function((DWORD_PTR)this->*pfn);

hehe, I was doing just tht right now... but I get a warning:

warning C4312: 'type cast' : conversion from 'DWORD' to 'CACMClass *' of greater size

the api has a parameter to pass a value to the callback function, it's DWORD dwInstance

As I know DWORD is unsigned long, and it should be enough to pass a class pointer right ?

Then why is the warning " ... of greater size" ?

Quote:

---

Originally posted by CVMichael
hehe, I was doing just tht right now... but I get a warning:

warning C4312: 'type cast' : conversion from 'DWORD' to 'CACMClass *' of greater size

the api has a parameter to pass a value to the callback function, it's DWORD dwInstance

1)As I know DWORD is unsigned long, and it should be enough to pass a class pointer right ?

Then why is the warning " ... of greater size" ?

---

1) Yes

2) Dunno, but try this APIFunction((DWORD)this);

I admit your case is special that your callback is going to be used in the class itself and I had never used acm multimedia functions myself.

Excuse me, all this while I am busy chatting online with a girl, I don't think I can help you, maybe you would want to search the C++ forum.

Thanks for you help...

I think it should work now, although I cannot test it yet, cuz the rest of the code is not done...

Remember to cast it back before using it

Code:

---

myclass* ptr = (myclass*) fdwOpen;//I assume fdwOpen contains this
ptr->mymemberfunction();

---

Really have to go now.;)

It just came to me that you need not pass the class pointer to the API Function because the Callback function which gets it, is a member function of the class itself. So just pass a NULL.

So the casting of the previous post is not needed as well.

Only if your Callback function is 'external' to the class, then you need to pass the class pointer.

Search the forum for callback and class, this has already been answered a few times.
The reason for the warning "of greater size" is due to a compiler setting. DWORD is unsigned long, a 32-bit value. A pointer is 32-bit too, but only for now. Soon there will be 64-bit systems where pointers are 64 bits long. But DWORD will still be 32-bit. Therefore VC++ warns you about this 64-bit portability problem.
DWORD_PTR however is an unsigned value which is the same size as a pointer.