callback in a class

[CVMichael]

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

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 ?

[transcendental]

You have mispelled CALLBACK.

[CVMichael]

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...

[transcendental]

How about trying this

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);
}

[CVMichael]

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

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

[transcendental]

What's that DWORD_PTR doing there?

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?

[CVMichael]

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

[transcendental]

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?

[CVMichael]

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

[CVMichael]

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

[transcendental]

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

[transcendental]

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

The class method will have to be static.

[CVMichael]

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 ?

[transcendental]

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);

[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

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" ?

[transcendental]

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.

[CVMichael]

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...

[transcendental]

Remember to cast it back before using it

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

Really have to go now.

[transcendental]

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.

[CornedBee]

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.